d4.2 – report on technical and quality of service viability
TRANSCRIPT
Document version: 1.2 Submission date: 2019-03-12
(H2020 730840)
D4.2 – Report on Technical and Quality of Service
Viability
Version 1.2
Final Version
Published by the MISTRAL Consortium
Dissemination Level: Public
This project has received funding from the European Union’s Horizon 2020 research and
innovation
programme under grant agreement No 730840
H2020-S2RJU-2015-01/H2020-S2RJU-OC-2015-01-2
Topic S2R-OC-IP2-03-2015: Technical specifications for a new Adaptable Communication
system for all Railways
Ref. Ares(2019)1779406 - 18/03/2019
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Document control page
Document file: D4.2 Report on Technical and Quality of Service Viability
Document version: 1.2
Document owner: Alexander Wolf (TUD)
Work package: WP4 – Technical Viability Analysis
Task: T4.3, T4.4
Deliverable type: R
Document status: approved by the document owner for internal review
approved for submission to the EC
Document history:
Version Author(s) Date Summary of changes made
0.1 Alexander Wolf (TUD) 2017-12-27 TOCs and content description
0.4 Alexander Wolf (TUD) 2018-01-31 First draft version
0.5 Carles Artigas (ARD) 2018-02-02 Contribution on LTE Security, Chapter 5
0.8 Alexander Wolf (TUD) 2018-03-31 Second draft version
0.9 Alexander Wolf (TUD) 2018-04-27 Release candidate
1.0 Alexander Wolf (TUD) 2018-04-30 Final version
1.0.1 Alexander Wolf (TUD) 2018-05-02 Formatting corrections
1.0.2 Alexander Wolf (TUD) 2018-05-08 Minor additions on chapter 6
1.1 Alexander Wolf (TUD) 2018-10-29 Minor additions after 3GPP comments
1.2 Alexander Wolf (TUD) 2019-03-12 Revision after Shift2Rail JU comments
Internal review history:
Reviewed by Date Summary of comments
Edoardo Bonetto (ISMB) 2018-04-19 Review, comments and remarks
Laura Masullo (SIRTI) 2018-04-19 Review, comments and remarks
Laura Masullo (SIRTI) 2018-04-29 Review
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Legal Notice
The information in this document is subject to change without notice.
The Members of the MISTRAL Consortium make no warranty of any kind with regard to this document,
including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose. The Members of the MISTRAL Consortium shall not be held liable for errors contained herein
or direct, indirect, special, incidental or consequential damages in connection with the furnishing,
performance, or use of this material.
The JU cannot be held liable for any damage caused by the Members of the MISTRAL Consortium or to
third parties as a consequence of implementing this Grant Agreement No 730840, including for gross
negligence.
The JU cannot be held liable for any damage caused by any of the beneficiaries or third parties
involved in this action, as a consequence of implementing this Grant Agreement No 730840.
The information included in this report reflects only the author’s view and that the JU is not responsible
for any use that may be made of such information.
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Index:
1 Executive summary ................................................................................. 9
2 Introduction .......................................................................................... 10 2.1 Purpose ............................................................................................... 10 2.2 Scope ................................................................................................. 10 2.3 Methodology ........................................................................................ 11 2.4 Outline ................................................................................................ 11
3 Requirements within Railway Communication Systems ......................... 12 3.1 Overview General Aspects ..................................................................... 12 3.2 Existing Definitions and Standardisations ................................................. 13
3.2.1 CENELEC definitions ...................................................................... 14 3.2.2 Safety requirements ...................................................................... 15
3.3 Retrospective: GSM-R ........................................................................... 16 3.3.1 Functional Requirements – Mandatory Applications features ............... 18 3.3.2 Functional Requirements – Optional Applications features .................. 22 3.3.3 Network Performance Requirements ................................................ 23
3.4 Voice Connectivity ................................................................................ 23 3.4.1 Call related services ...................................................................... 24 3.4.2 Railway specific services ................................................................ 24
3.5 Data Transmission (ERTMS/ETCS, EDOR) ................................................ 24 3.6 Cryptography ....................................................................................... 25
3.6.1 Encryption and authentication algorithms within GSM-R ..................... 25 3.6.2 EuroRadio .................................................................................... 25 3.6.3 Encryption of GSM-R traffic ............................................................ 26 3.6.4 Encryption weaknesses .................................................................. 26 3.6.5 Summary of railway specific services ............................................... 28
3.7 Overview of existing railway QoS definitions ............................................ 32 3.7.1 Specific railway Quality of Service requirements ............................... 32 3.7.2 Packet Core IP network QoS Management ........................................ 34 3.7.3 ETCS QoS Profile for GPRS ............................................................. 37
3.8 Conclusion on Requirements .................................................................. 38
4 QoS in IP-based Systems....................................................................... 41 4.1 Typical QoS implementation methods ...................................................... 42
4.1.1 Oversizing of network and bandwidth .............................................. 42 4.1.2 Reservation of bandwidth ............................................................... 42 4.1.3 Prioritisation of data packets .......................................................... 42 4.1.4 DiffServ - Differentiated Services .................................................... 43 4.1.5 Jitter buffer .................................................................................. 43 4.1.6 CoS - Classes of Service Class Application ........................................ 43 4.1.7 Prioritisation and queuing .............................................................. 43
4.2 Summary ............................................................................................ 44
5 QoS within public mobile radio networks .............................................. 45 5.1 QoS implementation in mobile technology sector ...................................... 45
5.1.1 Requirements on high service availability ......................................... 45 5.2 QoS mechanisms offered by 4G .............................................................. 46
5.2.1 Access Class Barring (ACB) ............................................................ 46 5.2.2 Allocation and Retention Priority (ARP) ............................................ 46 5.2.3 QoS Class Identifier (QCI) .............................................................. 47 5.2.4 4G QoS implementation ................................................................. 47 5.2.5 Requirements and Features of EPS security: ..................................... 50 5.2.6 LTE Security architecture ............................................................... 51
5.3 QoS mechanisms offered by 5G .............................................................. 54 5.3.1 QoS Model – general overview ........................................................ 54
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5.3.2 Quality Requirements in 5G Networks .............................................. 54 5.4 Architectural and procedural basics ......................................................... 54
5.4.1 Voice Services within 4G IP networks .............................................. 55 5.4.2 IP Multimedia Subsystem ............................................................... 55
5.5 3GPP Mission Critical Communication Enhancements ................................. 58 5.5.1 Quality of Service requirements for mission critical applications .......... 60 5.5.2 Derived QoS from Future Railway Mobile Communication System ....... 62
5.6 Summary ............................................................................................ 64
6 Railways QoS Implementation Requirements for future scenarios ........ 65 6.1 System architectural requirements ......................................................... 65 6.2 QoS for MISTRAL innovative services ...................................................... 66 6.3 Conditions for railway functionalities ....................................................... 68
6.3.1 Data Transmission ........................................................................ 68 6.3.2 Voice Transmission........................................................................ 69 6.3.3 Security requirements ................................................................... 73 6.3.4 Authentication concepts ................................................................. 74
6.4 Mapping of functional requirements ........................................................ 75 6.5 Conclusion on Technical viability ............................................................. 76
6.5.1 Connection to economic analyses within MISTRAL ............................. 76 6.5.2 Connection with other projects ....................................................... 77
7 Conclusion ............................................................................................. 79
8 List of tables, figures, and references.................................................... 80 8.1 Tables ................................................................................................. 80 8.2 Figures ................................................................................................ 80 8.3 References........................................................................................... 81
9 ANNEX ................................................................................................... 82
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Terms and definitions
Term Definition
3GPP 3rd Generation Partnership Project – the organisation responsible for the LTE
standard
4G 4th Generation cellular radio technology covers 3GPP standards from Release
8 to Release 14
5G 5th Generation cellular radio technology, including the evolution of LTE along
with new Radio Access technologies in ultra-high Frequency bands
Application Method that provides a solution for a communication need that is considered
necessary for current and future railway operations
Data
communication
Exchange of information in the form of data via a network, including video
transmission (excluding voice communication)
EIRENE
network
An EIRENE network is a railway telecommunications network, based on the
ETSI GSM standard, which complies with all related mandatory requirements
specified in the EIRENE FRS and SRS.
European
Railway Agency
The agency for railway safety and interoperability established by Regulation
(EC) No 881/2004 of the European Parliament and the Council of 29th April
2004 establishing a European Railway Agency.
GSM-R GSM-Railway is an international wireless communications standard for
railway communication and applications.
Network
registration
delay
Value of elapsed time from the request for registration to indication of
successful registration by CREG response (GSM-R)
Packet
Switched Data
(PSD)
All transmitted data is grouped into packets, which are transmitted via a
medium that may be shared by multiple simultaneous communication
sessions.
Quality of
Service (QoS)
QoS describes the quality of a communication service from the user's point
of view. The necessary QoS mechanisms must be implemented along the
entire transmission path between the network elements.
Transmission
interference
A transmission interference period is the period during the data transmission
phase of an existing connection in which, caused by the bearer service, no
error-free transmission of user data units is possible.
Voice
communication
Exchange of information in the form of voice communication, regardless of
the transmission method
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Abbreviations Specific Abbreviations of this document are listed below.
3GPP Third Generation Partnership Project
ASCI Advanced Speech Call Items
ATC Automated Train Control
ATO Automated Train Operation
BTM Balise Transceiver Module
CBTC Communication based train control
CCTV Closed Circuit Television
CS Circuit Switched
DoS Denial of Service
DSCP Differentiated Services Code Point
EAP Extensible Authentication Protocol
EDOR European Data Only Radio
EPC Evolved Packet Core
eLDA enhanced Location Dependent Addressing
eMLPP Multi-Level Precedence and Pre-emption
eREC enhanced Railway Emergency Calls
ERTMS European Railway Traffic Management System
ETCS European Train Control System
ETSI European Telecommunications Standards Institute
FRMCS Future Railway Mobile Communication System
FRS Functional Requirements Specifications
GBR Guaranteed Bit Rate
GGSN Gateway GPRS Support Node
GPRS General Packet Radio Service
GSM-R Global System for Mobile Communication – Railway
HSS Home Subscriber Server
IETF Internet Engineering Task Force
IM Infrastructure Manager
IMS IP Multimedia Subsystem
IP v4/v6 Internet Protocol (Version 4 / Version 6)
IPSec IP Security
IoT Internet of Things
ITU International Telecommunication Union
LTE Long Term Evolution (4G cellular system)
MAC Message Authentication Code
MME Mobile Management Entity
MORANE MObile radio for RAilway Networks in Europe
MS Mobile Station
MSISDN Mobile Station International ISDN Number
NGBR Non-Guaranteed Bit Rate
NSS Network Sub-System
OSI Open System Interconnection reference model
PCRF Policy and Charging Rules Function
PDN Packet Data Network
PER Packet Error Rate
PS Packet switched
PTT Push-To-Talk
QoS Quality of Service
RAN Radio Access Network
RSVP Resource Reservation Protocol
SDF Service Data Flow
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SIL Safety Integrity Level
SRS System Requirement Specification
TOS Type of Service
TSI Technical Specification for Interoperability
UE User Equipment
UIC Union Internationale des Chemins de Fer
URS User Requirements Specification
VBS Voice Broadcast Service
VGCS Voice Group Call Services
VoLTE Voice over LTE
VoIP Voice over IP
VRS Voice Recording System
WAN Wide Area Network
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1 Executive summary
A future migration of recent railway communication services to more modern, non-
exclusive Wireless Voice and Data Communication is currently being studied in
numerous working groups in the Telecom Industry, Railway Supply Industry and
normative Organisations mainly in terms of basic (technical) feasibility. However, a
coherent business scenario model has not been presented yet. This is mainly due to
several missing complex technical and economic issues that must be analysed. The
MISTRAL project will process these issues by aiming at these objectives. In addition to
economic considerations, MISTRAL also addresses the technical prerequisites, which are
necessary for the implementation of the new paradigms.
Today, the European Train Control System (ETCS) is the leading signalling system for
train control functionality. In the future, ETCS will be provided over new generation
(4G/5G) mobile networks.
It is necessary to analyse whether the specific railway safety requirements can be met
by the performance of the new networks. Special emphasis must be placed on the
integrity of ETCS data, i.e. ETCS data transmission must be protected against loss,
damage and manipulation. This deliverable considers various approaches to quality of
service analysis. These analyses are intended to clarify whether ETCS data integrity and
the functionality of other railway-specific services can be achieved even under difficult
conditions using the appropriate 4G/5G network QoS mechanisms.
The Global System for Mobile Communications-Railways (GSM-R) currently provides
communication between the ETCS elements. ETCS and GSM-R form the European Rail
Traffic Management System (ERTMS). GSM-R was developed specifically for railway
communication, but is now an out-dated technology with known limitations, especially
in the performance of data transmission mechanisms.
Inefficient use of network resources, insufficient capacity and limited support for
broadband data communication are the main problems of GSM-R. Alternative packet-
oriented technologies would bring higher capacity and efficiency. 4G and 5G network
technologies are under consideration to replace GSM-R in the future.
The convergence to an all IP based network will provide a more efficient use of the
network capabilities. Main drawbacks and challenges of this paradigm change are the
QoS assurance methods, guaranteed call setup times, proper service prioritisation and
IP security issues. The 4G and 5G QoS mechanisms have to be identified, discussed and
analysed if they fulfil existing and future railway requirements.
In order to define a comprehensive QoS definition for the future communication
technology, it is first necessary to analyse and define both functional and system-
specific technical requirements to the communication network. The definition of a QoS
vector for railway communication applications will be examined in the following
document and its applicability to new architectural network paradigms will be verified.
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2 Introduction
2.1 Purpose
The evolution of mobile telecommunication network standards is constantly being
driven forward. Increasing the data rates and improving the data transmission will
stimulate ideas to use the latest standards also for railway communication purposes as
well.
In the process of finding a replacement for GSM-R different mobile communication
standards were discussed within the MISTRAL project (D4.1). For the application of
future mobile radio systems within the railway sector and especially with the utilisation
of new paradigms (like Network-as-a-Service) certain individual service features and
their high technical requirements must be taken into account. The deliverable D4.2
focuses on the implementation of railway-specific requirements of the train radio
system on the technical characteristics of a modern mobile radio generation. The focus
is on quality of service and a possible feasibility study is performed.
2.2 Scope
The innovation cycles of technical railway systems are considerably longer than in other
industrial sectors. The reasons for this are, among other things, the longevity of
individual components, the complexity of the overall system, the compulsion for
compatibility (especially downward compatibility) of different technologies and the high
investment sums.
However, this makes it necessary to define the Quality of Service (QoS) requirements
to the communication layer as independent of the actual mobile radio standard as
possible. Although the change from circuit-switched transmission to packet-switched-
transmission necessitates a new conception of the QoS, it does not seem sensible to
determine a new QoS vector for each new telecommunications standard.
The idea of QoS for networks is to calculate, improve and guarantee in advance the
transfer rates, error rates and other transmission channel characteristics. QoS within
railway radio systems is particularly important for the continuous transmission of critical
low-latency data and also of high-bandwidth data in future, as this content is difficult to
transmit especially in public networks using common best-effort protocols. Quality of
service can be improved with various techniques such as packet prioritisation,
application classification and queue management.
The definition of a generic QoS vector for railway communication applications will be
examined in the following document and its applicability to new architectural network
paradigms will be addressed. This vector describes qualitatively and quantitatively the
characteristics and performance indicators of the transmission channel that meets the
specific requirements of future railway communication.
A capture of requirements is done, that leads to a clear definition of user requirements,
both functional and non-functional. This is a basis for a feasibility study to carry out an
initial assessment of the next generation system. The aim is to determine whether the
system meets its requirements and whether it is viable to implement as a railway
communication system. The study consists of defining the requirements for the new
system, identifying implementation options and assessing their impact.
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2.3 Methodology
The approach used for the analyses carried out to recommend a future QoS and
performance vector are the following. First, the functional and technical requirements of
the existing GSM-R train radio system are analysed. The system specifications of
ERTMS and existing standardisations in the railway domain are used as a foundation.
The existing requirements and principles of operation must also be implemented in a
corresponding way in future communication systems in order to guarantee efficient
railway operation. Furthermore, the network transmission functionalities provided by
future systems are evaluated. Modern mobile radio networks offer comprehensive QoS
functionalities that can be used in the future railway radio system. The results of both
analyses are compared and a new QoS vector is derived and specified, which can be
used for future railway communication.
2.4 Outline
Chapter 3 analyses the existing normative specifications, requirements and
implementations of functions and their quality of service demands of today’s railway
communication networks. An outline is given which specifications and standards are
relevant in the existing GSM-R train radio system. Derived from this, the functional
requirements for the train radio system are described and analysed.
The chapter 4 explains how Quality of service paradigms are fundamentally
implemented in modern IP-based network systems and which methods and algorithms
nowadays exist. Parameters that are particularly relevant for assessing the functionality
are explained in more detail. This chapter also forms the basis for the analyses in
Chapter 5.
Within the 5th chapter the QoS implementations of current public mobile phone
standards will be examined. Modern 4G and 5G mobile radio standards offer powerful
QoS frameworks on the network side, which contain flexibly adjustable prioritisation
functionalities for specific service demands. These capabilities and principles are
examined and the performance characteristics are identified.
The following chapter 6 combines the results of the previous chapters. An analysis
whether the existing stringent requirements can also be fulfilled in the future is
executed. Defining a concrete QoS vector for a future IP based railway communication
network is the aim of this section. The deliverable concludes with a summary and
overview of the results.
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3 Requirements within Railway Communication
Systems
3.1 Overview General Aspects
Basic principles and requirements on the development of railway voice and data
applications must first be specified, before specific technical details of the application
demands to the transmission technology can be addressed. Within the chapter 3
existing definitions and standardisations of functional and technical requirements for
railway communication are being discussed. The current ERTMS definitions are analysed
and requirements for the corresponding applications and communication methods are
evaluated.
First, this document analyses and highlights the end-to-end performance and
functionalities of the legacy GSM-R radio railway systems. This analysis serves as a
basic requirement to recognize necessary services, which a future railway
communication system has to provide. These functionalities are compared with the
technical performance features of new IP-based technology candidates (such as 4G/5G
networks) and conclusions with regard to the required feature implementation are
made. This document also contains novel introduced functionalities provided by the
MISTRAL project as innovative solutions for the development of railway services for
operational purposes as well as for railway passengers.
In order to define QoS within the railway communication system, an assessment must
first be made of which functions and subareas are of interest for the characteristics of
the data and voice transmission. This concerns the following questions:
• Which railway specific applications and services must be implemented within the
future railway communication network to meet todays and future demands?
• What are the requirements on QoS of these services that can be deduced from
this?
• Which QoS parameters are suitable for description?
• How can concrete values be defined quantitatively?
From a general point of view, the term QoS is derived from a series of measurable
values that indicates the total performance of a particular service. Basic criteria for
assessing the service performance of network applications are reliability (occurrence
and amount of transmission errors), throughput and delay requirements. Powerful QoS
management in mobile networks is particularly important to ensure that the
performance expectations of different users, which differ depending on the type of
application, must be met for critical applications on the network side. Varying levels of
performance will be required from users of different services.
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3.2 Existing Definitions and Standardisations
First, within this section an outline is given of which standardisations apply to the
railway communication system and which definitions exist.
Fundamentally GSM-R represents a special extension of the GSM standard for the
railway sector and includes additional railway-specific functions. The TSIs for Control
and Command ([19]) contain a complex structure of generic requirements on GSM-R
and ETCS. The references to underlying detailed technical specifications referring to
GSM-R are:
• EIRENE (UIC) Functional Requirements Specification FRS
• EIRENE (UIC) System Requirements Specification SRS
• MORANE (UIC) FFFIS for Euroradio
• Test Specifications for Mobile Equipment GSM-R
• GSM-R Version Management
The UIC project EIRENE developed a set of specifications for the European railways that
form part of the specification for technical interoperability as required by the EC
Directive for interoperability of the Trans-European high speed rail system.
The EIRENE specifications ([9]) were written down in the Functional Requirements
Specification (FRS) and the System Requirements Specification (SRS). These
standardisations include the comprehensive requirements for the system.
The FRS distinguishes between four basic service types, derived from requirements
defined by UIC. These elementary service categories are in particular:
• Voice services,
• Data services,
• Call related services,
• Railway operation specific services.
These basic functional groups are examined in more detail in the following sub-chapters
3.3 – 3.6 and the associated requirements for the communication network are derived
and explained in more detail.
The EIRENE Specifications define a radio system satisfying the mobile communications
requirements of the European railways. They encompass ground-to-train voice and data
communications, together with the ground-based mobile communications needs of
track-side workers, station and depot staff and railway administrative and managerial
personnel. The application of the specifications will ensure interoperability for trains and
staff crossing national and other borders between systems. It also intends to provide
manufacturing economies of scale wherever practical.
Together with the MORANE specifications the EIRENE are designed to provide the
minimum set of requirements necessary to ensure international interoperability
between GSM-R networks. The E-SRS covers summarized the following sections:
• Network services (GSM Phase 2 services, railway-specific services)
• Network planning
• Mobile Equipment specification (Core specification, cab radio specification,
general purpose radio, operational radio)
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• Numbering plan and call routing (Numbering plan requirements, constraints,
structure of functional Numbers, numbering plan)
• Subscriber management
• Modes of operation (standard mode, railway emergency calls, shunting mode,
direct mode)
With ERTMS/GSM-R the UIC started a project to complement the work specified in
EIRENE and MORANE. ERTMS is a system, formed by three elements:
• GSM-R, the telecommunication system and bearer for ETCS,
• ETCS as the European train control system
• The Traffic Management Layer.
The requirements defined in the above specifications for the existing railway
communication network form the basis for the analyses of this document. Future
technologies will have to measure up to both functional requirements and technical
requirements. Fulfilling existing quality of service values is essential.
However, it should be noted that existing quantitative specifications cannot be adopted
directly and that new requirements must be specified, which also flow into
standardisation. Due to the paradigm shift in future transmission technologies
(connection-oriented to packet-based transmission), new methods and evaluation
standards are taking effect.
3.2.1 CENELEC definitions
Furthermore, fundamental requirements of data transmission via open networks are
also being considered within these studies. At the European level, there exist a number
of standards which may be relevant to the safety of railway communications:
• IEC 61508, Functional Safety: Safety Related Systems;
• EN 50126, Railway Applications: Dependability for Guided Transport Systems;
• EN 50128, Railway Applications: Software for railway control and protection
systems;
• EN 50129, Railway Applications: Safety-related Electronic Railway Control and
Protection Systems;
• EN 50159-1, Railway Applications Part 1: Communication in Closed Transmission
Systems; Requirements for Safety-Related data communication,
• EN 50159-2, Railway Applications Part 2: Communication in Open Transmission
Systems. Requirements for Safety-Related data communication.
The standards are aimed at the development, maintenance and operation of systems.
They embody the concept of the safety lifecycle, in which the most important levels of
the safety guarantee are defined. Furthermore, the processes and procedures to be
applied in each phase according to the required safety integrity are specified.
The two EN50159 standards apply to communication systems for the transmission of
safety-relevant data. Since GSM-R is one implementation for data transmission, these
standards are not being directly applicable to GSM-R. However, they would apply to any
application that uses a data bearer (e.g. via GSM-R or successor technology) to
transmit safety-relevant data. Especially for IP-based transmission, the structural notes
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of CENELEC EN50159 “Safety-related communication in transmission systems” are also
retained.
An overview of safety-critical communication in public and private networks as well as
reference architectures is given.
The recent applicable standard for railway in the domain of communication, which
covers aspects of safety and also security, is the EN 50159 "Railway Applications -
Communication, Signalling and Processing Systems – Safety Related Communication".
Standard EN50159 is defining clearly what aspects of communication are covered:
Requirements needed to achieve a safety-related communication between safety
related equipment using a transmission system which was not necessarily designed for
that. The analyses take into account intended attacks by means of messages to safety-
related applications, but do not concern general communication security issues.
3.2.2 Safety requirements
Safety approvals for train communications systems are carried out on a national level.
However, several Member States, as well as the UIC Safety Committee have stated that
the GSM-R system itself does not possess any safety requirements.
There are no specific safety requirements on GSM-R and therefore do not need to be
linked to a specific safety integrity level (SIL), but certification for certain GSM-R
system elements is required.
Telecommunications services within railway sector are used to support safety-relevant
railway applications. However, safety is ensured by applications outside the
telecommunication systems or by operating procedures. Communication systems are
only used as a non-safety relevant carrier service. In order to ensure adequate safety
for specific railway applications using telecommunications services, the transmission
systems themselves have certain reliability requirements, but no special safety
requirements.
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3.3 Retrospective: GSM-R
According to the European Council Directive 91/449/EEG of 29 July 1991 concerning the
development of Railways in the European Community a telecommunications network
has to fulfil the following requirements:
1. be open and based on non-proprietary and standardized technology,
2. be nationally interoperable,
3. allow for international train traffic,
4. allow for open competition between train operators,
5. support new, standardized train control systems (ETCS),
6. be future-proof,
7. be able to support safety voice communication and other railway functionalities,
8. be able to support the different business needs of track owners and train
operators,
9. allow for competition between telecom suppliers.
These basic defined requirements are still valid and serve as a normative basis for the
assessment of the qualification of new communication technologies within the railway
sector.
In 1995 the International Railway Union (UIC) chose GSM as the most appropriate
platform for railways. Today, GSM-R, the Global System for Mobile Communication for
Railways, is a worldwide mobile communication system in the railway domain.
GSM-R is based on the most advanced and widely used 2nd generation GSM network
and is standardised by ETSI, the European Telecom Standards Institute and is
supported by the GSM Association (GSMA). However, the standard GSM specification
could not meet all the requirements for efficient railway communication in operation. It
was therefore necessary to identify and specify extra functionalities.
The GSM-R radio standard uses a specific frequency band around 800/900MHz. In
addition, the frequency bands 873-876 MHz (uplink) and 918-921 MHz (downlink) are
used as extenders for GSM-R at national level under the name Extended GSM-R (E-
GSM-R). Typically, GSM-R is implemented via dedicated base stations (BSs) near the
track, with distances between two adjacent BS of maximum 7-15 km to ensure higher
availability and reliability. GSM-R must meet stringent requirements for the availability
and performance of radio services.
The diagram Figure 1 shows an overview of the architectural framework of the services
that can be offered by the GSM-R network.
Several additional, railway specific features had to be described, standardized and
developed, in order to integrate the specific requirements of railways into existing
mobile communication. These functions are called ASCI features. ASCI stand for
Advanced Speech Call Items.
Specific ASCI features for implementation within the railway communication system
are:
• priority and pre-emption,
• voice group-calls,
• voice broadcast calls,
• railway emergency calls,
• fast call set-up for railway emergency calls,
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• functional numbering,
• location dependent addressing.
Figure 1: GSM-R service architecture {Ref [16]}
These features are described in more detail below. In principle, these functions are
regarded as the basic functional requirements for voice communication in railway
operations. Future technologies must also be able to reflect these specific
functionalities.
Nowadays, radio networks and railway-specific communication services for railways are
usually provided by the infrastructure manager. In a future scenario, it is probably
mandatory for private telecom providers to implement similar functionalities in order to
be able to map all previous GSM-R functions to a next-generation system. MISTRAL
developed different approaches on how an implementation strategy for train radio could
be implemented in the future. The main focus was on Network as a Service (NaaS)
approaches.
The above-mentioned features were developed together with ETSI and have meanwhile
been incorporated into the GSM standards. They are also partly developed by GSM
providers and made partially available to public operators in selected public networks.
Firstly, we will not go into the technical specification of the equipment technology and
the performance of the network level here. This description of the GSM-R system is
referenced in MISTRAL D4.1. Document D4.2 focuses on the functional requirements
and the QoS definitions, which are important as input variables for future analyses of
requirements to the next generation communication system.
In the following, all functional requirements for the railway-specific communication
network (voice and data) are discussed and summarized. This creates a basis for
identifying the suitability of new technologies and revealing possible shortcomings and
challenges in implementation.
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3.3.1 Functional Requirements – Mandatory Applications features
The applications described in this chapter form special functional requirements for the
future communication system. The network implementation might be done in a different
way, but the basic principles and paradigms have to be taken into account. These
requirements are derived from the legacy GSM-R technology.
Future enhancements of these basic mandatory functions are described in the MISTRAL
project and discussed in more detail in a further chapter.
3.3.1.1 Multi-Level Precedence and Pre-emption (eMLPP)
The functionality for Precedence and Pre-emption is implemented with eMLPP. It is a
mandatory functionality within the GSM-R system. The service consists of two parts:
Priority and Pre-emption.
The Precedence feature is responsible for assigning a priority level to a call, in
combination with a fast connection setup. Pre-emption is the seizure of resources used
by a call with lower priority, by a call with higher priority if no unused resources are
available. With Pre-emption it is also possible to involve a disconnection of a running
lower priority call in order to accept an incoming call with higher priority.
The standard system currently distinguishes between 5 different priority levels. The
Table 1 shows the differences between the functions and the individual priority levels.
Table 1: Priority regimes within GSM-R voice connections {Ref UNISIG}
eMLPP
priority
UIC description Pre-emption
0 Railway emergency calls 1 and below
1 Control-commands (safety) 2 and below
2 Public emergency and group calls
between drivers in the same area
3 and below
3 Railway operation
(calls between train drivers and
controllers) and Control-
command information
4
4 Railway information and all other
calls
-
3.3.1.2 Railway Emergency Calls
The Railway Emergency Call (REC) is required to alert railway personnel in a predefined
area in the event of an emergency. The railway emergency call is set up as a high-
priority connection. Railway emergency calls are divided into different groups for train
radio, shunting radio and general voice services.
Two types of railway emergency calls are defined:
• Train emergency calls (for railway emergencies without shunting operation),
• Emergency shunting calls (for railway emergency in shunting operation).
Railway emergency calls are not covering public emergency calls (e.g. 112) handled by
GSM-R with a lower eMLPP priority (2 vs. 0; see the previous section on eMLPP for
details).
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Within the EIRENE FRS also an optional enhancement of RECs (eRec, see 3.3.2.3) is
described. Extended localisation and referencing on specific tracks and train-run paths
should help to restrict the selection of railway emergency call recipients:
To minimise the impact on trains not affected by the incident it should be possible to
use an additional set of functionalities to enhance the operation of railway emergency
calls so as to define these areas to include or exclude joining, crossing and parallel
tracks and shunting areas.
3.3.1.3 Functional Addressing
Functional addressing allows a user or application to be reached under a number that
identifies its relevant function (e.g. driver of a specific train) and not the terminal. The
functional HLR (HLRf) controls the assignment of functional numbers to MSISDN. The
participant must register with his MSISDN and the functional number at the beginning
of the mission. Afterwards, it can be reached under this special function number.
There are different possible options for the implementation of a functional addressing
service:
• using Intelligent Network (IN) facilities,
• within the HLR of the GSM network using additional services,
• implementation of a dedicated switch with associated databases.
The IN-based solutions call routing is implemented separately from the communication
service provider. So called Service Switching Points (SSP) are used to detect whether a
calling party wishes to use IN functionality. If this is detected, then the information
provided by the calling party is transferred to the Service Control Point (SCP) which will
act upon the information provided.
If the IN solution is used within the GSM-R network, the caller can dial a Functional
Number (e.g. specific Number of Train plus additional service numbers to reach the
train driver directly), which is "trapped" by the SSP and forwarded to the SCP. The SCP
translates the functional number into the corresponding MSISDN. The SSP will then
make the call as desired.
Once the connection is established, the Functional Number is passed via the fixed
network and the MSC/SSP to the Service Control Point, where it will be translated into
the MSISDN number of the appropriate mobile equipment. Using this number, the call
will then be set up via the GSM network to the correct mobile extension.
3.3.1.4 Group & Broadcast Calls
Group calls in GSM-R are based on the GSM ASCI Voice Group Call Service (VGCS)
whereas broadcast calls are based on the GSM ASCI Voice Broadcast Service (VBS).
The Voice Group Call Service (VGCS) enables a group of subscribers to communicate
with each other in a predefined service area. The VGCS reaches all authorized users in
the defined area. The triggering mobile subscriber can leave the voice channel to
another subscriber the uplink can be changed several times. Operational use is the train
radio emergency call.
Voice Broadcast Service (VBS) group call is a call that reaches all authorized subscribers
in the predefined group call area. The calling party can speak, all other parties can hear
only. The group call is made within a defined group in a defined range. These
parameters are recorded in the Group Call Register (GCR).
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3.3.1.5 Location Dependent Addressing
Location-dependent addressing enables calls from the train to be forwarded directly to
the responsible dispatcher. This depends on the current position of the train. The driver
dials the dispatcher via a speed dial number. The call is forwarded to the responsible
dispatcher based on the cell identification. To be able to forward the call correctly, GSM-
R requires information about the train location.
The train passes various control areas within its journey. An automatic update process
is implemented within GSM-R to forward the driver's call to the right dispatcher at any
time. It’s not feasible for the driver to manually determine where the train is and then
enter this information into the handset before making the call, especially for emergency
calls.
3.3.1.6 Fast Call Setup
As defined in EIRENE FRS as a requirement, railway emergency calls must be set-up in
less than 2 seconds in 95% of cases. This feature is required to support this definition.
Important is the end-to-end delay of the radio call setup. The fixed network layer also
has an important role within the transmission chain for the connection setup. It could
delay the connection setup by 250ms. In the future, particular focus must also be
placed on these values, as new mechanisms are used in the paradigm shift from circuit-
switched to IP-based voice transmission, which also have a significant influence on call
set-up times and connection initiation procedures.
3.3.1.7 European Train Control System
One of the most important current GSM-R main application is the European Train
Control System (ETCS). Today GSM-R is the only available carrier for vital data
transmission in ETCS. In order to meet the ETCS requirements, the stricter
requirements for the GSM-R network described in the EIRENE specification must be
met. The quality of service (QoS) requirements are described in Subset 093.
The test conditions for the measurement of subset 093 compliance are described in the
UIC document (Subset 093 - GSM-R for ETCS QoS requirements).
The EuroRadio protocol (see 3.6.2) uses the carrier services of a GSM-R network to
transmit information between OBU and RBC. The service provider supplies these data
carrier services at defined interfaces. The cryptographic techniques of the EuroRadio
layer are described in section 3.6.2 and conclusions are drawn for future
implementation strategies.
The ERTMS/ETCS reference architecture is shown in Figure 2. The main functionality of
the logic for train control is in the Radio Block Centre (RBC). The RBC decides how the
trains run in their specific controlling area. The RBC receives necessary real-time
information about the monitored area from the train. Each ETCS train is equipped with
an ETCS On-board Unit (OBU). The OBU is responsible for the transmission and
processing of the information of the RBC. Important data such as train position, speed
and direction are transmitted. The RBC must also be informed about the condition of
the track elements on the track. Based on all this information, the RBC can decide
which trains are allowed to run on a particular train route and transmit these decisions
to the OBU via messages (specifically the ETCS movement authority (MA)). Different
message types can be exchanged within the ETCS (e.g. configuration messages,
location information). However, the most important message types, that provide the
core functionality of the system, are the location update and the MA.
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Figure 2: Reference architecture of train-side/track-side components of ERTMS/ETCS {Ref [6]}
ETCS ensures safe and efficient train operation. To achieve this aim, reliable and low-
latency communication between the ETCS elements is required. However, if ETCS data
transmission between the entities is disrupted or malfunctioning, trains have to be
stopped preventively until communication is restored. This situation is undesirable as it
leads to operational interference. In addition, any uncontrolled state always represents
a safety risk.
Due to the strict security and efficiency concerns, the underlying network must meet
high requirements for the performance of ETCS message transmission. Currently the
transmission is carried out via circuit-switched data calls in the GSM-R network.
Therefore, ETCS requirements have so far only been defined for circuit-switched
transmission. New generation mobile networks are packet-switched networks.
Therefore, the performance conditions of 4G networks data transmission capabilities will
not be directly comparable with current requirements.
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3.3.2 Functional Requirements – Optional Applications features
In addition to the mandatory functions to be implemented, the GSM-R specification also
contains functions that can be implemented optionally. The implementation of this
functionality is the responsibility of the respective infrastructure manager. In order to
consider the requirements of future transmission technology, the analysis of these
functionalities is carried out at this point.
3.3.2.1 Direct Mode
Within the EIRENE specifications an option for direct point-to-point communications
between personnel is defined. If the mobile network services provided by the GSM-R
core network become unavailable, this system can still be used for communication. This
method is known as Direct Mode.
The direct mode functionalities are:
• short range fall-back communications between train-side and track-side
personnel in the event of failure of all railway and/or public GSM services,
• short range communications for railway personnel in areas where no GSM
facilities are available at all. (Ref EIRENE FRS and SRS sections 15).
3.3.2.2 eLDA
Enhanced Location Dependent Addressing (eLDA) is an optional feature that improves
the accuracy of GSM-R Location Dependent Addressing and is specifically intended for
use in situations where the accuracy of cell dependent addressing is not sufficient. eLDA
functionality uses data from external systems (for example GPS, balises and trackside
train detection systems).
eLDA call setup times shall meet the call setup timing requirements for point to point
calls as specified in the EIRENE FRS specification. The eLDA system shall be possible to
transmit train location data over the air interface with a granularity equivalent to a
maximum distance of 1 meter.
3.3.2.3 eREC
With emergency calls in GSM-R (RECs), there is no distinction between specific track
lines possible. Therefore, when a REC is initiated in a particular area, the trains on the
other lines can also receive the REC. The consequence is that trains unnecessarily on
other tracks are affected by the emergency. This can be a big drawback in concerning
delays, particularly when a high-speed train is stopped to due to a conventional train.
eREC stands for enhanced Railway Emergency Calls and its purpose is to provide a
solution to this problem by enabling Railway Emergency calls to be set up only to the
subscribers/lines that are directly affected.
3.3.2.4 Shunting communication
Within the EIRENE FRS and SRS the requirements for shunting are defined. Shunting as
a GSM-R application is an optional service, but some shunting features are mandatory
in all cab radios. The shunting requirements have been written to integrate common
features to make shunting safer and more viable. These features include the link
assurance signal, protected group calls and shunting emergency calls.
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3.3.3 Network Performance Requirements
The infrastructure manager and GSM-R operator needs to check frequently the fault-
free operation of the network. It must be ensured that the requirements for the quality
of the radio network are fulfilled and QoS values are met. For network planning, the
coverage level is defined in terms of time and area where the minimum signal criteria
are achieved.
Selected relevant radio coverage criteria:
• The level of coverage should be at least 95% of the time over 95% of the
designated coverage area for a radio installed in a railway vehicle with an
external antenna.
• The land-based part of the system shall provide communications for mobiles
when stationary and when travelling at speeds up to the maximum allowable line
speed or 500 km/h.
• Received Signal Quality “rxQual UL & DL”: 95% of the results must be level ≤4
• Received Signal Level “rxLev UL & DL”: 95% of the results must be level ≥15
3.4 Voice Connectivity
Railway voice communication is quite similar to commercial mobile communication.
However, due to several railway specifics, there are some important differences in
terms of features and performance requirements.
The description of existing functional requirements regarding voice applications are
defined in EIRENE FRS chapter 2.2 and is limited to the different types of calls. The
following implementations are mentioned in detail:
• point-to-point voice calls
(voice calls between any two call parties, which shall allow both parties to talk
simultaneously)
• public emergency voice calls
(allow a user to make public emergency point-to-point voice calls. Such
emergency calls include ‘112’ calls and may not be used for railway
emergencies)
• broadcast voice calls
(these voice calls provide one-way voice communications from a single user to
multiple users in a pre-defined local area)
• group voice calls
(provide voice communications between a number of users in a pre- defined
local area, all of whom are members of the same call group. The composition of
call groups shall be able to be modified within the network)
• multiparty voice calls
(multi-party voice communications between up to six different parties. Any of
the parties involved in a multi-party voice call shall be able to talk
simultaneously).
Besides the additional features, railways defined requirements on call setup times.
These requirements ensure a sufficient level of service. Fast call setup times directly
contributes to railway safety: in the case of a railway emergency call a short setup time
ensures that all railway staff are quickly informed about a potentially dangerous
situation.
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Table 2: Requirements on voice call setup times {Ref [9]}
Type of call Call setup time
Railway emergency call <4s
High priority group calls <5s
All operational and high priority
mobile-to-fixed calls not covered by
the above
<5s
Other operational mobile-to-fixed
calls
<7s
Other operational mobile-to-mobile
calls
<10s
Low-priority calls <10s
3.4.1 Call related services
As mandatory implemented call related communication services, the following
applications are identified, following FRS (Ref FRS, Sec 2.4):
• Display of identity,
• Priority and pre-emption,
• Closed user groups,
• Call forwarding,
• Call hold,
• Call waiting,
• Charging information,
• Call barring.
These call related services are features that provide basic functionality related to user
interaction and operation for call devices.
3.4.2 Railway specific services
As mandatory implemented railway specific services, the following applications are
identified, following FRS (Ref FRS, Sec 2.5). The EIRENE network shall provide support
for the following railway specific services:
• Functional addressing including registration/deregistration,
• Location dependent addressing,
• Railway emergency calls.
3.5 Data Transmission (ERTMS/ETCS, EDOR)
As use cases of implemented data-only communication services, the following
applications are identified, following FRS (Ref FRS, Sec 2.3):
• text messages (optional)
(point-to-point and point-to-multipoint text messages from the ground to mobile
users)
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• general data applications
(range of data communications between the ground and mobile users, i.e.
timetable information; maintenance and diagnostic applications; mail and
messaging; remote database access)
• train control applications
(Where ERTMS/ETCS level 2 or 3 is implemented, the network shall be capable
of supporting data communications for that train control system with the
required quality of service. Communications for train control may be
characterised as low data rate per train; however, in some areas there will be a
high density of trains requiring simultaneous communications).
These functions are explained in more detail within this section and the requirements
for the communication network are specified.
3.6 Cryptography
3.6.1 Encryption and authentication algorithms within GSM-R
The legacy GSM-R-System is based on the GSM standard. Standardized cryptographic
methods are used for the application of network internal procedures for encryption of
the connection.
Securing the use of voice privacy (encryption) algorithms and authentication algorithms
for SIM cards is of interest to railways. Examples of standardized GSM cryptographic
algorithms are:
• A3 and A8 authentication algorithms,
• A5/3 encryption algorithm.
The A3 and A8 algorithms are open documented and freely available (see 3GPP TS
55.205 "Specification of the GSM-MILENAGE Algorithms”). A5 is closed source.
The ETSI GSM specifications only refer to the algorithms in general terms and do not
specify them in detail. The use of standardized voice privacy algorithms is mandatory
for a GSM-R network. Each railway is free to implement its own authentication
algorithms providing that there is no resulting loss in cross-border interoperability.
GSM-R uses the A5 suite of cryptographic ciphers, more specifically A5/1, a stream
cipher based on Linear Feedback Shift Registers (LFSRs). Optionally, the block cipher,
A5/3, may be supported. These ciphers are used for encryption of the communication
between mobile stations and the network plane (e.g. trains and handheld devices).
Mobile handsets and devices (e.g. cellular staff phone, cab radios) are authenticated
onto the network. However, due to the limited authentication functionality of GSM, the
base station is never authenticated to the handset.
3.6.2 EuroRadio
The EuroRadio protocol is a specific protocol layer between the GSM-R and Application
Layer protocol (see Figure 3). EuroRadio provides the authentication functionality and
integrity verification for ETCS messages sent and received via GSM-R. Thus, by adding
cryptographic MACs EuroRadio guarantees that messages received are authentic. GSM-
R provides the encryption methods between the train and network plane (mobile base
stations).
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Figure 3: Communication protocol stack architecture (TUD)
The MAC used in EuroRadio is derived from the ISO 9797-1 MAC Algorithm 3. This
cryptographic algorithm computes a MAC using DES and 3DES ciphers. The MAC is
computed on the length of the entire message, the ETCS ID of the recipient, the
message being sent and optional padding to ensure appropriate length for the MAC
computation. However, the specific EuroRadio MAC algorithm uses three distinct keys in
the final 3DES computation. The cryptographic keys used by EuroRadio to generate the
MACs are session keys. These keys are derived from a pre-shared 3DES key. They are
exchanged during the protocol handshake phase.
To ensure validity and integrity of ETCS messages, the EuroRadio layer is capable to set
different priorities of messages. Messages which have a “normal” priority, must have a
MAC computed and added to the payload. Messages with “high” priority do not contain
a MAC and bypass verification by the EuroRadio layer. They can include a very
restricted set of commands only (e.g. emergency stop).
3.6.3 Encryption of GSM-R traffic
GSM-R utilise the cryptographic mechanisms of GSM to protect messages. Security
vulnerabilities that exist against the GSM standard may be exploitable within GSM-R as
well. For example, channel jamming attacks could affect all operations within range of
the equipment for ERTMS. Also the interception of communications or an insertion of
messages into the communications channel might be possible. Unauthorized, inserted
messages could cause a train to stop and thus to influence the traffic operation within a
railway network completely.
The weaknesses of GSM encryption have been already evaluated. It was shown that the
A5/1 cipher used in both GSM and GSM-R could be broken with different attack
approaches. An export variant of A5/1, A5/2, was also shown to be weak and is
vulnerable to real-time attacks. Once the A5 key has been established, it is then
possible to recover traffic sent over the GSM-R link, including train control messages.
Another security risk is posed by so-called “IMSI Catchers”, which can get the TSMI and
some undisclosed ERTMS values (e.g. train ID) of an operational train.
3.6.4 Encryption weaknesses
EuroRadio MAC algorithm is the algorithm used to secure communication between trains
and track radio network equipment. On the ERTMS stack EuroRadio provides the safety-
critical function of message authentication.
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There are potential security vulnerabilities that could be exploited ([17][18]). This made
it possible to develop a key recovery attack by exploiting collisions in the MAC for
various messages. Cryptographic weaknesses of EuroRadio can be exploited if one of
the keys used to generate the MACs is successfully recovered. This vulnerability can be
combined in the Application Layer protocol to create a fake Movement Authority with a
valid MAC. These fake messages can be accepted by a train and possibly leading it in a
potential unsafe situation!
Although EuroRadio currently is safe for use in current ERTMS command and control
applications the recommendation is that EuroRadio should not be considered for use in
future high-bandwidth applications (e.g. video streaming, monitoring). High-bandwidth
applications reduce the time required to get collisions and thus session key recovery
becomes more likely. As a result, the MAC algorithm in EuroRadio should not be
considered for such applications.
Among other activities, the following measures are proposed ([18]) to achieve the
objectives of secure network for railway in the future:
• A restriction of utilisation of EuroRadio in high-bandwidth applications,
• Implementation of an alternative, more secure MAC Algorithm,
• Combining EuroRadio and Application Layer to achieve a secure message
sending/receiving service.
The limitations of the current GSM-R encryption strategies have already been confirmed
by railway infrastructure managers and public authorities. UK’s Rail Safety and
Standards Board (RSSB) and Network Rail concluded that EuroRadio is not safe to be
used with a transport protocol faster than GSM-R. For the future use of a next
generation mobile radio standard in the railway sector, it is essential to implement the
authorisation of subscribers with new, safe methodologies.
Approaches to more secure authentication are already included in the 4G/5G standards.
It is necessary to discuss how these more modern cryptographic methods can also be
used in the future railway communication networks. In chapter 5.2.6 we address
authentication concepts and analyse the possibility of an extended implementation for
railway applications.
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3.6.5 Summary of railway specific services
The following
Table 3 summarises in detail all functional requirements that currently exist for railway
specific communication via the GSM-R systems with regard to voice and data
connections. These requirements are based on the EIRENE FRS and are referenced to
the specification.
Future mobile radio standards will necessarily have to support these functional
requirements. The challenge is the implementation of specific voice services for railway
operations, especially the strict requirements for RECs. Individual requirements
regarding the mandatory quality of service will be further investigated in chapter 3.7.
Functions ERTMS Spec.Ref
ERTMS Functional Description
Data/Voice
Voice services EIRENE-FRSv7 2.2.1
This section describes the generic voice telephony services which are to be supported by the railway communication system: − point-to-point voice calls; − public emergency voice calls; − broadcast voice calls; − group voice calls; − multi-party voice calls.
V
Voice services EIRENE-FRSv7 2.2.2
All voice call services shall be able to operate between any combination of fixed and mobile equipment users.
V
Voice services - Point-to-point voice calls
EIRENE-FRSv7 2.2.3
The system shall support point-to-point voice calls between any two call parties.
V
Voice services - Point-to-point voice calls
EIRENE-FRSv7 2.2.4
The IP Communication System should support full duplex communications.
V
Voice services - Public emergency voice calls
EIRENE-FRSv7 2.2.5
The system shall allow a user to make public emergency point-to-point voice calls.
V
Voice services - Broadcast voice calls
EIRENE-FRSv7 2.2.7
The system shall support broadcast voice calls. V
Voice services - Broadcast voice calls
EIRENE-FRSv7 2.2.9
The composition of call groups shall be able to be modified within the network. A single user shall be able to be a member of one or more call groups.
V
Voice services - Broadcast voice calls
EIRENE-FRSv7 2.2.10
The local area over which broadcast calls shall be implemented shall be able to be modified within the network.
V
Voice services - Broadcast voice calls
EIRENE-FRSv7 2.2.11
It shall only be possible for the user who initiated the call to talk, other users can only listen.
V
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Voice services - Group voice calls
EIRENE-FRSv7 2.2.12
The system shall support group voice calls. V
Voice services - Group voice calls
EIRENE-FRSv7 2.2.14
The composition of call groups shall be able to be modified within the network. A single user shall be able to be a member of one or more call groups.
V
Voice services - Group voice calls
EIRENE-FRSv7 2.2.15
The local area over which group calls are implemented shall be able to be modified within the network.
V
Voice services - Group voice calls
EIRENE-FRSv7 2.2.16
It is acceptable that only one mobile user involved in the group call may talk at any time. In this case: − It shall be possible for controllers to speak at any time during the call. − A mechanism shall be provided by the system to arbitrate between those users wishing to speak within the group call.
V
Voice services - Multi-party voice calls
EIRENE-FRSv7 2.2.17
Any radio can transmit and receive data to and from several different IP sources and destinations addresses.
V
Voice services - Multi-party voice calls
EIRENE-FRSv7 2.2.18
Any of the parties involved in a multi-party voice call shall be able to talk simultaneously.
V
Data services EIRENE-FRSv7 2.3.1
The Communication System network will provide data services to support the following data applications: − text messages; (O) − general data applications; (M) − automatic fax; (O) − train control applications. (O)
D
Data services - Text messages
EIRENE-FRSv7 2.3.2
The network should support the transmission of point-to-point and point-to-multipoint text messages from the ground to mobile users. (O)
D
Data services - Text messages
EIRENE-FRSv7 2.3.3
The network should support the receipt of mobile-originated text messages by the ground. (O)
D
Data services - Text messages
EIRENE-FRSv7 2.3.4
If the text message facility is implemented, it shall not interfere with the ability of users to make or receive high priority voice or data calls. (M)
D
Data services - General data applications
EIRENE-FRSv7 2.3.6
The network shall support point-to-point data communications. D
Data services -
General data applications
EIRENE-
FRSv7 2.3.8
The network shall support data rates of at least 2.4 Kbit/s. D
Data services - Automatic fax
EIRENE-FRSv7 2.3.11
The network should support fax transmissions between the ground and mobile users. (O)
D
Data services - Automatic fax
EIRENE-FRSv7 2.3.12
Where fax functionality is provided, it shall be possible to interrupt the fax to make or receive a high priority voice or data call. (M)
D
Data services - Train control applications
EIRENE-FRSv7 2.3.13
Where ERTMS/ETCS level 2 or 3 is implemented, the network shall be capable of supporting data communications for that train control system. (M)
D
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Call related services
EIRENE-FRSv7 2.4.1
The Communication System will support the following call related services: − display of identity of called/calling user; (M) − restriction of display of called/calling user; (O) − priority and pre-emption; (M) − closed user group; (M) − call forwarding; (M) − call hold; (M) − call waiting; (M) − charging information; (O) − call barring. (M)
V
Call related services - Display of identity
EIRENE-FRSv7 2.4.2
It shall be possible for the equipment to display the identity of the called or calling party in the form of a standard telephone number.
V
Call related services - Display of identity
EIRENE-FRSv7 2.4.3
It shall be possible to display the identity of the called or calling party as a textual description of their function.
V
Call related services - Restriction of display of identity
EIRENE-FRSv7 2.4.4
It should be possible for the network to prevent the identity of certain users from being displayed on the mobile, either when being called, calling or both. (O)
V
Call related services - Priority and pre-emption
EIRENE-FRSv7 2.4.5
The network shall provide a mechanism whereby calls may be assigned one of a number of different priority levels.
V
Call related services - Priority and pre-emption
EIRENE-FRSv7 2.4.6
This mechanism shall allow calls with a higher assigned priority to override (pre-empt) existing calls of a lower priority.
V
Call related services - Priority and pre-emption
EIRENE-FRSv7 2.4.7
Pre-empted calls will be discontinued and the new call of a higher priority shall be connected instead.
V
Call related services - Closed user group
EIRENE-FRSv7 2.4.8
The group of users who may access the facilities of the Communication System shall be limited.
V
Call related services - Closed user group
EIRENE-FRSv7 2.4.9
Any user who is not within the list of allowed Communication System users shall not be able to gain access to any of the functions and services provided by the network. (M)
V
Call related services - Call forwarding
EIRENE-FRSv7 2.4.10
It shall be possible for an incoming call or data message for one user to be forwarded to another user using functionality provided by the network. (M)
V
Call related services - Call forwarding
EIRENE-FRSv7 2.4.11
In the case of voice calls, it shall be possible for the user who is attempting to forward a call to converse with the intended recipient prior to forwarding. (M)
V
Call related services - Call forwarding
EIRENE-FRSv7 2.4.12
There are a number of sub-classes of call forwarding to be supported by the network: − automatically forward the incoming call without any user interaction (unconditional); (M) − automatically forward the incoming call without user interaction if the user is busy in an existing call (busy); (M) − automatically forward the incoming call if there is no reply from the intended recipient (no reply); (O) − automatically forward the incoming call if the intended recipient cannot be contacted via the network (not reachable). (O)
V
Call related services - Call hold
EIRENE-FRSv7 2.4.13
The network shall allow the user to temporarily exit from an existing call by putting the call on hold. (M)
V
Call related services - Call hold
EIRENE-FRSv7 2.4.14
It shall be possible for the user to re-join the call which is on hold at any time. (M)
V
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Table 3: Functional requirements of railway communication system (Extracted from EIRENE FRS)
Call related services - Call waiting
EIRENE-FRSv7 2.4.15
The network shall provide the ability to inform a user, who is involved in an existing call, of attempts by other users to contact them. (M)
V
Call related services - Charging information
EIRENE-FRSv7 2.4.16
Where network services are chargeable, it should be possible for the network to provide information about call rates and on-going call charges. (O)
V
Call related services - Call barring
EIRENE-FRSv7 2.4.17
It shall be possible, using network management or maintenance facilities, to prevent individual users from: (M) − making calls to: − another network (fixed or mobile) (e.g. can only call on home network); − certain types of numbers within or external to the network (e.g. cannot call teleshopping numbers); − certain pre-defined telephone numbers (e.g. cannot call drivers and on-train users); − receiving calls from: − all other networks (fixed or mobile); − certain other networks (fixed or mobile); − certain types of numbers within or external to the network; − certain pre-defined telephone numbers.
V
Railway specific services
EIRENE-FRSv7 2.5.1
The System shall also provide support for the following railway specific services: − functional addressing including registration/deregistration (see section 11); (M) − location dependent addressing (see section 11); (M) − shunting mode (see section 14); (M) − Railway emergency calls (see section 13). (M)
V
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3.7 Overview of existing railway QoS definitions
This section is concerned with the specific definition of QoS implementation within the
GSM-R system and ERTMS/ETCS services. The individual parameters are described and
evaluated in detail.
In general, the quality of a voice or data transmission can be evaluated and controlled
in various ways. However, availability of transmission with a given coverage is a first
indicator for its quality. The end-to-end quality of transmission can be managed by
different IP QoS paradigms.
The QoS itself can be defined a series of parameters of network and service
performance indicators which determines the degree of service fulfilment. QoS
management in communication networks is based on the fact that different application
types and different performance levels are required by the users of different services.
Fundamental parameters are reliability (transmission link erroneousness) and
throughput/delay requirements. Services which having strict delay and reliability
requirements are regarded to real time classes, while services characterised by high
reliability and relaxed delay requirements are categorised as non-real time services.
Real time traffic classes can ask for guaranteed bitrate to ensure required service
performance while non-real time traffic classes cannot.
The ETCS application has pre-defined delay and reliability requirements, which are
specified in QoS specification in appropriate UNISIG subsets.
3.7.1 Specific railway Quality of Service requirements
This sub-section evaluates Quality of Service requirements based on UNISIG SUBSET-
093 and proposes a standardized QoS vector of existing GSM-R communication
networks.
It contains:
• Detailed definitions of the QoS parameters for the GSM-R bearer service,
• The interfaces on which the bearer service and the QoS parameters are
applicable.
The operation of the ETCS application has strict delay and high reliability requirements.
Since ETCS GSM-R circuit switched bearer is an end-to-end bearer service, it is not
enough to limit the quality of service requirements to the air interface. End-to-end
quality of service must be taken into account also for service access points.
In the context of end-to-end QoS the network may be regarded as the sub-system that
provides data connectivity between the mobile terminal and the external IP-based
network. The end-to-end transmission path between mobile terminal and RBC consists
of three segments:
• Radio Access Network
• Packet Core Network
• External IP based Packet Data Network
The communication network shall be able to support transparent train-to-trackside and
trackside-to- train data communications at train speeds up to 500 km/h e.g. in tunnels,
cuttings, on elevated structures, at gradients, on bridges and stations. Furthermore, the
required QoS parameters shall not depend on network load.
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Given the performance constraints of GSM-R, pre-conditions may be necessary to meet
the railway operational targets. The values shown in the following Table 4 characterise
the specified quality of service of the ETCS data connection according to UNISIG
SUBSET 093.
QoS Parameter Specified Value
Connection establishment delay of
mobile originated calls
< 8.5s (95%), ≤10s (100%)
Connection establishment error
ratio
<10-2
Maximum end-to-end transfer
delay
(30 byte data block)
≤ 0.5s (99%)
Connection loss rate ≤ 10-2 /h
Transmission interference period < 0.8s (95%), <1s (99%)
Error-free period >20s (95%), >7s (99%)
Network registration delay ≤30s (95%), ≤35s (99%), ≤40s (100%)
Table 4: Summary of QoS requirements {Ref [2]}
It is obvious that within the QoS parameters listed and specified in Table 4 a further
distinction is made between two subcategories. The registration of the network
connectivity itself and the characteristics of the specific connection.
First, relevant parameters for network registration are analysed. This is the first
essential step before applications are able to use the network. The registration process
to the network within GSM-R is independent of the type of applications to be used later,
i.e. data or voice transmission.
• Connection establishment delay is the value of elapsed time between the
connection establishment request and the indication of a successful connection
establishment. The value shall not depend on user data rate of the bearer
service.
• The connection establishment error ratio rates the number of unsuccessful
connection establishment attempts to the total number of connection
establishment attempts. With regard to the operational QoS targets, the ETCS
infrastructure should be designed in such a way that at least two consecutive
connection setup attempts are possible (recommended prerequisite for the ETCS
infrastructure).
The following parameters are related to data transmission over the established
connection.
• The Transfer delay is defined as a value of elapsed time between the request for
transfer of a user data block and the indication of successfully transferred end-
to-end user data block.
• The connection loss rate is specified as the number of unintentionally released
connections per cumulative connection time.
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• A transmission interference period TTI is the period during the data transmission
phase of an existing connection in which, caused by the bearer service, no error-
free transmission of user data units of 30 bytes is possible. An error-free period
shall follow every transmission interference period to re-transmit user data units
in error (e.g. wrong or lost) and user data units waiting to be served.
• The GSM-R network registration delay is a defined value of elapsed time from
the request for registration to successful registration. GSM-R network
registration delays >40s are evaluated as registration errors.
All these specified connection parameters form the QoS vector for circuit-switched data
connections in ETCS via GSM-R.
In the future, however, there will be a paradigm shift in data transmission. These
parameters will then no longer be applicable for next generation railway communication
networks (e.g. 4G/5G) and it is necessary to define other measurable characteristics.
QoS values for packet-based transmission technologies are already specified and in use.
These parameters concern the GPRS extension of the GSM-R. The following section
explains this extension and the quality of service expectations and definitions.
3.7.2 Packet Core IP network QoS Management
This sub-section introduces existing Quality of Service definition and requirements of
the Packet Core IP network (GPRS). The purpose is to provide principles for QoS
measures in PS-Mode. The basic need is to agree on a QoS profile that enables railway
communication via a IP-based communication network. The values proposed focus on
the 3GPP specified network elements (e.g. SGSN, GGSN, RNC and BSC) and not the
backbone network.
Today, railways and infrastructure managers are already enabled to use IP based GSM-
R enhancement GPRS/EGPRS to realise different operational services. Also ETCS over
GPRS/EGPRS is introduced.
QoS management includes methods to negotiate the quality to be provided, to prioritize
particular connections and to guarantee certain performance classes of higher priority
data flows at the expense of lower priority data flows.
However, QoS mechanisms allow infrastructure operators to realise the operation of
ETCS in PS-mode and other application at the same time.
Various QoS mechanisms are used along the end-to-end connection and it is necessary
to coordinate QoS management between all network segments. For example, in an
external IP network, differentiated services etc. can be implemented, which can also be
used in the Packet Core Network. In the GPRS/EGPRS network, the Gateway GPRS
Support Node (GGSN) can use QoS attributes to configure the differentiated edge
functionality to enable cooperation between the GPRS/EGPRS and the external IP-based
data network.
The concept of GPRS/EGPRS-QoS architecture was already standardized in 3GPP release
97/98. A QoS profile for a data transfer session between mobile phone and network can
be negotiated. The QoS concept in 3GPP Release 99 introduced a new set of QoS
parameters. Four new traffic classes have been introduced to differentiate between the
various service characteristics:
• Conversational traffic class is meant for services sensitive to delay and delay
variations.
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• Streaming class can compensate delay variations with the use of buffering
mechanisms.
• Interactive class is meant for applications that are sensitive to delay but less
sensitive to delay variations.
• Background class, low priority data transmission.
The requirements on reliability in interactive and background class are higher than in
conversational and streaming. The ranking between the four traffic classes is obvious,
background data flows have the lowest priority and will have less resources assigned
than conversational or streaming or interactive.
3GPP has specified a QoS concept and a QoS architecture for IP based networks. The
choice of QoS parameters is an important part of the QoS concept. The traffic class is
probably the most important QoS parameter. This parameter type divides the traffic
into distinct traffic classes, according to how delay sensitive typical services are.
All QoS parameters together form the so-called QoS profile. QoS profiles are associated
with each PDP context and define the characteristics of the connection through the
mobile network. The QoS characteristics affect the parameters in the radio network and
policing/shaping functions in the core network.
The QoS profile resides in the home PLMN HLR. QoS parameter values also work in a
multi-operator environment, which could also become relevant for the railway sector in
the future, if NaaS paradigm also applies to it. The QoS profile is defined per end-user
and per APN, that means a single end-user may have a separate QoS profile.
3GPP has only specified a model for the traffic classes and the other QoS parameters.
However, the detailed implementation is not described in the specifications and actually
vary from network vendor to vendor. The QoS parameters also have to work in a multi-
vendor environment.
3.7.2.1 QoS PARAMETER in Packet Core IP network
In this section an overview and description for QoS parameters in PS-Mode will be
given. The description includes the definition and the resulting effect of the parameter.
• Traffic class
The traffic class is an important QoS parameter. The traffic class parameter
maps different services onto different bearers so that the service requirements
can be met. The distinguish into specific traffic classes are made according to
how delay sensitive typical services are. The conversational class is the most
delay sensitive traffic class and the background class the least sensitive.
o Interactive traffic class
The idea behind the interactive class is to enable prioritisation between end-
users or services. Prioritisation does not offer any guarantees in form of bit
rate or delay. Instead a higher priority offers a higher portion of the available
resources in congestion situations.
o The background traffic
The background traffic class is the least delay sensitive traffic class and has
the lowest priority of all traffic.
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• Delivery order
The delivery order parameter indicates whether the bearer should provide in-
sequence delivery of Service Data Units (SDU) or not. The specifications indicate
a coupling to the reliability of the bearer. Using the delivery order is comparable
to guaranteeing the packet order in IP networks. In contrast to circuit-switched
connections, IP networks, in general, do not reorder packets or guarantee in-
sequence delivery of packets.
• Maximum bit rate (DL/UL)
Specifies the maximum allowed bit rate. In practice, the maximum bit rate
parameter defines the maximum bit rate for the mobile core network (CN). The
maximum value that can be used is of course limited by the bit rate the network
supports.
• Residual BER
Indicates the undetected bit error ratio in the delivered SDUs. If no error
detection is requested, residual bit error ratio indicates the bit error ratio in the
delivered SDUs. Generally speaking, the Residual BER parameter is used to
configure radio interface protocols, algorithms and error detection coding.
The reliability class maps to residual BER, delivery of erroneous SDUs and SDU
error ratio in QoS parameters. The recommended value for residual BER is
therefore 10-5 for background and interactive classes.
• SDU error ratio
Indicates the fraction of SDUs lost or detected as erroneous. SDU error ratio is
defined only for conforming traffic. For example, traffic that exceeds the
maximum bit rate and is hence dropped is not included in the SDU error ratio.
By reserving resources, SDU error ratio performance is independent of the
loading conditions, whereas without reserved resources, SDU error ratio is used
as target value.
General, the SDU error ratio is used to configure the radio interface protocols,
algorithms and error detection schemes.
• Traffic handling priority (THP)
The THP parameter is only specified for the interactive class and enables
prioritisation between bearers, which in turn enables service prioritisation
capabilities. THP specifies the relative importance for handling of traffic
belonging to a RAB compared to traffic belonging to other RABs. THP affects the
relative priority, which means that it affects the allocation of the bearers and the
retention of the bearer. In other words, it affects the operation of the packet
scheduler located in the RNC.
There are two different parameters that do the same prioritisation over the air
interface, precedence class (i.e. ARP) and THP (requires interactive traffic class)
in 3GPP Release 99 networks. The recommendation for priorities should be made
in a consistent way in network and it is recommended to use the same value for
the THP and ARP parameters for the interactive class.
• Allocation/Retention priority
The ARP parameter is a subscription parameter, which is not negotiated from the
mobile terminal. The priority is used for differentiating between bearers when
performing allocation and retention of a bearer. In situations where resources
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are scarce, the relevant network elements can use the ARP to prioritise bearers
with a high ARP over bearers with a low ARP when performing admission control.
These key parameters describe the QoS characteristics for certain services within
transmission in an IP-based (E)GPRS network. The following section shows how these
values are defined quantitatively for railway communication.
3.7.3 ETCS QoS Profile for GPRS
ETCS operation shall make use of streaming traffic class according to ETSI TS 123 107.
The data transport within the ETCS application is delay sensitive. Simultaneous
operation of ETCS and non-ETCS applications may cause unexpected transfer delay to
ETCS if the QoS profile parameters are treated always as best effort. Therefor a higher
priority and stringent QoS requirements in ETCS data transmission is demanded. It is
strongly recommended to provide an enhanced HLR QoS profile to be used for ETCS, to
meet the most demanding requirements that the ETCS application could impose and to
meet the performance requirements of the ETCS application. Other applications should
be provided with lower priority QoS profiles.
Table 5: Selected parameter of ETCS QoS Profile (PS-Mode) {Ref UNISIG}
3GPP ETCS QoS profile
parameters
QoS
parameter
settings
SDU error ratio 10-4
Traffic handling priority (THP) 1
Allocation/Retention priority (ARP) 1
Maximum bitrate [kbps] 64
Guaranteed bit rate [kbps]
( Up- and Downlink)
4
The values in this table are sufficient to meet the most demanding requirements that
the ETCS application could impose and to meet the performance requirements of the
ETCS application. It is strongly recommended to provide the HLR QoS profile to be used
for ETCS. Other non-ETCS applications should be provided with lower priority QoS
profiles.
3.7.3.1 Packet Core QoS Management
To ensure that resources required for a specific service are available within IP Networks,
it is also necessary to perform resource management. Where the resources for the IP
bearer service to be managed are not owned by the GPRS/EGPRS network, the resource
management of those resources need to be performed through an interaction with the
external network.
Interworking can be realised by packet marking or labelling along the flow path using
implemented standard QoS methods of IP networks e.g. DiffServ or MPLS (see section
4.1). DiffServ functionality classify packets of aggregated flows or "classes", based on
the DiffServ code point (DSCP) in the packet's IP header. DSCP (IETF RFC 2474) shall
be used to guarantee the interoperability between the different network types. The
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DiffServ function shall be aligned on the basis of PDP Context QoS parameters that a
user data packet between OBU and RBC contains QoS performance marking.
3.8 Conclusion on Requirements
Existing definitions and requirements for QoS measurements of the communication
connections within railway domain are clearly specified. However, the standardised QoS
connection values of the UNISIG subsets for the ETCS data connections are linked to
the existing GSM-R system definitions. The defined parameters refer to circuit-switched
connections or packet-based transmission with the GPRS/EDGE extension of the 2G
generation system. These definitions are valid and determine the quality of service and
network requirements to provide the required operational performance.
With the knowledge of which railway specific functions are using communication
services in the railway system and which requirements are set for characteristics of the
services and the transmission channel itself, further analyses on future technologies can
be carried out.
The Table 6 shows in detail all requirements on Quality of Service parameters regarding
the mobile network from the EIRENE and references the corresponding values.
For future communication systems, however, it will be necessary to define other QoS
characteristics. The parameters defined so far are no longer applicable in the new
networks or can be renewed by a much more stringent design. 4G and 5G have
powerful implementations to enable different QoS approaches for various applications.
In the future, these methods will monitor and influence the transmission quality of data
and voice connections to a much greater extent.
Especially with the focus on the main parameters of different key services within the
railway communication (Singalling, Critical data, Critical voice, Critical Video and
Passenger connectivity services) a QoS evaluation is mandatory.
In the following chapters we will discuss the basic principles of QoS in IP-based
networks and their derived implementations in current mobile networks. Novel
approaches are explained in detail and conclusions are drawn as to how the use of
these methods can influence the railway communications system of the future.
Functions ERTMS
Specification Ref
ERTMS Functional Description
Network configuration - Coverage and performance
EIRENE-FRSv7 3.2.2
The level of coverage should be at least 95% of the time over 95% of the designated coverage area for a radio installed in a vehicle with an external antenna. (O)
Network configuration - Coverage and performance
EIRENE-FRSv7 3.2.4
The communication system shall provide communications for mobiles when stationary and when travelling at speeds up to the maximum allowable line speed or 500 km/h, whichever is the lower. (M)
Network configuration - Call set-up time requirement
EIRENE-FRSv7 3.4.2
The required call set-up times shall be achieved in 95% of cases. (M)
Network configuration - Call set-up time requirement
EIRENE-FRSv7 3.4.3
Call set-up times for 99% of cases shall not be more than 1.5 times the required call set-up time. (M)
Network configuration - Call set-up time requirement
EIRENE-FRSv7 3.4.4
Set-up times shall include the time required for any translation of functional numbers internal to the IP communication system. (M)
Quality of Service requirements
SS-093 v230 6.3.1.5
The network shall provide a Quality of Service for ETCS data transfer that is at least as good as listed below. The parameters are valid for one end-to-end connection for one train running under all operational conditions.
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Quality of Service requirements
SS-093 v230 6.3.1.6
The required QoS parameters shall not depend on network load.
Quality of Service requirements
SS-093 v230 6.3.1.7
These performance figures reflect railway operational targets [EEIG 04E117].
Quality of Service requirements
SS-093 v230 6.3.1.8
QoS requirements are specified independently of the method of measurement (refer to [O-2475] for specification of testing).
Quality of Service requirements
SS-093 v230 6.3.1.11
Given the performance constraints of GSM-R, pre-conditions may be necessary to meet the railway operational targets of [EEIG 04E117]. If different operational QoS targets are required, then other pre-conditions on ETCS application may be necessary. Such a case is not covered by this specification and this aspect of ETCS System Performance becomes the responsibility of whoever specifies different operational targets.
Quality of Service requirements
SS-093 v230 6.3.2.1
Connection establishment delay is defined as: Value of elapsed time between the connection establishment request and the indication of successful connection establishment.
Quality of Service requirements
SS-093 v230 6.3.2.2
In case of mobile originated calls, the delay is defined between the request by command ATD and indication by the later of the two events response CONNECT or transition of DCD to ON.
Quality of Service
requirements
SS-093 v230
6.3.2.3
The connection establishment delay of mobile originated calls
shall be <8.5s (95%), ≤ 10s (100%).
Quality of Service requirements
SS-093 v230 6.3.2.4
Delays >10s shall be evaluated as connection establishment errors.
Quality of Service requirements
SS-093 v230 6.3.2.5
The required connection establishment delay shall not depend on user data rate of the asynchronous bearer service.
Quality of Service requirements
SS-093 v230 6.3.2.6
The required connection establishment delay is not valid for location dependent addressing.
Quality of Service requirements
SS-093 v230 6.3.3.1
The Connection establishment error ratio is defined as: Ratio of the number of unsuccessful connection establishment attempts to the total number of connection establishment attempts.
Quality of Service requirements
SS-093 v230 6.3.3.2
“Unsuccessful connection establishment attempt” covers all possible types of connection establishment errors caused by end-to-end bearer service.
Quality of Service requirements
SS-093 v230 6.3.3.3
Connection establishment delays >10s shall be evaluated as connection establishment errors.
Quality of Service requirements
SS-093 v230 6.3.3.4
The GSM-R networks should be designed in such a way, that at least two consecutive connection establishment attempts will be possible (pre-condition on GSM-R networks), e.g. regarding GSM-R radio coverage related to maximal possible train speed.
Quality of Service requirements
SS-093 v230 6.3.3.5
If the operational QoS targets of [EEIG 04E117] are wanted, then the ETCS infrastructure should be designed in such a way, that at least two consecutive connection establishment attempts will be possible (Recommended pre-condition for ETCS infrastructure).
Quality of Service requirements
SS-093 v230 6.3.3.6
The connection establishment error ratio of mobile originated calls shall be <10-2 for each attempt.
Quality of Service requirements
SS-093 v230 6.3.4.1
The end-to-end transfer delay of a user data block is defined as: Value of elapsed time between the request for transfer of a user data block and the indication of successfully transferred end-to-end user data block
Quality of Service requirements
SS-093 v230 6.3.4.2
The delay is defined between the delivery of the first bit of the user data block at the service access point of transmitting side and the receiving of the last bit of the same user data block at the service access point of the receiving side.
Quality of Service requirements
SS-093 v230 6.3.4.3
The end-to-end transfer delay of a user data block of 30 bytes shall be ≤ 0.5s (99%).
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Quality of Service requirements
SS-093 v230 6.3.5.1
The Connection loss rate is defined as: Number of connections released unintentionally per accumulated connection time.
Quality of Service requirements
SS-093 v230 6.3.5.2
The requirements for connection loss rate varies depending on ETCS system variables such as T_NVCONTACT and the possible train reactions after connection loss (see section 10.5).
Quality of Service requirements
SS-093 v230 6.3.5.3
If the operational QoS-targets of [EEIG 04E117] are wanted, then the ETCS infrastructure should be designed in such a way, that at least the following conditions are fulfilled (Recommended pre-condition for ETCS infrastructure): • T_NVCONTACT ≥ 41s and • M_NVCONTACT different to train trip and • a new MA reach the OBU before standstill.
Quality of Service requirements
SS-093 v230 6.3.5.4
If the connection establishment error ratio is <10-2, then the connection loss rate shall be <10-2/h.
Quality of Service requirements
SS-093 v230 6.3.6.1
A transmission interference period TTI is the period during the data transmission phase of an existing connection in which, caused by the bearer service, no error-free transmission of user data units of 30 bytes is possible.
Quality of Service requirements
SS-093 v230 6.3.6.2
A transmission interference happens, if the received data units of 30 bytes deviate partially or completely from the associated transmitted data units.
Quality of Service requirements
SS-093 v230 6.3.6.3
The transmission interference period shall be < 0.8s (95%), <1s (99%).
Quality of Service requirements
SS-093 v230 6.3.6.4
An error-free period shall follow every transmission interference period to re- transmit user data units in error (e.g. wrong or
lost) and user data units waiting to be served.
Quality of Service requirements
SS-093 v230 6.3.6.5
The error-free period shall be >20s (95%), >7s(99%).
Quality of Service requirements
SS-093 v230 6.3.7.1
The GSM-R network registration delay is defined as: Value of elapsed time from the request for registration to indication of successful registration by +CREG response.
Quality of Service requirements
SS-093 v230 6.3.7.2
The GSM-R network registration delay shall be ≤ 30s (95%), ≤ 35s (99%).
Quality of Service requirements
SS-093 v230 6.3.7.3
GSM-R network registration delays > 40 s are evaluated as registration errors.
Table 6: QoS requirements on railway communication systems (Extracted from EIRENE FRS)
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4 QoS in IP-based Systems
In future different approaches to defining QoS requirements in railway specific
communication networks will certainly be necessary. The completely (end-to-end)
packet-based data transmission via modern IP networks makes this development
necessary. Within this chapter we will first analyse how QoS is implemented as standard
in IP networks, which paradigms and methods exist and which basic principles are also
used within the transmission networks. The applicability of these principles must be
examined in more detail.
Normally, all data packets in a network are treated equally according to the best-effort
principle. In a packet-oriented network, the individual data packets can travel at
different speeds and on different paths. As long as low-priority messages and data files
are mainly transferred, transmission problems rarely occur. However, when real-time
applications such as Voice over IP, video streaming or particularly critical services are
used, delays or packet losses have a negative effect on transmission characteristics
between subscribers. For example, by partially interrupted language or missing image
sequences in a video stream. In comparison, it is barely noticeable when a text
message arrives at the recipient a few seconds later.
A low bandwidth, poor transmission characteristics and different utilisation lead to the
discarding or delayed delivery of data packets. As a consequence, data transmission is
disrupted.
Because TCP/IP separates the application layer from the transmission layer and makes
it independent, no communication takes place between these layers. The transmission
systems are not able to distinguish between a voice packet and a normal data packet.
The OSI layer model ensures that the protocols work independently of each other on
the different layers. However, this approach causes problems in audio and video
transmission.
So-called QoS methods try to eliminate this deficiency and to mark data packets with
service classes that are assigned to certain applications. In this way, an attempt is
made to define service characteristics at application level and to pass them down
through the protocols. In the TCP/IP world, QoS describes the quality of a
communication service from the user's point of view. The network service quality is
often defined by the parameters bandwidth, delay, packet loss and jitter.
The network load influences the quality of the transmission. For example, how long it
takes for a data packet to reach the recipient. Therefore one tries to mark data
packages with appropriate service classes. Prioritized data packets are preferably
forwarded in routers or switches. However, this only makes sense if all network
components and subnetworks support the same traffic classes and priority rules. For
QoS to function, the necessary QoS mechanisms must be implemented along the entire
transmission path between the participants. Of course, this is only possible if the
network belongs to a single organisational unit, which is not the case on the Internet.
It is not enough to implement QoS features within the network architecture. Whoever
introduces QoS methods should take their measurability into account. Quality
improvements in the network should always be measured before and after. If
something is to be improved, it must be determined before doing so what and how it
can be improved. For this purpose, quality must be checked with suitable measuring
and monitoring tools. For example, the available bandwidth for certain applications
must be continuously monitored.
Criteria for the quality of the transmission are for example packet delays, packet loss
rate and jitter. Depending on the application, further quality characteristics must be
examined and measured.
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4.1 Typical QoS implementation methods
Quality of Service implementation can be achieved through a variety of coordinated
methods. Some of them are:
• Prioritized transmission of certain data packets
• Connection-oriented protocol below the IP layer
• More bandwidth than required (oversizing the network)
• Reservation of bandwidth for specific applications
These basic methods and their technical backgrounds are examined and explained in
the following sub-chapters in more detail.
4.1.1 Oversizing of network and bandwidth
Formerly it was the practice not to implement dedicated QoS algorithms, but to provide
much more bandwidth than was practically necessary for the services within the
network. This approach only works if there is a too low bandwidth demand. However,
the bottlenecks along the entire transmission path must be taken into account. If the
bandwidth requirement increases over time, even more bandwidth must be made
available. An approach that means very high technical effort, especially with increasing
user counts. A priority differentiation between different services is not performed here.
If critical applications are used, this implementation approach is not suitable.
4.1.2 Reservation of bandwidth
In order to achieve a high QoS, it is common practice to reserve and guarantee the
available bandwidth for certain applications. Other applications are given lower priority
and then receive less bandwidth. One implementation approach of this method is the
Resource Reservation Protocol (RSVP). This makes it possible to reserve bandwidth for
specific applications. RSVP is a typical QoS measure.
When designing a data network that is also to be used for time-critical applications such
as Voice over IP or video transmission, the runtime of data packets must be kept as
short as possible. To achieve this, data packets containing language are preferably
processed during forwarding in network nodes (e.g. routers, switches). Mechanisms for
compliance with runtime variations are provided by the network. In the IP network, this
is done via the Resource Reservation Protocol (RSVP).
4.1.3 Prioritisation of data packets
The definition of specific traffic classes is necessary in order to prioritise certain data
packets. The traffic class is defined according to a fixed quality of service. Data packets
of a higher traffic class are then also transmitted with higher priority.
However, prioritisation only works where the traffic classes apply. If prioritized data
packets leave a special network type and enter other network architectures, other
traffic classes may apply here.
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4.1.4 DiffServ - Differentiated Services
DiffServ is a standardised QoS method for prioritizing traffic for real-time applications
over IP. Each data packet is assigned to a specific traffic class. Data packets of a higher
traffic class are given preference over a lower traffic class. DiffServ is an IETF standard.
It is one of the class-based QoS mechanisms and is used on layer 3 of the OSI model as
a supplement to IP. It is a method for prioritizing data traffic for real-time applications
over IP. Each data packet is assigned to a traffic class. Transmitting data packets of a
higher traffic class are given higher priority over a lower traffic class. With DiffServ, the
sender classifies and marks the data packages. The routers on the way to the receiver
evaluate this mark.
The possibilities to define QoS in the IP header are very limited. DiffServ is a model to
give independent systems priorities for traffic control. The advantage: DiffServ does not
change the IP packet. Only the ToS field in the IP header is interpreted differently. The
ToS field in the IP header is therefore also called Differentiated Services Code Point
(DSCP).
As DSCP provides information about the QoS request for a packet, this functionality is a
basic principle in IP QoS management.
4.1.5 Jitter buffer
Jitter is the term used to describe the difference in packet runtimes between sender and
receiver. Runtime differences in the data packets are particularly negative for time-
critical data transmission in real-time applications (voice and video transmissions).
Jitter buffers are used to avoid differences in runtime. A jitter buffer can compensate for
these irregularities to a certain extent. It records all real-time data packets and returns
them in a smooth flow.
4.1.6 CoS - Classes of Service Class Application
Classes of Service defines different classes of specific data transfers to which
application-oriented data packets are assigned. Each class corresponds to a priority,
which is used to decide which data packets are to be transmitted preferentially. It
should be noted that the amount of data in the high traffic classes must be limited,
otherwise no transmission is possible on overloaded connections for low-priority data
packets. The implementation of CoS is usually made more difficult because different
CoS rules exist in the different networks of the network operators. Each network
operator sets its own definitions here.
4.1.7 Prioritisation and queuing
When designing a data network that can also be used for time-critical data
transmission, the runtime of the data packages must be kept as short as possible. To
achieve this, specific data packets with different priorities are processed. So-called
prioritisation and queuing mechanisms ensure this.
Weighted Fair Queuing (WFQ) is such a queuing method where small packet streams
are preferred. In this functional procedure, each priority has a guaranteed minimum
bandwidth. WFQ distributes the available bandwidth dynamically among the various
data streams. This prioritisation mechanism is used directly in network core elements,
routers or Layer 3 switches. This gives voice or video data priority when it is
transmitted through the IP network.
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4.2 Summary
Quality of Service is a method of controlling the data traffic of networks with the aim of
ensuring that data from certain services are delivered to the receiver according to
specified quality parameters. QoS procedures ensure a higher level of network
reliability.
The introduced basic implementation strategies within IP networks will also be applied
to QoS approaches within next generation mobile networks. They serve as a basis for
implementing powerful QoS frameworks in 4G/5G networks. This enables network
providers to implement and offer differentiated QoS strategies.
In the next chapter, the QoS implementations of the mobile radio standards will be
examined in more detail. The basic paradigms and performance expectations for the
control of the connection quality are shown, as well as the suitability for future railway
communication are indicated.
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5 QoS within public mobile radio networks
5.1 QoS implementation in mobile technology sector
Quality of Service has become an important part of network planning and design when
deploying public wireless networks for data and voice services. There are public
subscribers which are using mobile radio services for critical operations and furthermore
there are subscribers who just want to use broadband internet with low latencies.
One of the future trends in next-generation solutions for communication in the railway
sector is the use of commercial 4G (and later 5G) networks. The MISTRAL project deals
with different scenarios and approaches to implement a NaaS paradigm for railway
communication networks. But will these 4G/5G networks are able to offer the high
service availability and high quality of service offered by existing standardized
technologies such as GSM-R?
This chapter focuses on the functions for prioritizing data connection, one of the key
criteria for high service quality. Within this chapter the QoS frameworks of 4G and 5G
networks are introduced. Current narrowband systems already offer versatile
prioritisation mechanisms.
5.1.1 Requirements on high service availability
In general there are several core features that make a mobile network compatible with
critical communications requirements. These include the following:
• High availability and reliability
The network must be designed, implemented and maintained with the aim of
high availability and reliability. Typical methods are additional, redundant base
stations to cover public safety and improve resistance through rigid single-point
of failure analysis, including power supply to radio sites (cover radio access,
transmission, core elements, terminals and applications)
• Prioritising public safety users
The priorities of the network must be designed to provide critical users with
radio and other network resources in all situations. In practice, this means giving
them higher priorities than consumers or business users.
• Prioritisation of traffic classes
Mission Critical users have different demands on the handling of means of
transport, so that users can have different priority levels for different
applications: different applications, such as Push-To-Talk (PTT) voice services,
time-critical data queries or video transmission demands higher different
prioritisation levels
• Management in special conditions
Under extreme conditions, extremely high traffic volumes can occur in networks
that go far beyond the normal level. These situations may be caused by
accidents or disasters. Exceptional circumstances are naturally unpredictable and
difficult to plan. Therefore, the networks must be able to adapt very quickly to
the extraordinarily high mission-critical load.
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5.2 QoS mechanisms offered by 4G
Mobile broadband IP communication for mission-critical users can be provided with 4G
(LTE) technology, standardized by 3GPP and implemented by telecom providers.
The time lines of development and standardization of different recent and future
telecommunication standards are considered in MISTRAL D4.1. Furthermore, the
document provides a comprehensive review of current requirements and performance
aspects of railway communication systems for main-line railway domain. Possible future
railway communication technology candidates and recent trends have been analysed
there.
The technology provides QoS functions that are able to set priorities and traffic
management to certain users. Today, 3GPP begins work on the 5G standard.
Development is expected to take several years, but the first high-level architectural
concepts are already taking shape.
The 4G high-level architecture includes user equipment (UE), wireless access, E-UTRAN
and core network elements. It also supports collaboration with 2G (GERAN) and 3G
(UTRAN) network access.
The approach of QoS definitions in 4G is based on so called bearers. A 4G bearer is a
specific transmission mode via the infrastructure and radio interface with a pre-defined
capacity, latency time and packet loss. Depending on the transmission requirements, a
specific bearer can be assigned either dynamically or statically. When a terminal
connects to the network, it always receives a static default owner. Additional dedicated
carriers may also be set up on the basis of the subscription or added later.
Especially for mission-critical communications, it is mandatory that the communication
service itself remains available even if the network or parts of it are congested. Within
the 4G standard a set of tools for managing QoS and prioritisation for specific
subscribers and users is specified.
Three main functionalities regarding QoS are realized within this toolkit: Access Class
Barring, Allocation and Retention Priority and QoS Class Identifier. These functions are
first introduced and described in more detail below and then the implementation of the
methods in the 4G network is examined in more depth.
5.2.1 Access Class Barring (ACB)
The Access class barring function is used to prioritise different terminal classes in the
radio interface when connecting to the network. If the wireless interface is massively
overloaded, a terminal may not even be able to give the network its identity or the
status of its request caused by channel interference of other devices.
If such a state is detected, a base station can block certain classes of terminals.
Different Access Classes (AC) are defined with values from 0 to 15. All consumer
terminals have an access class from AC0 to AC9, and the terminals for public safety can
also be equipped with AC14 (emergency) or AC12 (security). Consumer terminals that
make an emergency call can also have AC10.
Only terminals with AC12/AC14 and terminals that attempt a public emergency call can
access the network. The other terminals are blocked either for a certain period of time.
5.2.2 Allocation and Retention Priority (ARP)
The Prioritisation of data transfer is an important function block for QoS control.
Prioritisation in bearer admission control for attached terminals using ARP has three
components:
1. Priority (1-15) where 1 is the highest value. (public safety may use values 1-8)
2. Pre-emption capability indicator (PCI)
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3. Pre-emption vulnerability indicator (PVI)
Allocation and Retention Priority is part of QoS profile definition within the Home
Subscriber Server (HSS) for the default bearer. ARP values are used for deciding
whether a bearer can be admitted or pre-empted if there is insufficient transmission
capacity available. ARP is not used by schedulers or packet queue management.
5.2.3 QoS Class Identifier (QCI)
These methods allow prioritisation in scheduling and queuing of data flows, with
specified values of 1-254. Each value is associated with three attributes: Priority,
Packet delay budget, Packet error loss. Possible QCI values between 1-127 are reserved
for standardisation. 3GPP has already standardised some of these for different
application categories and requirements (see Table 27). QCI 9 is used for the best
effort traffic and most of the current commercial user data flows are classified for QCI
9.
More detailed considerations are given in section 5.2.4.
For public safety services QCI values have been standardised starting within 3GPP
rel.12 (compare chapter 5.5):
• QCI 65 for Guaranteed Bit Rate (GBR) bearers used for the mission critical user
plane (e.g. MCPTT voice),
• QCI 69 for Non-guaranteed Bit Rate bearers (NGBR) used for mission critical
signalling,
• QCI 70 for Non-guaranteed Bit Rate bearers used for mission critical data.
The 4G QoS toolbox implemented on the network side can be utilised to prioritise
specific user classes (e.g. mission-critical services) and data transmission streams in
LTE networks. It makes it possible to flexibly and extensively control QoS functionalities
in the 4G network.
5.2.4 4G QoS implementation
Service provider should be able to differentiate between subscribers and provide QoS
requirements depending on the subscriber level. That implies that the service provider
should have knowledge about which type of subscriber is a certain user and which
services requirements are demanded, that, will allow to assign properly the radio
resources.
The LTE network classifies user traffic in different service data flow (SDF) each of them
having different QoS classes based on the type of service that is being provided. SDFs
are delivered through EPS bearers to their destination, EPS bearers can be seen as
virtual connections with a specific QoS service characteristic.
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Figure 4: SDF and EPS Bearers {Ref [22]}
An SDF refers to a group of IP flows associated with a service that a user is using, and
a EPS bearer refers to IP flows of aggregated SDFs that have the same QoS class.
There are specific packets filters that are preconfigured by network operators in
accordance with the policy and classified in 5 values: source IP, destination IP, source
port, destination port and protocol ID. All SDF with the same QoS requirement are
aggregated in a EPS bearer with the same QoS class and mapped by the P-GW, and
then delivered to the UE or vice versa.
Different services have different QoS classes. Appropriate QoS policies (e.g. priority,
bandwidth control, etc.) are applied to these SDFs before they are delivered to the UE.
There are two types of EPS bearer (see Figure 5): default bearer and dedicated bearer.
When a UE attaches to the LTE network a default EPS bearer is assigned with a
standard QoS requirements, for example internet. If the UE starts a service that the
bearer cannot identify, a new EPS bearer is created on demand, the maximum number
of EPS that a UE can have is 11. The default bearer lasts until the UE is detached from
the network and there is one per each PDN that the UE is connected. When the UE
attach to the network the HSS provide the subscribe information to the MME related to
the UE and selects a P-GW for data flow and activates the EPS bearer depending on the
subscribed QoS.
Figure 4 shows EPS bearers and SDFs when downlink IP flows are delivered to a UE
through EPS. The IP flows arriving at a P-GW through a PDN are filtered to SDFs
templates (kind of packet filters). In the figure, IP flows 1, 2 and 3 are filtered to SDFs
1, 2 and 3, respectively while IP flows 4 and 5 are filtered to SDF 4. Different QoS
policies are applied to each SDF and then the SDFs are mapped to EPS bearers as
classified by using Traffic Flow Template (TFT). SDFs 1 and 2 are mapped to the default
bearer and SDFs 3 and 4 are mapped to the dedicated bearer, all destined to the UE.
Upon arrival at the UE, the IP flows are all sent to their destination applications.
In the LTE network QoS parameters are defined both in service level and bearer level.
SDF aggregate refers to a group of SDFs which have the same QCI (QoS class
identifier) and ARP (Allocation and Retention Priority) and belongs to the same EPS
session. QCI and ARP are the basic QoS parameters applied to SDF and EPS.
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Figure 5: 4G/5G Bearer QoS concept (TUD)
QCI serves as a reference that indicates the performance characteristics of EPS and
SDF. There are as well GBR, MBR and AMBR parameters that specify the bandwidth
utilisation. The QCI values 1 to 9, indicates nine different QoS performance
characteristics of each IP packet, i.e. packet delay, packet error rate. Maximum Bit Rate
(MBR) and Guaranteed Bit Rate (GBR) indicates Bandwidth. AMBR (Aggregated
Maximum Bit Rate) indicates the maximum total bandwidth for multiple EPS bandwidth
allocation. There are two types of AMBR: APN-AMBR, the maximum bandwidth that can
be shared by all non-GBR bearers in a PDN, and UE-AMBR, the maximum bandwidth
that can be shared in a UE. A UE can be connected to more than one PDN, in which
case the total APN-AMBR of all PDNs cannot exceed the UE-AMBR.
LTE has a big granularity and is able to recognize and differentiate specific types of
traffics. The entity which provides this characteristic is the Policy and Charging Rules
Function (PCRF) and enables the network core to shape bandwidth and implement QoS
manage resource allocation.
The 3GPP has defined a series of standardized QCI types, which are summarized in
Table 7.
QCI Resource
Type
Priority Packet
Delay
Budget
Packet
Error
Loss
Rate
Services
1 GBR 2 100 ms 10-2 Conversational voice
2 4 150 ms 10-3 Conversational Video
3 3 50 ms 10-3 Real time Gaming
4 5 300 ms 10-5 Non-conversational video
(Buffered Streaming)
5 Non-GBR 1 100 ms 10-5 IMS Signalling
6 6 300 ms 10-5 Video (Buffered streaming) TCP
based services (www, E-mail)
7 7 100 ms 10-3 Voice, Video (live streaming)
8 8 300 ms 10-6 Video, TCP based services
(iwww, E-Mail, ftp) 9 9 300 ms
Table 7: Pre-defined Quality class indicators in LTE (3GPP) {REF [11]}
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This list of QCI values contains standardized values for a typical implementation in LTE.
Additional QoS parameter vectors can be defined and implemented for special
applications. 3GPP has already issued a recommendation for QCI definitions especially
for Mission Critical Services. Table 8 extends the above table with introduced Mission
Critical specific values. An overview of all pre-defined QCI values and characteristics can
be found in Table 27.
QCI Resource
Type
Priority Packet
Delay
Budget
Packet
Error
Loss
Rate
Services
65 GBR 0.7 75ms 10-2 Mission Critical user plane Push
To Talk voice (e.g., MCPTT)
66 2 100ms 10-2 Non-Mission-Critical user plane
Push To Talk voice
67 1.5 100ms 10-3 Mission Critical Video user plane
69 Non-GBR 0.5 60ms 10-6 Mission Critical delay sensitive
signalling (e.g., MC-PTT
signalling, MC Video signalling)
70 5.5 200ms 10-6 Mission Critical Data (e.g.
example services are the same
as QCI 6/8/9)
79 6.5 50ms 10-2 V2X messages
Table 8: Quality class indicators for Mission Critical service (3GPP) {REF [11]}
The exemplary expansion of the QCIs for Mission Critical Services is a decisive
advantage for the later implementability of the functions in the 5G network
infrastructure. The more stringent QoS requirements of railway-specific applications can
only be met with this. Simultaneously, the table shows exemplarily how adaptable the
QoS management is within the 3GPP specifications.
QCIs 65 and 69 are particularly interesting for use in the railway domain. For the
transmission of ETCS telegrams (railway signalling) a short latency and extremely low
packet error rates are expected. The data rate, on the other hand, is not primarily
relevant due to small transmission capacities. This is achieved by class 69.
Voice communication also requires low latencies, as defined in QCI 65. On the other
hand, the error rate of the package does not necessarily have to be low, as error-
tolerant methods are used by the speech codec. In this context, the bandwidth is of
greater importance. The higher the available bandwidth, the more powerful and better
quality codecs can be used. For uninterrupted voice communication, it is also necessary
to guarantee this bandwidth.
5.2.5 Requirements and Features of EPS security:
The following list provides an overview of implemented features in the 4G/LTE that refer
to the security aspects of the connection and the connection setup:
• EPS provided a high level of security.
• EPS should provide protection against threats and attacks.
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• EPS shall support authenticity of information between the terminal and the
network.
• Appropriate traffic protection measures should be provided.
• EPS shall ensure that unauthorized users cannot establish communications
through the system.
• EPS shall allow a network to hide its internal structure from the terminal.
• Security policies shall be under home operator control.
• Security solutions should not interfere with service delivery or handovers in a
way noticeable by end users.
• EPS shall provide support for lawful interception.
• 3GPP Rel-99 (or newer) USIM is required for authentication of the user towards
EPS.
• USIM shall not be required for re-authentication in handovers (or other changes)
between EPS and other 3GPP systems, unless requested by the operator.
• EPS shall support IP Multimedia Subsystem (IMS) emergency calls (ECs).
• EPS shall provide several appropriate levels of user privacy for communication,
location and identity.
• Communication contents, origin and destination shall be protected against
disclosure to unauthorized parties.
• EPS shall be able to hide user identities from unauthorized parties.
• EPS shall be able to hide user location from unauthorized parties, including
another party with which the user is communicating.
5.2.6 LTE Security architecture
The EPS must be able to interwork with legacy systems, so these adaptations have to
be done in a backward-compatible way. In addition to the adaptation of legacy systems,
new extensions and enhancements have been introduced in the EPS security
architecture.
ASME (Access Security Management Entity) is an entity that receives top-level key(s),
from an HSS, to be used in an access network. In EPS an MME takes the role of ASME
and generates a KASME that is used as the top-level key to be used in the access
network. The MME, on behalf of an HSS, conducts mutual authentication with a UE
using KASME.
After the identification of the UE, the Mobility Management Entity in the serving network
fetches authentication data from the home network. Next the MME triggers the
Authentication and key agreement (AKA) protocol with the UE. The result is that MME
and the UE share a secret key, KASME.
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Figure 6: EPS security Architecture {Ref [21]}
Now the MME and the UE are able to derive further keys from the KASME. Two derived
keys are used for confidentiality and integrity protection of the signalling data between
the MME and the UE. This is represented in the figure by the arrow with ‘Non-Access
Stratum (NAS) protection’.
Another derived key is transported to the base station (evolved NodeB, eNB). From this
key three more keys in the base station and in the UE are derived. Two of these keys
are used for confidentiality and integrity protection of the signalling data between the
eNB and the UE – see the arrow with ‘AS protection’ (Access Stratum). AS security is
purposed to ensure secure delivery of data between an UE and an eNB over radio links.
The third key is used for confidentiality protection of the user plane (UP) data between
the eNB and the UE – see the arrow with ‘UP encryption’.
There is also confidentiality and integrity protection for the signalling and user data
carried over the interface between the base station and the core network (EPC). The
signalling data is transferred between the UE and the MME over the S1-MME interface
while the user data is transferred between the UE and the Serving Gateway (S-GW)
over the S1-U interface. If cryptographic protection is applied to the S1-interfaces, the
protection mechanism used is IPsec.
The X2-interface between two base stations is similarly protected by IPsec with keys
that are not specific to the UE in case cryptographic protection is applied.
Confidentiality and integrity protection mechanisms are embedded in the protocol stack
of EPS. Figure 7 shows the relevant signalling plane protocols.
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Figure 7: EPS signalling plane protection {Ref [21]}
Figure 8: EPS user plane protection {Ref [21]}
For signalling and user data, the confidentiality protection between the UE and the base
station is in charge of Packet Data Convergence Protocol (PDCP). As seen in the Figure
8 there is no integrity protection for user data between eNB and UE. IPsec is used on
both signalling and user data on X2 and S1 interfaces.
The main challenges regarding security of the transmission are in the coding and
integrity protection of vital railway application messages (e.g. ETCS) and user data.
Due to the IP basis of the LTE network, the security risks are directly associated with
the implementation of powerful security protocols for secure IP transport. Mechanisms
for mutual authentication of the devices and the network, and for encryption and
verification of message integrity in data communication between Terminal and e-
NodeB. The IMS architecture provides mechanisms for secure IP Data transport and SIP
signalling are also encrypted.
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5.3 QoS mechanisms offered by 5G
3GPP is constantly working on the standardisation of 5G. The high-level system
architecture is already formulated in the normative standard draft and includes the QoS
concept. The 5G QoS concept will be flow-based. Packets are classified and marked with
QoS Flow Identifier (QFI). There will be two types of flows: one with standardized QoS
profiles and the other with operator-specific QoS profiles. For the first one, only the QFI
value is used in the network. For the latter one, QoS attributes are also signalled
between the network elements. The 5G QoS flows are mapped in the Access Network to
Data Radio Bearers (DRBs), unlike 4G where the mapping is 1:1 between EPC and radio
bearers. Anyway, the 3GPP work on the 5G standard is expected to refine the QoS
concept over the next few years.
5.3.1 QoS Model – general overview
The 5G QoS model supports a QoS flow based framework. This approach provides both
QoS flows that require guaranteed flow bit rate and QoS flows that do not require
guaranteed flow bit rate. The 5G QoS model also features reflective QoS, where no
explicit signaling is needed for the configuring of QoS rules – the network decides which
QoS characteristics are applied.
Basically, the QoS flow is the finest granularity of QoS differentiation in the PDU
session. A QoS Flow ID (QFI) is used to identify a QoS flow in the 5G system. User
Plane traffic with the same QFI within a PDU session receives the same traffic
forwarding treatment (e.g. scheduling, admission threshold). It can be applied to PDUs
with different types of payload, i.e. IP packets, unstructured PDUs and Ethernet frames.
The QFI shall be unique within a PDU session.
5.3.2 Quality Requirements in 5G Networks
During the evolution of QoS management mechanism in 3GPP (GSM/UMTS/LTE)
networks there was a migration from QoS management at the user equipment level to
the QoS management at the network level. This approach to QoS management will be
maintained in 5G networks as well.
QoS management mechanisms in 5G networks should provide video and VoIP traffic
prioritisation towards Web-search traffic and other applications tolerant to quality. The
service of streaming video transfer without buffering is very sensitive to network delay,
so one of the most important parameters that determine QoS requirements is the total
packet delay budget (PDB), which is formed on the RAN air interface, and is treated as
the maximum packet delay with a confidence level of 98%.
5.4 Architectural and procedural basics
At this sub-section the architecture of 4G, especially the implemented services for voice
transmission and data prioritisation will be examined. The aim of these studies is to
proof whether existing railway applications (voice functions, with focus on critical
services, e.g. railway emergency calls) can be implemented within the next standards.
Railway applications demand specific functionalities as introduced in section 3.3. The
ASCI enhancements are one example for this. 4G/5G network features and mechanisms
to implement the railway functionalities are introduced and summarized within this sub-
chapter.
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5.4.1 Voice Services within 4G IP networks
The ASCI enhancements within GSM-R networks for specific railway voice services are
core functionalities for railway operation. The voice service capabilities within the 4G
3GPP standards are major challenges that must be analysed.
The technological solutions for delivering voice over LTE are based on the IP Multimedia
Subsystem (IMS), with the implementation known as Voice Profile for Voice over LTE
(VoLTE). The main characteristics of this solution and the advantages and
disadvantages from the railway point of view are discussed. Table 9 gives an overview
of the main expected advantages of the VoLTE for railway applications.
Table 9: VoLTE capabilities overview
VoLTE capability
Seamless Continuity of Voice Services Single Radio-Voice Call Continuity
Mechanisms
Support of IMS Based Service Priority and QoS handling of different Voice
types (i.e. staff communication, RECs)
Support of 4G/5G QoS mechanisms
Specific qualification for railway
applications
Supports transparent handover between LTE
and GSM/UMTS for migration phase
Provides end-to-end IP services
Supports simultaneous voice / data
transmission
Low latency
Short Call Setup Times
Compared to the 2G or 3G network capabilities used up to now for voice telephony,
VoLTE offers significant improvements such as reduced power consumption, improved
voice quality and faster connection setup. In contrast to GSM, voice communication via
VoLTE is packet-based. VoLTE is distinguished above all by its better voice quality and
faster call set-up. Usually the AMR-WB codec commonly is used, depending on the
network operator. The narrowband AMR-NB codec is also supported for compatibility
purposes.
Before the introduction of VoLTE, it was not possible to establish a standard voice
connection via the LTE network. The circuit-switched fall-back (CSFB) was introduced. If
the mobile network signals a call to the mobile phone, the modem of the device
switches from LTE to GSM in order to establish the circuit-switched call. This procedure
can be important in the future migration scenario between GSM-R and a successor
technology.
5.4.2 IP Multimedia Subsystem
The IP Multimedia System within 4G networks (IMS) is an IP-based architecture for
delivering multimedia services independently of the underlying access network. IMS
offers several functionalities, such as interworking with circuit-switched networks, user
roaming, and negotiation of QoS requirements.
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The IMS functionalities are necessary for providing voice communication via 4G
networks. VoLTE defines a specific subset of the IMS functionalities required to provide
a standardized voice communication service over the LTE network.
The basics of the IMS are described in the "Technical Specifications" of the 3GPP
standardisation committees. The specifications of IMS can be found in 3GPP TS 23.228
and 3GPP TS 23.002 (overall architecture). IMS also forms the basis for the Next
Generation Network Release 1 described by ETSI.
IMS supports the following services:
• packet-switched connections between two or more subscribers,
• collaboration between the circuit-switching and packet-oriented domains,
• end-to-end point negotiation of service quality (QoS),
• service-based cost accounting,
• provision of the home network environment in foreign networks,
• support of different media types,
• fast and flexible creation of services through predefined service components
("service enabler"),
• services should be independent of the access network.
Figure 9: IMS architecture within LTE Evolved Packet Core (Spirent)
The standard describes how multimedia connections and services are controlled by an
IMS core network. Application servers provide the services via open interfaces to the
core network. Signalling, transport and security mechanisms were defined for this, but
not the services themselves. Today, IMS is a step towards the Next Generation Network
(NGN). IMS brings all communication services together on one technical platform to
enable versatile IP-based communication services.
IMS is based on SIP, the signalling protocol. The connection between the different
network types is created by so-called media gateways. The user can access services
and applications from any network. The difference between wired and wireless networks
disappears. This is known as "Seamless Mobility". In such a network, it no longer
matters which terminal device, which content is accessed or which services are used.
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There will still be different networks, but they will only be based on the Internet
Protocol (IP).
The main challenge in the implementation are the unstructured services and systems.
On the one hand, there is the transmission of voice and SMS. On the other hand, data
services and combined voice and data services.
IMS actually consists of four layers:
• Application Layer (Service/Application Layer)
• Session Layer (IMS Layer)
• Transport Layer
• Access layer (terminals)
The aim of IMS is that the applications in the core network no longer communicate with
every end device, but only with the session layer, which acts as middleware. Another
advantage is that the user is no longer tied to a specific terminal device.
The core architecture of 3GPP IMS (see Figure 10) is described in the specification 3GPP
TS23.228. It describes various elements of the architecture.
• User Agent (UA)
• Call Session Control Function (CSCF)
• Home Subscriber Server (HSS)
• Application Server (AS)
• Media Gateway (MGW)
• Media Gateway Controller (MGC)
• Signalling Gateway (SGW)
Figure 10: IMS core elements (Spirent)
The IMS architecture simplifies infrastructure management. Network operators can
expand their range of services at any time without interfering with the network. The
basic functions are set up on a central server. These functions are required for a large
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number of applications. This includes authentication, billing, QoS and registration.
Flexible access to various functions enables new services to be provided much faster.
5.4.2.1 IMS Signalling
The central element for the connection establishment of multimedia sessions is the
Session Initiation Protocol (SIP). It is a protocol for IP-based communication services
such as voice and video telephony or group calls. It does not matter whether it is a
voice or data connection.
SIP is a text-based protocol used by clients and servers to control their connections.
With SDP, media description, codec, ports and transmission direction are exchanged.
With RTP, the media streams are transmitted in real time. In parallel to RTP, RTCP is
used to exchange important control information about the RTP media stream between
client and server.
But before signalling starts, the User Agent must first establish a connection to a proxy
(P-CSCF). After that the User Agent has to register. The aim is to establish a unique
assignment between the unique public ID of the user and the user agent. The user is
always addressed using the public ID. While this is unique, the user's IP address can
always be different, or the user is registered in several places on the network at the
same time.
5.5 3GPP Mission Critical Communication Enhancements
The standardisation of current wireless technologies is an ongoing process that is
constantly being driven forward by the 3GPP standardisation initiative. In the recent
years, progress has been made in the definition of mission critical communication1,
which is particularly important in the railway sector.
Mission Critical Push To Talk (MCPTT) was the first development step of 3GPP Mission
Critical (MC) Services and functionalities. In Release 14 (2017), 3GPP added
supplementary MC Services and enhancements to its repertoire of standardized
applications, specifically:
• Enhancements to MCPTT,
• MCData,
• MCVideo,
• General framework which facilitates standardizing additional MC Services.
The Release 14 specification on MC Services required a large set of new protocol
additions and new security functionality. The introduced MC Services were split into
smaller, self-contained features. These MC Services introduced in Rel-14 offer stand-
alone functionality that enriches the existing base of services, so the providers don’t
have to wait for the completion of additional standardized functionality in Rel-15.
With the extension of the functionalities in MCCore and additional features introduced in
Release 16 (e.g. MCData and MCVideo enhancements), all necessary functional
requirements of railway communication are to be addressed for the first time.
A detailed list of selected Mission critical functionalities, that may be of special interest
for use in the railway communication, completed in Rel-13, Rel-14, Rel-15 and Rel-16 is
shown in Table 10:
1 http://www.3gpp.org/NEWS-EVENTS/3GPP-NEWS/1875-MC_SERVICES
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Rel-13 MCPTT
(completed 2016)
Rel-14 MC Services
(completed 2017)
Rel-15 MC Services
(completed 2018)
Rel-16 MC
Services
(in progress)
User authentication and
service authorisation
User authentication
and service
authorisation
Enhanced MCPTT
group call setup
procedure with
MBMS bearer
MCCore Services
Common
Functionalities
Enhancements
Affiliation and de-
affiliation
Extended Location
Features
Enhanced Location
management,
information and
triggers
Enhancement of
MCPTT
(e.g. Enhanced
Broadcast group
call, Multiple
simultaneous
users)
Group calls on-network
and off-network (within
one system or multiple
systems, pre-arranged
or chat model, late
entry, broadcast group
calls, emergency group
calls)
(Dynamic) Group
Management
Interconnection
between 3GPP
defined MC systems
MCData additions
(e.g. Emergency
Alert, Enhanced
Status,
Accessing list of
deferred
communications)
MCPTT security
Group Call (including
emergency group
calls, imminent peril
group calls,
emergency alerts)
Interworking with
legacy systems
MCVideo additions
(e.g. Emergency
and imminent peril
private
communications,
Broadcast Group
Call)
Simultaneous sessions
for call
Transmission and
Reception Control
Enhanced handling of
MCPTT Emergency
Alerts
Pre-established sessions
Short Data Service
(SDS)
Enhanced Broadcast
group call
Location configuration,
reporting and triggering
MC Security
framework
Broadcast Group Call
Table 10: 3GPP Mission Critical services and enhancements in Release 13 to 16 (3GPP, TUD)
These extensions for mission critical communication introduced by 3GPP offer the
possibility to implement the stringent requirements of railway-specific data transmission
into the 4G/5G network. Only with the help of these additional functionalities it is
possible to handle the railway QoS requirements and also to perform the data
transmission over public commercial networks in a future NaaS scenario for railway
communication.
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5.5.1 Quality of Service requirements for mission critical applications
As introduced within the functional requirements, in railways a number of commonly
used applications exchange information via the railway communication mobile network.
These applications comprises data- (e.g. Train Protection and Automation), voice- (e.g.
staff calls, calls between drivers and controllers) and video-applications (e.g. driver
CCTV). Each application type has a different quality of service requirement. A general
approach of QoS requirements for common types of applications (not railway specific)
can be found in [15]. This study covers, QoS definitions, Classification of applications
(real-time, non-real-time) and QoS metrics for different classes of applications.
3GPP is following a progressive definition of future network technologies that can
transmit mission critical data with the QoS definition ([10],[11]). The application that
transfers data within the networks is assigned to certain categories according their
demands on network resources.
A qualitative classification of the QoS characteristics for different applications is defined
in the following table.
Application
Category
Service Attribute Session
establishment
Session Loss
Rate Latency
peer to peer
Packet
Loss Rate
Voice Low
Normal
Normal
-
Critical Voice Low
Low
Immediate
-
Video Normal Low Normal -
Critical Video Low Low Immediate -
Very Critical
Data
Ultra-Low Ultra- Low Immediate -
Critical Data
(future
applications)
Low Ultra- Low Immediate -
Critical Data
(legacy
applications)
Normal Low Normal <10-2/h
Non-Critical
data
Normal Low Normal -
Messaging Best Effort Low Normal - Table 11: QoS categorisation for different application types (3GPP) {Ref [10]}
To ensure the required communication quality for a specific application demand, the
network requests the resources required for the communication service from the
underlying 3GPP transport.
The transport resources are characterized by latency, reliability, guaranteed bit
rate/unguaranteed bit rate and priority of the communication service. If no specific
resource characteristics are required for a particular communications service, the
mobile network can use a predefined standard. Each feature of the communication
service resource can be requested independently by the application.
As a result of the request the mobile network system can offer different values than
those requested, but which match the required QoS application categories (Table 11).
According to the 3GPP specification ([11]), a differentiation of the following application
categories for transmission services will be considered in the future:
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• Voice
user to user or multiuser communication;
• Critical Voice
follows the voice conversational pattern but requires immediate session setup;
• Video
visual train remote control or observations purposes; Video requires low jitter;
• Critical Video
door monitoring by the Driver, passenger surveillance, driverless mode in urban
rail;
• Very critical data
for future rail applications;
• Critical data
follows the response pattern and requires high reliable transport. This category
comprises future and legacy applications e.g. ETCS;
• Non-Critical data
used for the exchange of railway system or communication relevant information;
requires high reliable transmission and preservation of the response pattern;
• Messaging
for the exchange of non-critical short information messages, recorded voice (for
example voicemail), data, pictures, video; requires reliable transmission.
To achieve the QoS applicable to each application category, specific priority levels are
required to differentiate communication priority. The priority handling of a
communication service includes the assignment of a priority to a communication and
the handling also includes the interruption of a running communication with lower
priority in order to enable an incoming communication with higher priority. The
appropriate used priority level depends on the user initiating the communication. The
communication can have the priority level selected by the user at setup or the priority
level is predefined at network registration.
In order to meet the different application characteristics such as real-time or critical
data, further quantitative specification is necessary. 3GPP recommends the following
values for the mapping of service quality for the specific categories (see Table 12).
Mapping the service attributes latency and reliability between the functional
requirements and the communication network system and their target values are
prerequisites for the implementation of QoS.
Service Attribute System
Requirement
(according 3GPP)
Service
Attribute value
Latency Ultra-Low ≤10ms
Low ≤100ms
Normal ≤500ms
Best Effort >500ms
Transport reliability - Packet Loss (%)
Ultra-Low 99.9999%
Low 99.9%
Normal 99%
Session
Establishment
Immediate <1s
Normal <3s (typical) Table 12: Quantitative Mapping of service and QoS attributes (3GPP) {Ref [10]}
The attributes are defined on the following basic aspects. Latency quantifies the end-to-
end user data transport delay between the involved communication entities and are
classified:
• Low: Delays harm the functioning of the application.
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• Normal : Delays do not harm the sequence and progress of the application.
Reliability defines the amount of sent network layer packets, which are successfully
delivered to a given node within a specified time constraint, divided by the total number
of sent network layer packets. Two levels are to be taken into account:
• High: The packet loss at transport level is exceptional rare.
• Normal : The packet loss at transport level is seldom.
The Session Establishment time defines the time to initiate a voice or data
communication session with the application that is sufficient to perform the specific
railway operation.
• Immediate: A session is set up time critically <1s
• Normal : A typical session setup time does not harm the specific railway
application (typically <3s).
In order to meet the different application characteristics also for real-time applications
and critical data, further detailing is necessary. The requirements for ultra-low latency
and particularly ultra-high reliability are expected especially for mission-critical
applications.
5.5.2 Derived QoS from Future Railway Mobile Communication System
In 2012 the UIC set up the Future Rail Mobile Communications System (FRMCS)
project. The aim was to prepare the introduction of a successor of GSM-R. These first
steps started with an evaluation of the actual situation, included several studies on
railways’ needs and ended with the delivery of a first set of “User Requirements
Specification”.
The UIC driven FRMCS project and the 3GPP standardisation body work closely
together. The UIC is an international association of railway companies. Besides, the
3GPP works as a standardization body for communication technologies. The UNIFE
Unitel committee, which gathers the main suppliers of railway telecommunication
systems, is also involved within the activities about FRMCS specifications. Mobile
network operators are not participating in the UIC working groups.
The results of the FRMCS study are incorporated in the following standards and
recommendations. Within the FRS a set of technology independent user requirements
for the FRMCS are defined. The various inputs were received from globally railway
communities.
The technology independent user requirements for the future radio mobile
communications are defined in the form of individual application use cases. Each of
these applications has been defined to provide or support an identified communications
demand that is considered necessary for current and future railway operations.
These use cases are differentiated of the following railway application types:
• Critical Communication Applications,
• Performance Communication Applications,
• Business Communication Applications,
• Critical Support Applications.
Of particular interest are the generic approaches for QoS and critical support functions,
which are the prerequisites for the actual critical railway applications. In Table 13 the
main groups are named and briefly described.
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Critical Support
Applications
Description
Assured Voice
Communication
Indication as soon as an end-to-end voice communication link is
broken or as long as the end-to-end communication link is active. Multi user talker
control
Limits the number of simultaneous talkers in a multi-user voice
communication and mitigates the risk of miscommunication. Role management
and presence
A user shall be able to register and deregister to one or more
functional identity/ies. Location services The system shall be able to store and provide the location of the
user(s) or devices. Authorisation of
communication
The system shall be configurable, so that access to voice and
data communications can be controlled QoS Class
Negotiation
The system shall be able to assign different QoS classes in order
to fulfil the level of communication quality required by the
applications. Safety application
key management
communication
Authentication of the source of the messages received as a
trusted source, and shall be able to detect corruption of the
messages received.
Assured data
communication
Indication to the users as soon as an end-to-end data
communication link is broken or as long as the end-to-end
communication link is active. Table 13: Overview of Critical Support Applications (FRMCS) {Ref [10]}
With the help of these predefined methods it is possible to introduce critical railway
applications also within 4G networks. These supporting methods can be implemented
after they have been incorporated into the 3GPP standard. The committees work closely
together.
Furthermore, FRMCS also provides a qualitative approach for the definition of a QoS
recommendation for the respective use cases. Table 14 shows a selected set of use
cases with different characteristics and compares the recommended QoS
characteristics.
Application Type Symmetry Up/Down
Latency Bandwidth Reliability Setup Train Speed
Voice
Communica
tion
Voice 50/50 Low Low High Normal High
Public
Emergency
Call
Voice 50/50 Low Low High Normal High
Railway
Emergency
Communica
tion
Voice 50/50 Low Low High Immediate High
ATO Data 50/50 Normal Low High Immediate High
Critical
Realt-time
Video
Data 95/5 Low High High Normal High
Wireless
internet for
passengers
Data 20/80 Normal High Normal Normal High
Table 14: QoS of different selected FRMCS application types (FRMCS) {Ref [10]}
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In comparison with the findings of 3GPP-QoS analyses from Table 11 a qualitative
statement about the required bandwidth of the application, the maximum train speed
and the expected symmetry for the upload and download is also evaluated here. The
following values from Table 15 can be assigned to these qualitative aspects, which
generate a plausible service characteristic.
Table 15: Quantitative Mapping of additional service attributes {Ref [10]}
Service Attribute System
Requirement
Service
Attribute value
Bandwidth High 5000 kbit/s
Medium 500 kbit/s
Low 50 kbit/s
Very-Low 5 kbit/s
Train speed High >250 km/h (up to 500 km/h)
Normal >40 km/h
<250 km/h
Low <40 km/h
5.6 Summary
Quality of Service (QoS) has become an important part of network planning and design
when deploying 4G/5G networks for specific data and voice services.
Especially for network users who demand LTE services for critical operations, 4G was
designed to meet these increased data and application demands with reliable
connections parameters. A highly-flexible QoS framework was introduced to allow
assignment of different priorities for certain applications or services.
QoS is implemented and applied to a set of specific bearers with defined priority
regimes, specified with certain QCI parameters. This framework provides the platform
for implementing special functions that were previously only available via a dedicated
network, as we have learned from the example of GSM-R.
Newer approaches such as Network Function Virtualisation (NFV) follow the aim to
virtualise parts of mobile networks and have obviously some overlap with the
functionality of 5G Network Slicing. The main advantage of the Network Slicing
approach is that it provides a logical virtual end-to-end network for a specific user
segment. No existing QoS-based solution can offer this. With the introduction of
Network slicing concept in 3GPP TS22.889 standardized extensive possibilities are made
available, which will facilitate the QoS definition and alignment for railway
communication networks.
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6 Railways QoS Implementation Requirements for
future scenarios
This chapter serves as a derivation of the preceding main chapters. With the knowledge
of which requirements exist for different specific voice and data railway services and
which methods are available in future IP based standards, an attempt is now being
made to combine the results. The focus is also on whether the necessary technical
prerequisites can be provided in a future scenario. The conclusion whether the
requirements that currently exist for railway-specific services can also be applied in
NaaS scenarios and if so, which preconditions exist are examined.
6.1 System architectural requirements
Future railway communication technologies will most likely be based on 5G network
technology. Already in MISTRAL D4.1 the new technical approaches of this new
generation were mentioned and the differences in design to 4G networks were
highlighted.
Current standardisations of 3GPP (starting with Release 13) are already starting to
incorporate special functionalities and requirements for the domain of mission critical
applications into the specification. Especially the use cases of the railway domain are
considered relevant for our analysis in MISTRAL project.
This section is dedicated to current developments and highlights the basic approaches
for implementing the railway QoS principles (and other possible mission critical
application domains) in the 5G specification.
The future 5G network will support many commercial applications with specific priority
demands. These special services share strict common QoS characteristics such as
latency and packet loss rate, but do not necessarily require the same priority regime.
On the other hand, voice-based services (e.g. operational railway calls, railway
emergency call) may have common QoS features as they apply to normal public voice
communication, but with different priority requirements. These mechanisms, which
enable the decoupling of the priority of a particular communication from the associated
QoS characteristics such as latency and reliability, will be implemented by the 5G
network. The priority of each service can be different for a user of that service,
depending on operational requirements and regional or national regulations.
Therefore, the 5G system should provide a flexible framework for prioritising and
implementing priorities in services (e.g. Emergency, Public Safety, Railway domain).
The future network shall deliver the ability to provide the required QoS (e.g. in terms of
defined values for reliability, latency and bandwidth) for specific services and prioritize
resources when required to meet service requirements. Existing QoS and policy
frameworks handle latency times and improve reliability through traffic engineering.
The 5G network will operate in a heterogeneous environment with several access
technologies, different types of UE, etc. It is necessary to implement a harmonised QoS
and guideline system applicable to several types of access.
For QoS control in future railway communication networks, End-to-End QoS (e.g. RAN,
Backhaul, core network, Network to Network Interconnect) is required to achieve the
user demands (e.g. ultra-low latency, ultra-high bandwidth).
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6.2 QoS for MISTRAL innovative services
The MISTRAL project defined different application scenarios and innovative functions for
the railway sector that will transmit data via future communication networks. The
analyses focused on different fields of application and user groups. A detailed
description of the innovative services is provided in MISTRAL deliverable D2.1.
At this point the presented application areas are linked with the previously introduced
application categories for specific communication parameters and QoS requirements.
The Table 16 shows the results of the analyses.
It is obvious that the generic approaches of QoS classification are also applicable to the
identified innovative services. The general distinction between voice and data
transmissions with different priority and requirement levels enables a logical integration
of services into specific QoS classes.
In order to analytical break down the identified service categorisation, a clear distinction
can be made between railway-related services (mission critical communication),
business-related analysis services (IoT and maintenance data, with low bandwidth and
latency demands) and services for passengers (bandwidth related demands).
While QoS for mission critical applications must be defined much stringent in terms of
latencies and reliability parameters, on the other hand, the quality of service with
regard to high bandwidth plays the major role in services for passengers, such as the
provision of on-board entertainment services. Especially if a larger number of
passengers use the services simultaneously on trains, a high bandwidth can be
expected here.
MISTRAL Innovative Service/Application
Comm. vector
Type of Trans-
mission
QoS require-ments
Service Attributes
List of innovative services described in deliverable D2.1
Application Category (linked to 3GPP TR22.889)
Latency
Pac-ket Loss
Session establish-ment
Band-width
Moving Block (signalling)
Train-To-Infrastruc
ture
Data communica
tion
Critical Data
(future application)
Low Ultra-Low
Immediate
Low
Data
communication
Very
Critical Data (future application)
Ultra
-Low
Ultra-
Low
Immedi
ate
Low
GOAx (signalling) - ATO - ATP
Train-To-Infrastruc
ture
Data communica
tion
Critical Data
(legacy application)
Normal
Low Immediate
Low
Data communica
tion
Critical Data
(future application)
Low Ultra-Low
Immediate
Low
Improved railway emergency calls
Train-To-Infrastruc
Voice call Critical Voice
Low Low Immediate
Medium
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ture Video call Critical Video
Low Low Immediate
High
Precise and certified timing and train position (signalling)
Train-To-Infrastruc
ture
Data communication
Non-critical data
Normal
Low Normal Low
Voice, Data communication,
Messaging
Voice, Non-critical data,
Messaging
Low-Normal
Low-Normal
Normal Medium
IoT-enabled maintenance in railways (real time and smart
monitoring TCMS)
Train-To-Infrastructure
Data communication
Non-critical data
Normal
Low-Normal
Normal Low
Bidirectional real- time monitoring and recovery of trains information and voice
communication
Data communication
Non-critical data
Normal
Low-Normal
Normal Low
Monitoring infrastructure
Train-To-Infrastructure
Data communication
Critical data
Normal
Low Normal Medium
Real-time video
surveillance (on-board)
Train-To-
Infrastructure
Video Video Nor
mal
Low Normal High
Cost analysis of
operational and maintenance (Big-Data)
Train-To-
Infrastructure
Data
communication
Non-critical
data
Nor
mal
Low Normal Low
Statistical analysis of safety-related failures of systems and devices
Train-To-Infrastructure
Data communication
Non-critical data
Normal
Low Normal Low
Ad-hoc
operational data dispatching
Train-To-
Infrastructure
Data
communication
Non-critical
data
Nor
mal
Low Normal Low
Automated Train-to-Train
communication (information about track
conditions, obstacles, weather, etc.)
Train-To-Infrastruc
ture
Data communica
tion
Non-critical data
Normal
Low Normal Low
Additional information (multi-modal
seamless experience)
Train-To-Infrastructure
Data communication,
Messaging
Non-critical data,
Normal
Low Normal Medium
Messaging Messaging Best Effort
Low Normal
Electronic ticketing
Train-To-Infrastruc
ture
Data communica
tion
Non-critical data
Normal
Low Normal Low
On board entertainment
Train-to-mobile
Data communication
Non-critical data
Normal
Low Normal High
One-stop Train-to- Data Non-critical Nor Low Normal Medium
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shopping gateway application
mobile communication
data mal
Context aware marketing and
services
Train-to-mobile
Data communica
tion
Non-critical data
Normal
Low Normal Medium
Context aware shopping
Train-to-mobile
Data communication
Non-critical data
Normal
Low Normal Medium
Railway services elicited from the
metro context
Data communica
tion
Non-critical data
Normal
Low Normal Low
Assisted decision-making: Optimal choice for the
transport and route (criteria:
more comfortable, faster, cheaper, better connections, etc.)
Data communication
Non-critical data
Normal
Low Normal Low
Real-time timetable
information (enhanced connection and routing information)
Data communica
tion
Non-critical data
Normal
Low Normal Medium
Provide precise
positioning data of trains(e.g. for maps/guides in emergency cases)
Train-to-
mobile
Data
communication
Non-critical
data
Nor
mal
Low Normal Low
Seamless services available on board
and at the station (business)
Train-to-mobile
Data communica
tion
Non-critical data
Normal
Low Normal Medium
Table 16: MISTRAL innovative services and QoS attributes mappings (TUD)
6.3 Conditions for railway functionalities
This section is intended to provide an overview of the basic conditions and requirements
for the transfer functions of the network when railway-specific functions and
applications are used. Prerequisites are defined for data and voice connections, as well
as overlapping features, such as cryptographic requirements for transmission or
authentication features.
6.3.1 Data Transmission
The minimum functional and performance requirements for a 4G-based data
communication network for railway domain can be derived from [13]. Table 16 shows a
summary of the most important parameters regarding network deployment.
Items Description
Coverage
and performance
Network coverage: 98%
Communication up to a train speed of 500 km/h
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Handover
success
seamless data transmission,
handover success rate shall be 99%
Connection drop
rate
a call disconnection rate less than 0.01 times per
hour
Train control data
transmission
>99% data reliability to transmit data for train control
Train control data shall have highest priority.
Network
redundancy
network equipment, core equipment and server shall
be designed to be redundant for stability and
availability.
Table 17: Minimum requirements on railway network performance
In particular, all functionalities and performance requirements with the exception of the
high speed (500 km/h) mentioned above were fully supported by LTE.
Promising Mission Critical Services functionality (see Chapter 5.5) that extends 3GPP
Rel. 16 to meet more demanding requirements have to be taken into account for future
networks. In this context, these requirements to future railway network technology can
be regarded as basic functional and performance requirements.
Application
Category
Service Attributes Session
establishment
QCI
Latency
peer to
peer
Reliability Priority
Critical Data
(future
applications)
≤100ms 99.9999% 0.5 <1s 69
Critical Data
(legacy
applications)
≤500ms 99.9% 5.5 <3s 70
Non-Critical data ≤500ms 99.9% 9 <3s 9 Table 18: Derived QoS vector for Data transmission
The values shown in Table 18 can be derived from the qualitative QoS requirements
recommended by 3GPP. These values for data communication are considered a
prerequisite for the minimum quality of service of the respective application class.
Particularly with regard to critical data transmission (e.g. railway signalling application),
the high level of reliability required for transmission must be taken into account.
6.3.2 Voice Transmission
The future transmission technology in the railway sector will be completely IP-based.
Especially in the transmission of voice, this will result in a complete paradigm shift
compared to GSM-R. Voice transmission will be carried out using Voice over Internet
Protocol (VoIP) technology, which enables voice calls over an IP communication
network instead of a standard dedicated (circuit-switched) channel. VoIP has high
demands on transmission quality.
For the IP transmission procedure this means that there are high requirements on
latency, jitter and packet loss rate (as low as possible). There are also data throughput
requirements (as high as necessary) that relate to the desired audio quality and depend
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on the speech codec used. Particularly for voice transmission, the influences of packet
loss, distortion, echoes and degraded voice quality due to packet delay is particularly
significant.
6.3.2.1 Service quality requirements
The ITU has specified a reference model, in order to determine the expected quality of a
voice transmission, which defines a classification that can be correlated to the
demanded voice quality. The ITU specification defines different QoS classes and
guidelines for assessing service quality in IP terms, which class a particular type of
application should use. This assessment of quality can be derived from certain
transmission parameters.
Regarding voice services, the total end-to-end latency is the sum of delays in the
specific network areas from the speaker's terminal to the listener's terminal: within the
devices and network connections over which the voice traffic routes. Many factors
contribute to the end-to-end latency, which are discussed below:
• Network latency
Generically, a circuit-switched Public Switched Telephone Network (PSTN)
typically has a relatively low delay, which is primarily dependent on the
transmission distance. Typically, this latency is <100ms.
Latencies and delays within IP networks are primarily determined by the
performance of network elements (e.g. routers): Buffering, the queue and the
switching or routing delay are important factors. Specifically, the IP network
delay consists of packet recording delay, switching/routing and delay and waiting
time.
• VoIP device delay
Due to the signal processing on both the sending and receiving side of a
connection, VoIP gateways and VoIP terminals contribute significantly to the
end-to-end delay caused by the signal. This processing is necessary to enable
the coding and compression of the analogue speech signal into a digital signal.
The choice of voice codec implementation is important. The more effectively the
device is adapted to the speech codec used, the shorter the processing times
within the devices. On the other hand, powerful codecs can also achieve a lower
network load if smaller data packets can be generated by higher compression.
Various optimized speech codecs are available for transmission. Specific codecs
are optimized for the respective transmission medium and the requirements for
the QoS characteristics of the transmission. On the receiver side, voice packets
are intentionally delayed to compensate for variations in packet transmission
times (variation of packet run times is called "jitter"). Also packets that are
created with constant time intervals usually also come to the recipients with a
randomly allocated distribution due to the different buffering and queue times
within the network, because different transmission paths are normal within IP
networks.
Voice codecs (see 6.3.2.2) require a constant and seamless data flow. It is
recommended to perform voice transmissions with a guaranteed bit rate. The
use of jitter buffers is also required for jitter smoothing.
Above all, the use of mechanisms that prioritize voice traffic over other traffic on
the network can significantly reduce jitter and increase voice quality.
• Packet loss rate
If the network or a network segment is overloaded, packets may get lost and the
speech signal cannot be decoded. With real-time voice information, the packets
must arrive in a relatively short time to reconstruct the voice signal, otherwise it
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would lead to incomprehensible voice transmission. In general, packet loss must
be less than 1% if audio quality is not to be affected. Greater than 3% is clearly
noticeable as a reduction in the quality of the language.
• Echo and Distortion
Echo and distortion problems are usually the result of insufficient service quality
of the transmission path. The errors are mainly handled in the VoIP gateways
and terminals. In the network, small delays, low jitter and low packet loss help
to counteract these problems.
These characteristics must be considered and evaluated in order to define requirements
for voice transmission.
6.3.2.2 Qualified voice codecs
A voice codec converts analogue speech into digital bit streams and back. In addition,
voice codecs usually use compression methods that filter redundant or less important
information to reduce the required transmission bandwidth.
Various voice codec algorithms were specified and defined e.g. within ITU-T. The
compression performance of a voice codec is a compromise between voice quality,
processing power of the equipment, the acceptable delay and required network
bandwidth. In general, the greater the bandwidth reduction, the higher the
computation costs of the codec for a certain level of quality.
In addition, a greater saving in bandwidth usually leads to a higher computing delay
and thus to a significant increase in end-to-end delay. The influence of a codec on the
voice quality is also influenced by the packet size, packet loss and possible errors
caused by the correction mechanisms used by the codec itself.
The standard codec for GSM-R is currently the Enhanced Full Rate Codec (GSM-EFR),
with a bandwidth requirement of 12.2 kbps. However, this codec will no longer be used
in future VoIP networks. New codecs with better performance and adaptability to
different bandwidths are in demand.
The AMR Wideband (AMR-WB) codec specifications is applicable for voice applications in
converging wired and wireless networks and standardised by ITU. Within the AMR-WB
speech codec nine different rates are defined of 23.85, down to lowest bitrate of 6.6
kbps. The standard supports dynamic adaptation to network conditions through lower
bit rates in the event of network overload or deterioration while maintaining audio
quality. AMR-WB also works together with VMR-WB, a 3GPP2 broadband voice standard
that is mandatory for broadband telephony and multimedia messaging services.
As a codec for voice transmission that offers quality requirements suitable for future
networks such as 5G and variable performance options, the AMR-AWB codec is the
recommended implementation.
6.3.2.3 Voice Group calls
The group call functionality is part of the EIRENE/ERTMS standard and is used, for
example, for shunting work, for commercial operation and for emergency railway calls.
With GSM-R, group calls currently use the Voice Group Call Service (VGCS). VGCS uses
a single voice channel to all receivers (downlink) in a cell and activates the uplink only
for the speaking person, initiated by a switch.
However, in a packet-switched network, group calls can be realized with individual
bidirectional calls to each receiver. Alternatively, multicast functionality can be used to
send efficiently to the group in a single transmission. The benefit of using a multicast
solution depends on the technology and implementation. In the most effective case, the
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bitrate request for the group call can be reduced to the same requirement as a single
VoIP call.
6.3.2.4 Summary for Voice transmission
The following Table 19 shows the combined requirements which can be defined for a
standard voice connection for operational railway utilisation:
QoS channel indicator Quality characterisation
Bandwidth/Throughput
Depends on the used speech
codec and protocols.
AMR-WB codec is about 60
kbps
Guaranteed bitrate is
recommended
Latency
<150ms,
ideally <40ms
High priority regime class
Jitter
<25ms
Packet Loss rate
<0.5%,
ideally <0.1%
Table 19: Existing QoS recommendation for voice transmissions
A more generalized approach to integrate VoIP with different codec types into a
quantitative QoS assessment is described in the Table 28. However, these requirements
are less restrictive and not completely applicable to the railway communication.
Application
Category
Service Attribute Session
establishment
QCI class
Latency
peer to peer
Reliability Priority
Voice ≤100ms 99% 2 <3s
1
Critical
Voice
≤100ms 99.9% 0.7 <1s
65
Video ≤150ms 99.9% 4 <3s 2
Critical
Video
≤100ms 99.9% 1.5 <1s 67
Table 20: Derived QoS vector for Audio and Video transmission
The values shown in Table 20 can be derived from the qualitative QoS requirements
recommended by 3GPP. These values for voice communication are considered as a
prerequisite for the minimum quality of service of the respective application class.
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6.3.3 Security requirements
In addition to the requirements for QoS features, security against cyber-attacks on the
network infrastructure will also be an important issue in the future, especially with focus
to the NaaS transition. It is necessary to investigate which security functions and
cryptographic approaches can be used in the future railway communication network.
The security framework of the communication network shall protect against attacks and
threats, like misuse, Denial of Service (DoS), unauthorized access to services,
interception, man-in-the-middle attacks, replay attacks and intended data modification.
General risks that may occur during the transmission of safety-relevant are analysed in
EN50159.
The followings risks could be caused by intentional influence on the communication
channel. A classification of these attacks can be made as follows:
• Jamming
A blocking or interference method, which can be used in wireless data
networks to partially or completely disrupt information flows.
• Denial-of-service attack (DOS)
Intentional overload of the system, by flooding the targeted network
resource with massive requests
• Spoofing and redirection of messages to other destination
This attack misleads the transmitted information to the receiver. This is
done by broadcasting not counterfeit data signals or genuine “corrupted”
signals captured and modified in a different source of information.
• Penetration into the communication system by an attacker stopping the
transmission system.
The availability of the communication system of signalling applications are indispensable
for safe railway operation. Railway signalling applications are designed in such a way
that, in the event of a communication failure (e.g. due to message cancellation,
intentionally denial of service), they react safe-side (stopping railway train operation).
Requirements regarding the transmission of safety-related messages are defined within
EN50159 as efficient measure against repetition, insertion, re-sequencing and delay,
based on sequence number, timestamps and safety code.
These methods are of course implemented at the application or transport level, but are
not significant for the IP-based communication system itself. These procedures do not
prevent the attack itself, but ensure that the application reacts fail-safe by reacting to
the attack.
Furthermore, it is not possible to prevent an attacker from creating safety-related
messages with a correct timestamp, sequence number and safety code (requires access
to the specification of the signalling application). For this reason, EN 50159 also
requires the avoidance of masquerade through the use of cryptographic techniques to
ensure the senders authentication.
Although not directly part of the EN 50159 standard, confidentiality is known to be
important to reduce the knowledge of any attacker and thus avoid knowing how to
efficiently control his attack.
The communication system itself is not a safe system. As a result, the railway signalling
application (and other services with vital and mission critical data transmission) itself
must implement countermeasures for transmission failures. The requirements to be met
by IP-based communication systems in terms of security and safety can be summarised
as follows:
• Availability of the communication (e.g. redundancy, diversity),
• Stringent authentication of the communication modules,
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• Integrity of the transmitted messages,
• Confidentiality of the exchanged messages,
• Limited latency and jitter,
• Securing of the control plane.
However, the security of the transmitted information itself is handled at a higher
(application) level and does not fall within the IP-based communication system. The
security of the railway application that uses the communication network must be
guaranteed at higher levels.
6.3.4 Authentication concepts
To ensure that the right application is using the right priority and QoS profile, every
application must support an authentication procedure prior to the session
establishment.
The 3GPP 4G/5G network security features are implemented at the following
subsystems:
• SIM cards (UICC)
ICC performs the cryptographic operations for authentication
• Air Interface Protection (performing user-to-network security)
• IP Backhaul Protection
The 4G network entity authentication is as defined by TS 33.102. User data and
signalling data confidentiality is done according to the standard 3GPP TS 33.401. The
following security features related to entity authentication are provided within 4G:
• user authentication:
the property that the serving network corroborates the user identity of the user
• network authentication:
the property that the user corroborates that he is connected to a serving
network that is authorised by the user's HE to provide him services; this includes
the guarantee that this authorisation is recent.
To achieve these objectives, entity authentication occurs at each connection set-up
between the user and the network. Two mechanisms have been included: an
authentication mechanism using an authentication vector delivered by the user's HE to
the serving network, and a local authentication mechanism using the integrity key
established between the user and serving network during the previous execution of the
authentication and key establishment procedure.
According 3GPP TS 33.401 LTE authentication is mutual authentication performed by
and between a user and a network based on EPS Authentication and Key Agreement
(AKA) procedure. The mutual authentication is a major advantage compared to one-
sided (user-plane) authentication in the GSM network and the only solution to eliminate
existing vulnerabilities and threat vectors (see 3.6.4).
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6.4 Mapping of functional requirements
A summary and mapping of requirements from existing requirements specifications and
functionalities of new communication technologies takes place within this chapter.
Table 21 shows the mapping of the functional requirements created from the supported
GSM-R functionalities analysed at the beginning. For this purpose, the functionalities
offered by an LTE network equipped starting with Rel. 13 standard according to 3GPP
(see 5.5) were compared. With the introduction of mission-critical services, special
functions were introduced that will primarily cover the railway specific functionalities in
the network. The mapping of these special capabilities is finally applied. The table
provides a comparison of the differentiated functional implementations of different
recent 3GPP releases.
Table 21: Mapping of functional railway requirements between GSM-R and 4G/5G networks (TUD)
Function implementation
GSM-R 4G (Rel. 12) 5G (Rel. 15,16)
incl. MC capabilities
Mandatory Priority and Pre-
emption
(eMLPP)
Access Class Barring
mechanisms (ACB),
QoS mechanisms
(ARP).
Enhanced QoS
mechanisms (ARP)
Mandatory Fast Calls Set-up IMS based VoLTE +
Access Class Barring
MC Voice functionality
(MCPTT)
Mandatory Railway
Emergency Calls
(REC, e-REC)
Emergency and
critical safety voice
services (IMS)
MC Emergency
priority call,
MC Localisation
Services
Mandatory Voice Group Call
Service (VGCS)
IMS based VoLTE +
IMS based Push to
talk Over Cellular
(PoC) +
Enhanced Multimedia
Broadcast Multicast
Service (eMBMS)
MC Group call
functionality,
MC Broadcast calls
Mandatory Voice Broadcast
Calls (VBS)
VoLTE + eMBMS: IP
multicast of voice
services
MC Broadcast calls
Mandatory Functional
Addressing (FN)
Session Initiation
Protocol (SIP)
Addressing
Session Initiation
Protocol (SIP)
Addressing
Mandatory Location
Depending
Addressing
(LDA, eLDA)
Localisation Services
(3GPP Rel. 10)
MC Localisation
Services
(3GPP Rel. 15)
Optional Data Exchange
(SMS)
IMS based SMS
Service
IMS based SMS
Service
Optional Direct Mode N/A On-/Off-network
communication
functionality
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6.5 Conclusion on Technical viability
The analyses confirm that future transmission standards according to current 3GPP
definitions are capable meeting the high QoS requirements of railway applications.
4G LTE networks, as well as future 5G networks, offer different approaches and several
types of parameter classes which are used to implement traffic prioritisation and QoS
management. These methods need to be implemented according to 3GPP specifications.
Traffic prioritisation for mission critical users is a new domain for public communication
providers, which so far providing mostly non-prioritised services.
High QoS requirements for mission critical services cannot be met with the QoS
features offered by existing public 2G/3G networks. This highlights the need for
enhanced 4G or 5G network availability and coverage.
The aim of the study was to establish a specific QoS vector for railway applications. In
the first part of the document, all functional and technical requirements for railway
communication that are currently specified were analysed. These requirements were
compared and mapped with the functions provided by existing 4G and future 5G
networks.
Together with future innovative applications identified in MISTRAL, a requirement vector
has been specified for the communication technology to which a QoS vector can be
derived. In its analyses, MISTRAL introduces different implementation scenarios
regarding the design of future railway-specific communication networks. The NaaS
scenario seems to offer very promising technical and economically interesting solutions
for railways and mobile network provider.
Railway infrastructure managers may in future share the same communication system
infrastructure (i.e. with public commercial service operators or other mission-critical
users). In addition, different categories of applications of a railway company must share
the network infrastructure. The separation and prioritisation of the different
communication types is an essential prerequisite. The ability to provide diverse
applications with very differentiated authorisation regimes and performance values
within a network is an enormous advantage of 4G networks. However, only with the
implementation of Mission Critical service capabilities can a quality of service vector be
achieved that enables the complete replacement of the existing GSM-R network.
Railway applications that exchange critical information for railway operations require
secure radio access and transport resources. The 5G's end-to-end network capability for
network slices allows a design to achieve these stringent requirements. End-to-end
transport resources can be allocated and ensure the separation of communication
between railway infrastructure manager and railway undertakings.
For this reason, end-to-end network slicing, which includes 3GPP radio access (4G and
beyond) and core functions such as session, mobility and transport management
functions, must be supported by the future communications system. Moreover, end-to-
end network slicing offers scalability and flexibility to introduce new innovative
applications in the railway domain. It is necessary that the specific railway network slice
can be handled independently in terms of system functionality, priority, data rate
(guaranteed and maximum bit rate), latency and communication security.
6.5.1 Connection to economic analyses within MISTRAL
The project MISTRAL falls within the Shift2Rail Joint Undertaking Multi-Annual Action
Plan (MAAP) and has the goal to analyse a feasible and viable migration towards a new
management model for the railway communication system. To achieve this goal, all the
technical and economic issues have been analysed with the aim to create an adaptable
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train-to-wayside IP communication technology. MISTRAL focuses its analyses on the
possibility, to shift from the network as an asset (NaaA) scenario to a network as a
service (NaaS) paradigm. This approach can affect not only the costs, (e.g., reduce
CapEx and increase OpEx) but also the creation of new services and new markets that
can attract both new and old operators.
A quantitative analysis has been leveraged in MISTRAL D3.2 to generate the main
results of the deliverable. The aim was to identify a set of plausible and real values for
the data and to define a total cost of ownership function linked to all the technical and
economic consideration. The results of the D3.2 confirmed the considerations about
NaaA and NaaS with 4G*-MC deployment. In particular a promising solution would be in
deploying a new infrastructure that can use both 4G*-MC and 4G for the railway
communication network.
This conclusion also forms the specific link to deliverable D4.2. It was shown that only
with the introduced extensions of the 3GPP MC services an implementability of the
existing and future requirements for IP based railway communication networks becomes
possible. However, the implementation of comprehensive NaaS approaches becomes
particularly interesting for MNOs with the introduction of standardised network
virtualisation functions of the upcoming 5G networks. Here a technical separation of the
network functionalities and thus also of the transmission characteristics of different
applications and services can be realised more comprehensive way.
6.5.2 Connection with other projects
The Roll2Rail project [20] aims to develop key technologies for innovations in the field
of future rail vehicle technology. The results should serve to increase operational safety
and reduce life cycle costs.
Within this project, the selection and validation of a radio transmission technology, in
particular LTE technology for wireless train backbone network, was analysed. The
Roll2Rail project identified various requirements for the transmission system in order to
accomplish this task. The areas Train Control and Monitoring System (TCMS) and On-
board Multimedia and Telematics Subsystem/Services (OMTS) are covered. For these
both domains a set of requirements to be fulfilled by applications working in each
domain was specified. From the TCMS standard point of view, there exist five different
Data Class which are: Process, Message, Supervision, Best Effort and Streaming with
different demands on the transmission. These requirements applicable for the
communication technology are introduced in Table 22.
Domain Data
Class
Data size Data rate
need
Cycle
time
Latency
max max min max
TCMS Process 1432 Bit 10 Mbit/s 16ms 80ms
Message 65388 Bit 10 Mbit/s Not
relevant
250ms
Supervision Not
relevant
10 Mbit/s 50ms 250ms
Best Effort Not
relevant
10 Mbit/s N/A N/A
Streaming N/A N/A N/A N/A
OMTS Process 1432 Bit 10 Mbit/s 16ms 80ms
Message 65388 Bit 10 Mbit/s Not
relevant
250ms
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Supervision Not
relevant
10 Mbit/s 50ms 250ms
Best Effort 4 GByte 10 Mbit/s N/A N/A
Streaming Not
relevant
100 Kbit/s
(Audio),
8 Mbit/s
(Video)
– 0.1s
(Audio),
0.5s
(Video) Table 22: Identified requirements for communication technology from roll2rail {Ref [20]}
The identified parameters, especially regarding the latencies, are comparable to those
described and specified in the section 6.3. It can be recognized that with the utilisation
of modern 4G networks and the implemented powerful QoS mechanisms these
characteristic values can be also achieved and monitored. However, differences can be
seen in the expected data rate. While the FRMCS assumes the highest expected data
rate for railway applications with 5 Mbit/s (see Table 15), the values identified within the
Roll2Rail project are twice as high. At this point a suitability of 3GPP networks can also
be recommended for the Roll2Rail project with regard to the required QoS
characteristics.
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7 Conclusion
The project MISTRAL has the goal to analyse a feasible and viable migration towards a
new management model for the railway communication system. To achieve this goal, all
the technical and economic issues have been analysed with the aim to create an
adaptable train-to-wayside IP communication technology. This deliverable D4.2 focuses
on technical viability and aims to establish a specific QoS vector for IP-based
communication in railway transmission systems.
The convergence to an all IP based network will provide an efficient use of the network
capabilities. The QoS methods, guaranteed call setup times, proper service prioritisation
and IP security capabilities, offer advantages over the previous GSM-R system. The 4G
and 5G QoS mechanisms were identified. With the introduction of mission critical
communications services, the standardization bodies open the door for the railways to
make a transition to the new network generation.
Both functional aspects of railway communication and their technical requirements can
be offered and handled in modern 3GPP mobile radio networks.
Comprehensive QoS methods for the future communication networks meet the
requirements of stringent QoS parameters of railway functions. The definition of a QoS
vector for railway communication applications for voice and data applications according
to the recent 3GPP standardisations have been examined. In comparison, 4G MC-
enabled networks not only offer the possibility of a very differentiated prioritization of
service features, they also use improved cryptographic methods and authentication
mechanisms. This will in addition enable higher safety standards to be met in the
future.
With network slicing mechanisms within 5G networks an implementation of
virtualisation is introduced that allows multiple virtual networks to operate on a
common physical network infrastructure. The aim is to enable a physical mobile
operator to partition its resources so that very different users with specific applications
and requirements for the transmission network can use a single physical infrastructure.
With focus on the railway sector, the sharing of a certain physical network, in order to
simultaneously allow application of Internet of Things (IoT) for maintenance purposes,
mobile broadband applications for passengers (MBB) and critical applications with high
QoS requirements (e.g. automatic train protection), is interesting. These applications
obviously have very different transmission characteristics.
The implementation of mission critical features, the flexible adaptation of QoS features
within the future network generation and the introduction of network slicing will change
communication in the railway sector. Most of all, these prerequisites will enable NaaS
application scenarios that have been presented in MISTRAL.
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8 List of tables, figures, and references
8.1 Tables
Table 1: Priority regimes within GSM-R voice connections {Ref UNISIG} ...................................................... 18 Table 2: Requirements on voice call setup times {Ref [9]} .............................................................................. 24 Table 3: Functional requirements of railway communication system (Extracted from EIRENE FRS) ............ 31 Table 4: Summary of QoS requirements {Ref [2]} ........................................................................................... 33 Table 5: Selected parameter of ETCS QoS Profile (PS-Mode) {Ref UNISIG} ................................................ 37 Table 6: QoS requirements on railway communication systems (Extracted from EIRENE FRS) ................... 40 Table 7: Pre-defined Quality class indicators in LTE ....................................................................................... 49 Table 8: Quality class indicators for Mission Critical service (3GPP) {REF [11]} ............................................ 50 Table 9: VoLTE capabilities overview .............................................................................................................. 55 Table 10: 3GPP Mission Critical services and Enhancements in Release 13 to 15 (3GPP) .......................... 59 Table 11: QoS categorisation for different application types (3GPP) {Ref [10]}.............................................. 60 Table 12: Quantitative Mapping of service and QoS attributes (3GPP) {Ref [10]} .......................................... 61 Table 13: Overview of Critical Support Applications (FRMCS) {Ref [10]} ....................................................... 63 Table 14: QoS of different selected FRMCS application types (FRMCS) {Ref [10]} ....................................... 63 Table 15: Quantitative Mapping of additional service attributes {Ref [10]} ..................................................... 64 Table 16: MISTRAL innovative services and QoS attributes mappings .......................................................... 68 Table 17: Minimum requirements on railway network performance ................................................................ 69 Table 18: Derived QoS vector for Data transmission ...................................................................................... 69 Table 19: Existing QoS recommendation for voice transmissions .................................................................. 72 Table 20: Derived QoS vector for Audio and Video transmission ................................................................... 72 Table 21: Mapping of functional railway requirements between GSM-R and 4G/5G networks (TUD) ........... 75 Table 22: Identified requirements for communication technology from roll2rail {Ref [20]} .............................. 78 Table 23: Minimum performance requirements of railway communication systems {Ref [13]} ....................... 82 Table 24: Minimum functional requirements for LTE based railway communication system {Ref [14]}.......... 84 Table 25: FRMCS – Service and Service Attributes requirements (qualitative) {Ref [10]} ............................. 85 Table 26: FRMCS – Service attribute mapping (quantitative) {Ref [10]} ......................................................... 85 Table 27: Standardised QCI characteristics (including MC services) {Ref [11]} ............................................. 87 Table 28: QoS Metrics for Voice over IP (quantitative) {Ref: [15]} .................................................................. 88
8.2 Figures
Figure 1: GSM-R service architecture {Ref [16]} ............................................................................................. 17 Figure 2: Reference architecture of train-side/track-side components of ERTMS/ETCS {Ref [6]} ................. 21 Figure 3: Communication protocol stack architecture (TUD) .......................................................................... 26 Figure 4: SDF and EPS Bearers {Ref [22]} ..................................................................................................... 48 Figure 5: 4G/5G Bearer QoS concept (TUD) .................................................................................................. 49 Figure 6: EPS security Architecture {Ref [21]}................................................................................................. 52 Figure 7: EPS signalling plane protection {Ref [21]} ....................................................................................... 53 Figure 8: EPS user plane protection {Ref [21]} ............................................................................................... 53 Figure 9: IMS architecture within LTE Evolved Packet Core (Spirent) ........................................................... 56 Figure 10: IMS core elements (Spirent) ........................................................................................................... 57
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8.3 References
[1] UNISIG SUBSET 88: “ETCS Application Levels 1 & 2 - Safety Analysis”, Parts 0-3
[2] UNISIG SUBSET 93: “GSM-R Interfaces Class 1 Requirements”, V2.3.0, 2005-10
[3] CENELEC - EN 50129: “Railway application – communication, signalling and
processing systems – safety related electronic systems for signalling”, CENELEC
2002
[4] ERTMS User Group: “ETCS/GSM-R quality of service – Operational analysis”,
2005
[5] ETSI TS 103 328: “Railway Telecommunications (RT); GPRS/EGPRS
requirements for ETCS”, V1.1.1, 2015-12
[6] P.Stanley: “ETCS for Engineers”, 1st Edition, Eurail Press 2011
[7] Aleksander Sniady, José Soler, Mohamed Kassab, Marion Berbineau: “Ensuring
Long-Term Data Integrity”, IEEE Vehicular Technology, pp. 60-70; 2016
[8] Shift2Rail: “MISTRAL: Communication System for Next-generation Railways”,
Project reference 730840,
[9] “System requirement specification” UIC Project EIRENE, UIC CODE 951, Ver.
15.4; 2014
[10] 3GPP TR 22.889: “Study on Future Railway Mobile Communication System”;
V16.2.0, 2018-03
[11] 3GPP TS 22.203: “Technical Specification Group Services and System Aspects;
Policy and charging control architecture (Release 15)”; V15.2.0, 2018-03
[12] 3GPP TS 33.102: “Technical Specification Group (TSG) SA; 3G Security, Security
Architecture, 1999-04
[13] TTA TTAK.KO-06.0370, “User Requirements for LTE-Based Railway
Communication System”, 2014-10
[14] TTA TTAK KO-06.0-369, “Functional Requirements for LTE-Based Communication
System”, 2014-10
[15] IOS Press, “QoS Requirements of Network Applications on the Internet”, 2004
[16] Kapsch, “White Paper: European Railway Traffic Management System – ERTMS”,
2012
[17] FRANEKOVÁ, et.al.: “Safety analysis of cryptography mechanisms used in GSM
for railway”, 2011
[18] Chotia, Ordean, Thomas, Ruiter: “An Attack Against Message Authentication in
the ERTMS Train to Trackside Communication Protocols”, 2017-02
[19] EC, “Commission Decision on the technical specification for interoperability
relating to the control-command and signalling subsystems”, 2012-01
[20] Roll2Rail Project, Internet URL: http://www.roll2rail.eu, 2017-10
[21] Forsberg, Horn, Meller, Niemi: “LTE Security”, WILEY, 2012
[22] Netmanias: “LTE Technical Documents”, Internet URL:
http://www.netmanias.com, 2018-03
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9 ANNEX
Items Description Remarks
Coverage
and performance
Coverage shall be continuous from a time and space
perspective, and the temporal and spatial range to
guarantee stability shall be more than 98% based on
the vehicle being equipped with an external antenna.
The network shall be able to accommodate the mobile
terminal for railway communication.
The system shall be able to provide communication
when moving at track speed limit or 500 km/h,
whichever is lower.
Call setting time Railway emergency call <1s (90%), <2s(99% or
more)
Broadcasting or group call <1s (90%), <2.5s(99% or
more)
All voice/video calls that do not correspond to the
above
< 3.5s(90%), < 5s(99% or more)
*
External PSTN
connection not
considered
Handover
success
The network shall be able to have seamless data
transmission, and the handover success rate shall
be 99% or more.
Call access success The call access success rate shall be 99% or more.
Connection drop rate The system shall be able to guarantee a call
disconnection rate less than 0.01 times per hour
during a lengthy call
Train control data
transmission
The network shall guarantee more than 99% data
reliability to transmit data for train control.
Train control data shall have top priority.
Network redundancy The network including eNB equipment, core equipment
and server shall be designed to be redundant for
stability and availability.
However, the
application scope
of redundancy is
determined by
the operator.
Broadcasting and group
call area
Radio equipment in a restricted area can participate in
broadcasting and group call, and the radio equipment
out of broadcasting and group call area during call
shall be excluded from call.
Table 23: Minimum performance requirements of railway communication systems {Ref [13]}
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Service Detailed
service Description
Mandatory/Op
tional
(M/O) Voice
service
Individual
voice call
The system shall support voice calling between
two callers.
M
Public
emergency
call
The system shall allow the user to make a
public emergency call.
M
Broadcasting
voice call
The system shall support a broadcasting call. M
Group voice
call
The system shall support a group call. M
Multi-party
voice call
The system shall support a multi-party call
between at least 3 different parties.
M
Data
service
Multimedia
message
service
The network shall support point-to-point and
point-to-multi-point message transmission from
the ground to mobile radio equipment users.
M
General data
service
The system shall support broadband data
communication between ground and mobile
radio equipment users.
M
Train control
service
The system shall support seamless data
communication for stable train control.
M
Video
service
Individual
video call
The system shall support video calling between
two callers.
M
Group video
call
The system shall support group video calling. M
Video
information
transmission
The system shall support the video information
transmission function related to safe train
operation.
M
Call
related
service
Receiver
/Caller
ID display
The equipment shall display the receiver or
caller ID in the form of a standard telephone
number.
M
Receiver
/caller ID
display
restriction
The system shall allow the ID of a specific user
to be prevented from being displayed on the
mobile radio equipment.
O
Priority and
pre-emptive
right
A function in which a call is allocated to the
member who has top priority among the
members who have different priority levels
shall be provided.
M
Closed user
group
The user group who can access the Korean
railway integrated radio network from outside
shall be restricted.
M
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Service Detailed
service Description
Mandatory/Op
tional
(M/O) Call transfer The incoming call or data message for one user
shall be transferred to other devices in the
network.
M
Call holding The network shall allow the user to hold a call
temporarily from an existing call.
M
Call waiting The network shall be able to notify the user of
the existing call that another user is attempting
to access.
M
Charging
information
When there is a network service charge, the
network shall be able to provide the information
on call charge and ongoing call charges.
O
Call
restriction
The system shall be able to restrict a call using
the network management or maintenance
facility.
M
Automatic
answering
service
A call shall be answered automatically
according to the priority of an incoming call.
M
All voice/
Video call
recording
A call shall be answered automatically
according to the priority of an incoming call.
M
Railway
specializ
ed
service
Functional
addressing
The system shall provide the addressing system
in which the Controller can set communication
with the train driver using train number
M
Location-
dependent
addressing
The system shall provide the location-
dependent addressing system in order to
identify the destination number, which varies
depending on the location of users
M
Railway
emergency
call
The network shall provide the system to handle
a voice call with high priority for a railway
emergency call
M
Shunting
mode
The network shall provide the system to
regulate and control the user’s access to the
function and features of mobile radio
equipment being used for shunting mode
communication
M
Direct
communicati
on
The system shall support direct communication
between terminals in the event that an LTE-
based railway communication service is not
normally available due to a failure in eNB.
O
Table 24: Minimum functional requirements for LTE based railway communication system {Ref [14]}
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Application
Category
Service Attribute
(according to Table
12.10-1)
Session
establishment
Session Loss
Rate
Latency
peer to
peer
Packet
Loss Rate
Voice Low
Normal
Normal
NA
Critical Voice Low
Low
Immediate
NA
Video Normal Low Normal NA
Critical Video Low Low Immediate NA
Very Critical
Data
(Note 1)
Ultra-Low Ultra- Low Immediate NA
Critical Data
(future
applications)
Low Ultra- Low Immediate NA
Critical Data
(legacy
applications)
Normal Low Normal <10-2/h
Non-Critical
data
Normal Low Normal NA
Messaging Best Effort Low Normal NA
Note 1: The applicable vehicle speed to be considered is in the range of 0-10
kmh-1.
Table 25: FRMCS – Service and Service Attributes requirements (qualitative) {Ref [10]}
Service Attribute FRMCS - Functional
Requirement
FRMCS – System
Requirement
Service
Attribute value
Latency Low Ultra-Low ≤10ms
Low ≤100ms
Normal Normal ≤500ms
Best Effort >500ms
Transport reliability -
Packet Loss (%)
High Ultra-Low 99.9999%
Low 99.9%
Normal Normal 99% Table 26: FRMCS – Service attribute mapping (quantitative) {Ref [10]}
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QCI Resource Type
Priority Level
Packet Delay
Budget
Packet Error Loss
Rate
Example Services
1 (NOTE 3)
2 100 ms (NOTE 1, NOTE 11)
10-2 Conversational Voice
2 (NOTE 3)
GBR
4 150 ms (NOTE 1, NOTE 11)
10-3 Conversational Video (Live Streaming)
3 (NOTE 3, NOTE 14)
3 50 ms (NOTE 1, NOTE 11)
10-3 Real Time Gaming, V2X messages Electricity distribution - medium voltage (e.g. TS 22.261 [51] clause 7.2.2) Process automation - monitoring (e.g. TS 22.261 [51] clause 7.2.2)
4 (NOTE 3)
5 300 ms (NOTE 1, NOTE 11)
10-6 Non-Conversational Video (Buffered Streaming)
65 (NOTE 3, NOTE 9,
NOTE 12)
0.7 75 ms (NOTE 7, NOTE 8)
10-2
Mission Critical user plane Push To Talk voice (e.g., MCPTT)
66 (NOTE 3, NOTE 12)
2
100 ms (NOTE 1, NOTE 10)
10-2
Non-Mission-Critical user plane Push To Talk voice
67 (NOTE 3, NOTE 12)
1.5
100 ms (NOTE 1, NOTE 10)
10-3
Mission Critical Video user plane
75 (NOTE 14)
2.5 50 ms (NOTE 1)
10-2 V2X messages
5 (NOTE 3)
1 100 ms (NOTE 1, NOTE 10)
10-6 IMS Signalling
6 (NOTE 4)
6
300 ms
(NOTE 1, NOTE 10)
10-6
Video (Buffered Streaming) TCP-based (e.g., www, e-mail, chat, ftp, p2p file sharing, progressive video, etc.)
7 (NOTE 3)
Non-GBR 7
100 ms
(NOTE 1, NOTE 10)
10-3
Voice, Video (Live Streaming) Interactive Gaming
8 (NOTE 5)
8
300 ms
(NOTE 1)
10-6
Video (Buffered Streaming) TCP-based (e.g., www, e-mail, chat, ftp, p2p file
9 (NOTE 6)
9 sharing, progressive video, etc.)
69 (NOTE 3, NOTE 9,
NOTE 12)
0.5 60 ms (NOTE 7, NOTE 8)
10-6 Mission Critical delay sensitive signalling (e.g., MC-PTT signalling, MC Video signalling)
70 (NOTE 4, NOTE 12)
5.5 200 ms (NOTE 7, NOTE 10)
10-6 Mission Critical Data (e.g. example services are the same as QCI 6/8/9)
79 (NOTE 14)
6.5 50 ms (NOTE 1, NOTE 10)
10-2 V2X messages
80 (NOTE 3)
6.8 10 ms (NOTE 10, NOTE 15)
10-6 Low latency eMBB applications (TCP/UDP-based); Augmented Reality
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NOTE 1: A delay of 20 ms for the delay between a PCEF and a radio base station should be subtracted from a given PDB to derive the packet delay budget that applies to the radio interface. This delay is the average between the case where the PCEF is located "close" to the radio base station (roughly 10 ms) and the case where the PCEF is located "far" from the radio base station, e.g. in case of roaming with home routed traffic (the one-way packet delay between Europe and the US west coast is roughly 50 ms). The average takes into account that roaming is a less typical scenario. It is expected that subtracting this average delay of 20 ms from a given PDB will lead to desired end-to-end performance in most typical cases. Also, note that the PDB defines an upper bound. Actual packet delays - in particular for GBR traffic - should typically be lower than the PDB specified for a QCI as long as the UE has sufficient radio channel quality.
NOTE 2: The rate of non-congestion related packet losses that may occur between a radio base station and a PCEF should be regarded to be negligible. A PELR value specified for a standardized QCI therefore applies completely to the radio interface between a UE and radio base station.
NOTE 3: This QCI is typically associated with an operator controlled service, i.e., a service where the SDF aggregate's uplink / downlink packet filters are known at the point in time when the SDF aggregate is authorized. In case of E-UTRAN this is the point in time when a corresponding dedicated EPS bearer is established / modified.
NOTE 4: If the network supports Multimedia Priority Services (MPS) then this QCI could be used for the prioritisation of non-real-time data (i.e. most typically TCP-based services/applications) of MPS subscribers.
NOTE 5: This QCI could be used for a dedicated "premium bearer" (e.g. associated with premium content) for any subscriber / subscriber group. Also in this case, the SDF aggregate's uplink / downlink packet filters are known at the point in time when the SDF aggregate is authorized. Alternatively, this QCI could be used for the default bearer of a UE/PDN for "premium subscribers".
NOTE 6: This QCI is typically used for the default bearer of a UE/PDN for non-privileged subscribers. Note that AMBR can be used as a "tool" to provide subscriber differentiation between subscriber groups connected to the same PDN with the same QCI on the default bearer.
NOTE 7: For Mission Critical services, it may be assumed that the PCEF is located "close" to the radio base station (roughly 10 ms) and is not normally used in a long distance, home routed roaming situation. Hence delay of 10 ms for the delay between a PCEF and a radio base station should be subtracted from this PDB to derive the packet delay budget that applies to the radio interface.
NOTE 8: In both RRC Idle and RRC Connected mode, the PDB requirement for these QCIs can be relaxed (but not to a value greater than 320 ms) for the first packet(s) in a downlink data or signalling burst in order to permit reasonable battery saving (DRX) techniques.
NOTE 9: It is expected that QCI-65 and QCI-69 are used together to provide Mission Critical Push to Talk service (e.g., QCI-5 is not used for signalling for the bearer that utilizes QCI-65 as user plane bearer). It is expected that the amount of traffic per UE will be similar or less compared to the IMS signalling.
NOTE 10: In both RRC Idle and RRC Connected mode, the PDB requirement for these QCIs can be relaxed for the first packet(s) in a downlink data or signalling burst in order to permit battery saving (DRX) techniques.
NOTE 11: In RRC Idle mode, the PDB requirement for these QCIs can be relaxed for the first packet(s) in a downlink data or signalling burst in order to permit battery saving (DRX) techniques.
NOTE 12: This QCI value can only be assigned upon request from the network side. The UE and any application running on the UE is not allowed to request this QCI value.
NOTE 13: Packet delay budget is not applicable on NB-IoT or when Enhanced Coverage is used for WB-E-UTRAN (see TS 36.300 [19]).
NOTE 14: This QCI could be used for transmission of V2X messages as defined in TS 23.285 [48]. NOTE 15: A delay of 2 ms for the delay between a PCEF and a radio base station should be subtracted from the given
PDB to derive the packet delay budget that applies to the radio interface.
Table 27: Standardised QCI characteristics (including MC services) {Ref [11]}
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Table 28: QoS Metrics for Voice over IP (quantitative) {Ref: [15]}