eps-introduction and architecture.pdf

Study Program

Master Telecommunications and Internet Technologies

Course

Application Prototyping

LECTURE NOTE

Version: 1.1

Datum: 11. 06. 2010

EVOLVED PACKET

SYSTEM (EPS) Introduction and Architecture

DI Franz Edler

EPS: Introduction and Architecture

Author: DI Franz Edler page: 2 / 42

CONTENTS:

1. Overview ...................................................................................................................................... 4

1.1. Content of the course ............................................................................................................ 4 1.2. Structure of the course .......................................................................................................... 4

1.3. Preconditions and further readings and exercises ................................................................ 4

1.4. Questions and exercises ........................................................................................................ 4

1.5. Target audience ..................................................................................................................... 4

2. Introduction to EPS ...................................................................................................................... 5 2.1. Principle architecture of EPS ................................................................................................ 5

2.2. Mobile network evolution..................................................................................................... 5 2.2.1. Motivation for EPS ........................................................................................................ 6

2.2.2. Building Blocks of EPS ................................................................................................. 6

2.2.3. Evolution of Data Rates ................................................................................................. 7

2.2.4. EPS overview ................................................................................................................. 8 2.2.5. Voice services in EPS .................................................................................................. 10

2.2.6. Seamless Mobility and Convergence with all-IP ........................................................ 11

3. Evolved System Architecture .................................................................................................... 13 3.1. 3G Architecture................................................................................................................... 13

3.2. EPS Architecture................................................................................................................. 14

3.2.1. Requirements ............................................................................................................... 14

3.2.2. Features ........................................................................................................................ 15 3.2.3. Architecture overview ................................................................................................. 15

3.2.4. Mobility Management Entity (MME) ......................................................................... 16

3.2.5. Serving Gateway (S-GW) and PDN-Gateway (P-GW) .............................................. 16

3.2.6. Home Subscriber Server (HSS) ................................................................................... 17 3.2.7. Evolved Packet Data Gateway (ePDG) ....................................................................... 17

3.2.8. Policy Charging Rule Function (PCRF) ...................................................................... 18

3.2.9. eUTRAN ...................................................................................................................... 18

3.2.10. Tracking concept ....................................................................................................... 19 3.2.11. Redundancy and distributed architecture .................................................................. 20

3.3. EPS Interfaces and Protocols .............................................................................................. 21

4. Registration ................................................................................................................................ 23 4.1. Authentication and Security ............................................................................................... 23

4.1.1. Life of a Mobile ........................................................................................................... 23

4.1.2. High Level View of network attachment .................................................................... 23

4.1.3. Initial Attach Request .................................................................................................. 24 4.1.4. Authentication .............................................................................................................. 25

4.1.5. Security ........................................................................................................................ 26

4.1.6. Subscription Information ............................................................................................. 28

4.2. Default Bearer Setup ........................................................................................................... 28

5. Service Data Flows and QoS ..................................................................................................... 31

5.1. Introduction to Service Data Flow and EPS Bearer ........................................................... 31 5.1.1. High Level View .......................................................................................................... 31

5.1.2. Services and Service Data Flow .................................................................................. 32

5.1.3. Service Data Flow and EPS Bearers ........................................................................... 32



5.1.4. EPS Bearer ................................................................................................................... 33

5.1.5. Downstream and Upstream Packet Classification ....................................................... 33

5.2. QoS ..................................................................................................................................... 34

5.2.1. Bearer classes ............................................................................................................... 34 5.2.2. QCI classes .................................................................................................................. 35

5.2.3. Differentiated Service .................................................................................................. 36

5.3. Service Addition and Dedicated Bearer Setup ................................................................... 37

5.3.1. Service Addition Example ........................................................................................... 37

5.3.2. PCRF links SDF and EPS Bearers .............................................................................. 38 5.3.3. Dedicated Bearer Creation ........................................................................................... 38

5.4. PMIPv6-based EPS Bearer ................................................................................................. 40

6. Exercises and Questions ............................................................................................................ 41

7. References .................................................................................................................................. 42 7.1. Books .................................................................................................................................. 42



1. OVERVIEW

1.1. CONTENT OF THE COURSE

… to be added …

1.2. STRUCTURE OF THE COURSE

… to be added …

1.3. PRECONDITIONS AND FURTHER READINGS AND EXERCISES

… to be added …

1.4. QUESTIONS AND EXERCISES

At the end of each part the student can find some questions which should help to get feedback on

the core points of the course. The student should be able to answer the questions and exercises at

the end of the course.

1.5. TARGET AUDIENCE

The target audience of this course are students on bachelor degree in the upper classes on

telecommunications systems and students for the master degree of “Telecommunications und

Internet-technology”.



2. INTRODUCTION TO EPS

2.1. PRINCIPLE ARCHITECTURE OF EPS

Figure 1 shows the principle architecture of the Evolved Packet System (EPS) without going into

details. It should help to understand the various terms used in standardisation and their history.

EUTRAN EPC

EPS

+

Figure 1: Principle architecture of EPS

End of 2004 3GPP started to work on a new mobile network system. The first studies were

carried by two working groups, one dealing with evolution of the radio access network (LTE,

Long Term Evolution) and another working focussed on the evolution of the core network (SAE,

System Architecture Evolution). Both groups worked in a coordinated manner and therefore the

term LTA/SAE was used to describe this initiative.

The result of both groups was:

a new radio network technology and architecture called EUTRAN (Evolved UMTS

Terrestrial Radio Access Network), and

a new core network architecture called EPC (Evolved Packet Core).

Both components together (EUTRAN and EPC) comprise the EPS (Evolved Packet System).

These are the official terms, but LTE is still used for the radio network.

2.2. MOBILE NETWORK EVOLUTION

We can observe the phenomenon of an ongoing evolution process of wireless networks. With the inclusion of IP connectivity in the service portfolio of mobile networks an ever increasing need

for higher data rates and capacity has been created.

The existing 3G networks cannot fulfill this demand because there are three limiting factors of

3G networks:

Data rates: Current 3G networks offer theoretical data rates of 14 Mbps in the downlink

and 5.7 Mbps in the uplink using a 5 MHz wide channel. However, these data rates are not practical data rates for several reasons. One of the key reasons is that the 5 MHz

channel is also used to offer voice services, so the resources are divided between voice

and data users. The new radio technology of LTE will enable data rates of 50 to 100

Mbps in practice and therefore a much better end-user experience.

Delay: The latency or delay in current 3G networks is in the range of 50 to 100 ms. This results in end-to-end delays of over 200-300 ms. To offer a rich multimedia experience,



we need to reduce the delays in radio networks to below 10 ms. This results in a better

user experience for real-time multimedia and video telephony/conferencing applications.

Architecture: The current 3G network continues to utilize the circuit and packet data

networks. They were designed to handle voice services in a circuit fashion and medium

rate data services. This limits the provision of feature-rich multimedia services.

2.2.1. MOTIVATION FOR EPS

The main aspects which drive the evolution of existing 3G networks towards 4G are:

Rapid growing of IP traffic

High performance with reduced cost

Seamless mobility

Enhancements like High Speed Downlink Packet Access (HSDPA) and High Speed Uplink

Packet Access (HSUPA) make 3GPP access technology competitive today. To ensure the

competitiveness of 3GPP systems for the next ten years and beyond, LTE offers the long term

evolution of 3GPP access technology.

To meet the needs of the rapidly growing IP data traffic over the air, to reduce the cost per bit

and enable in parallel high performance a complicated network architecture and unnecessary

interfaces have to be avoided.

Reduction in the cost can be achieved by simplifying the wireless network, utilizing unified

protocols of IP, and reducing the number of network nodes.

2.2.2. BUILDING BLOCKS OF EPS

The enhancements offered by the EPS address all three main areas of a mobile network

Air Interface

Radio Network Architecture

Core Network

These enhancements are shown in Figure 2 below and summarized as follows:

Use of Orthogonal Frequency Division Multiplexing (OFDM) and advanced multiple

antenna techniques over the air interface: These techniques significantly increase the spectral

efficiency. The air interface allows many users to experience data rates in excess of 1 Mbps.

The peak supportable data rates are more than 100 Mbps (as high as 260 Mbps) in a 20 MHz

spectrum.

Distributed (as opposed to centralized) architecture: Distributed radio network architecture

allows reduced latency since most dynamic decisions are made “locally”. An IP-based radio

network results in an easy-to-scale network and provides scalability and reduction in the cost.

IP-based radio and core network: This allows introduction of new services easily via the IP

Multimedia Subsystem (IMS). However, the use of IMS is not mandatory and an operator

can offer numerous services even without IMS.



OFDMAOFDMA

Multiple

Antenna

Techniques

Very high

spectral

efficiency

Air Interface

OFDMADistributed

IP based

Reduced

Latency,

Scalability

Radio Network

Architecture

OFDMAIMS

IP based

Scalability,

Cost efficiency,

Services

Core Network

Evolved

Packet

Systen

Figure 2: Areas of enhancements within the EPS

2.2.3. EVOLUTION OF DATA RATES

Figure 3 shows the evolution of data rates in mobile networks during the last 20 years.

There is also a difference between the maximum achievable data rates (theoretical value) and the

practical advertised data rates.

1990s 2000 2003 2006 2007 2008+

Maximum 56k 473k 14M DL 5,7M UL 42M / 11,4M

Advertised 14k 60 – 80k 400 -700 k tbd tbd

GSMGPRS/

EDGE

UMTS/

HSDPA

UMTS/

HSUPA

UMTS/

HSPA+

Maximum 115k 614k 2,45M DL 3M / 1,8M

Advertised 14k 60 – 80k 400 -700 k 400k – 800k

IS-95 1xEV-DO

Rev 0

EV-DO

Rev A

Figure 3: Evolution of data rates in mobile networks

The figure shows the evolution of data rates from 2G networks (GSM, GPRD/EDGE) to 3G

networks (UMTS). HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed

Uplink Packet Access) are UMTS enhancements that enable more optimal use of bandwidth and



much higher data rates. Further enhancement of HSPA with Multiple Input and Multiple Output

techniques and a higher order modulation scheme resulted in HSPA+.

It also shows the CDMA2000 technology which is an evolution of the IS-95 system.

CDMA2000 1x and 1xEV-DO are two different options for 3G technologies. CDMA2000 is not

well known in Europe, as it used mainly in American, African and Asian countries Mobile

WiMAX is the competing 4G technology with LTE.1

An important fact is that the EPS (LTE+EPC) has been defined as an evolution path for both 1xEV-DO and UMTS/HSPA networks. Also mobile WiMAX (not shown here) may be

connected to the EPC, but it is handled as a non 3GPP based network.

It should be mentioned that the ITU has defined requirements for an advanced radio technology

network called “IMT-Advanced system”2 which will be the 4th generation (4G). The LTE

technology does not fulfill all the requirements of this definition (e.g. peak data rates up to 1 Gbps) is therefore not a 4G technology, but it is sometimes called as a pre-4G technology.

3GPP is already working on an “LTE Advanced” specification which will be available in

2010/2011 and offered as a candidate for 4G.

2.2.4. EPS OVERVIEW

The EPS (Evolved Packet System) is an end to end enhancement to the existing UMTS

architecture. EPS represents a migration from the traditional hierarchical system architecture to a flat architecture that minimizes the number of hops and distributes the processing load across the

network. Some more details can be seen in Figure 4.

eNodeB

EUTRAN EPC

EPS

EUTRAN- eNodeB

- reduced number of nodes

- OFDMA

- SC-FDMA

- adaptive modulation

(up to 64 QAM)

EPC- all IP

- interworking with 3GPP

and non 3GPP networks

- More efficient QoS and

security

MME

P-GWS-GW

HSS

Figure 4: EPS overview

The evolved packet system (EPS) is made up of the evolved packet core (EPC) and evolved

UTRAN (EUTRAN). The EUTRAN has an evolved Node B (eNB) and a new air interface based

on Orthogonal Frequency Division Multiple Access (OFDMA) in the downlink and Single

Carrier Frequency Division Multiple Access (SC-FDMA) in the uplink.

The EUTRAN air interface is a new packet-only wideband radio with flat architecture. The EPC

is a packet switched data solution which supports interworking across different 3GPP and non-3GPP access technologies. The number of nodes and the interfaces are reduced in the LTE

architecture to cut down on the latency, the cost and the complexity.

1 See: http://en.wikipedia.org/wiki/List_of_CDMA2000_networks 2 IMT = International Mobile Telecommunications Advanced

http://en.wikipedia.org/wiki/List_of_CDMA2000_networks



The reduction in complexity can be seen by comparing the in Figure 5 with the EPS architecture

in Figure 6.

GSM radio network

(GERAN)

UMTS radio network

(UTRAN)

BSC

RNC

MSC/

VLR

SGSN

A

Gb

Iu-PS

Iu-C

S

GGSN

External

Voice

Network

External

Data

Networks

BTS

NodeB

GMSC

BTS Base Transceiver Station

MSC Mobile Switching Center

GMSC Gateway MSC

RNC Radio Network Controller

SGSN Serving GPRS Support Node

GGSN Gateway GPRS Support Node

GERAN GSM EDGE Radio Access Network

UTRAN UMTS Terrestrial Radio Access Network

CS-CN

PS-CN

HLR

HLR

Figure 5: 2G/3G network architecture

EPC

S1-U

S6a

S11

S5/S8

External

Data

Networks

(PDNs)

SGi

eNodeB

LTE

S-GW

MMEHSS

P-GW

Figure 6: EPS network architecture



The two main differences between the packet network part of 2G/3G and the EPS network

architecture are:

Flat network architecture in EPS:

No central controller for eNodeB: the functions of the RNC are distributed between eNodeB,

MME and S-GW

Clear separation between signaling and data:

The data connections go directly to the S-GW and signaling (for mobility and radio session)

is handled by MME. This allows a better scaling of the network equipment. If data rate

requirements are increasing due to e.g. the introduction of a new service like mobile TV then only the capacity of the S-GW and P-GW have to be prepared accordingly.

2.2.5. VOICE SERVICES IN EPS

The EPS architecture does not have a separate circuit-switched network (CS) as in 2G/3G

networks. All services are packet-switched (PS) and therefore also voice calls have to be handled

natively by VoIP and IMS. This is one of the key changes due to the evolution towards an all-IP

based core network.

In EPS both voice and data services are provided by one consolidated network. Operators will be able to provide voice and data services with a single unified core network. As a result, separation

between the circuit switched and packet switched networks will disappear.

A network operator who does not already have an IMS based voice service in place is required to

introduce IMS in parallel to the introduction of LTE. At least this is the proposed strategy of

GSMA3. GSMA supports an industry initiative called VoLTE (Voice over LTE4) and creates a

profile for minimum IMS functions to be supported by an operator to guarantee interoperability

and roaming.

But how is IMS included in the EPS architecture? Looking to Figure 6 the IMS network is one of

the PDNs (Packet Data Networks) where an operator offers access to. That means that IMS

signaling and media is transparently carried throughout LTE and EPC networks and handed over

to an IMS network as one of the possible PDNs. This is depicted in Figure 7 below.

EUTRAN EPC IMS

IMS signalling and session data

Figure 7: EPS and IMS as a dedicated Packet Data Network (PDN)

The VoLTE initiative was a clear message against another initiative, which proposes to continue re-using the CS-infrastructure of 2G/3G by tunneling TDM oriented voice-traffic via PS access

network (VoLGA5). Despite of some hype at start of the VoLGA initiative it meanwhile seems

to be clear that the VoLGA initiative will not be supported by most of the operators. The main

3 GSMA = GSM Association ; http://www.gsmworld.com/ 4 VoLTE initiative: http://www.gsmworld.com/our-work/mobile_broadband/VoLTE.htm 5 VoLGA initiative: http://www.volga-forum.com/

http://www.gsmworld.com/

http://www.gsmworld.com/our-work/mobile_broadband/VoLTE.htm

http://www.volga-forum.com/



issue of separate solutions is that due to the handset market and the requirement of roaming a

market fragmentation (VoLTE and VoLGA) seems to be economically infeasible,

But even when the strategy is clear and the industry is moving towards an IMS based service

infrastructure the complexity of seamless handover between 2G/3G CS-based voice services and

IMS based voice service in EPS must not be underestimated. 3GPP has therefore decoupled the introduction of LTE/EPC from the introduction of IMS by defining a so called CS-Fallback

solution. All the details of the integration of voice-services and inter-RAT6 technology handover

will be covered by another part of the lecture.

2.2.6. SEAMLESS MOBILITY AND CONVERGENCE WITH ALL-IP

Before the details of the EPC architecture are explained the advantages of the EPS architecture

based on an all-IP network should be highlighted again.

For operators, these integrated networks offer reduced operating costs. They also enable

operators to offer integrated multimedia services combining voice and data services. As every communication network moves to IP-based networks, seamless mobility can be achieved by

moving to All-IP-based networks.

During the introduction the 3GPP based access networks (GERAN/UTRAN/EUTRAN) have

been mentioned only. But in next chapter the inclusion of non 3GPP networks will also be

explained. As an overall picture the target network infrastructure offered by EPS looks as

presented in Figure 8 .

Services

(e.g. IMS)

All-IP Core (EPC)

WLAN

Hot-Spots

UTRAN

GERANEUTRAN WiMAX

Figure 8: Seamless Mobility with all-IP network

For instance, the user should be able to move from a Wireless LAN connection (voice and data)

to a cellular network connection while leaving for work. Once the user enters the office, the

connection should seamlessly be transferred from a cellular network to an enterprise network.

The idea has been around since 2G networks were introduced. However with LTE it is close to getting realized. IMS enables using the same service with any access technology. LTE

interworking enables handovers across different access technologies. These two together make

seamless mobility no more a dream but a reality.

6 Inter-RAT = Inter Radio Access technology; this means change of radio technology during operation



Another is the growth of IP networks. As every type of wireless network moves towards IP

networks, and cellular networks develop an all-IP core network architecture, mobility between

various types of radio networks can be achieved by all-IP networks.

The network architecture shown above is based on IMS and inter-technology interworking. This

enables true “convergence” of services. Convergence is a phenomenon that allows the subscriber to access any service, anywhere, using any device and any access network. IP convergence fully

exploits the potential of IP to implement convergence.

In practice, IP convergence allows the user to obtain numerous services while stationary or while

on the move using a variety of devices on different radio access networks.



3. EVOLVED SYSTEM ARCHITECTURE

3.1. 3G ARCHITECTURE

Figure 9 (again) shows the typical architecture of a 2G/3G network which usually has two types

of core networks, a circuit switched-core network (CS-CN) and a packet switched-core network

(PS-CN). Both core networks are usually supported by two radio networks GERAN (2G) and UTRAN (3G). Voice services are supported using the circuit switched network and packet

services are supported using the packet switched core network. The radio interfaces support

bursty traffic for the packet domain and traditional telephony traffic for the CS domain

GSM radio network

(GERAN)

UMTS radio network

(UTRAN)

BSC

RNC

MSC/

VLR

SGSN

A

Gb

Iu-PS

Iu-C

S

GGSN

External

Voice

Network

External

Data

Networks

BTS

NodeB

GMSC

BTS Base Transceiver Station

MSC Mobile Switching Center

GMSC Gateway MSC

RNC Radio Network Controller

SGSN Serving GPRS Support Node

GGSN Gateway GPRS Support Node

GERAN GSM EDGE Radio Access Network

UTRAN UMTS Terrestrial Radio Access Network

CS-CN

PS-CN

HLR

HLR

Figure 9: 2G/3G network architecture

The GSM EDGE Radio Access Network (GERAN) consists of Base Transceiver Stations (BTS)

and Base Station Controller (BSC). The UMTS Terrestrial Radio Access Network (UTRAN)

consists of Node Bs and Radio Network Controllers (RNCs).

The MSC/VLR (Mobile Switching Center/Visitor Location Register) is the key element in the

circuit switched core network.

The Serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN) are

the two key elements in the packet switched core network. The Home Location Register (HLR)

maintains the database for the domains and may be used in common. Initial releases of UMTS

used an ATM (Asynchronous Transfer Mode) backbone. Later releases moved towards an IP

backbone.



With the advent of VoIP (Voice over Internet Protocol) UMTS started moving towards

supporting even voice services over the packet domain, with the assistance of IMS (IP

Multimedia Subsystem). Using only one core network to support both voice and packet services

reduces the CAPEX and OPEX cost for the operators.

But as the legacy CS-CN cannot be switched off on an instant 3GPP has defined the voice call continuity (VCC) feature to enable handover between the two domains. VCC relies on IMS and

enables seamless mobility. It also requires support of inter-radio-access-technology (inter-RAT)

interworking.

Moving from a circuit-switched core network for a voice call towards a packet-switched core

network brings up the issue of latency. The reduction of latency of the packet switched network

(LTE and EPC) has been one major requirement of the EPS architecture.

3.2. EPS ARCHITECTURE

3.2.1. REQUIREMENTS

The main requirements of EPS are summarized in Figure 10.

The usual core network requirements like low latency, good QoS support and enhanced security

features apply also for the EPS. But decreased complexity is also a key requirement of the EPS.

Main EPS

requirements

Low latency

Decreased Complexity

- No CS-core-network

- Direct Link between core

and NodeB

Good QoS support

Enhanced security

Figure 10: EPS requirements

The EPS doesn't have a circuit switched core network and the radio access network (LTE) does

not support legacy TDM oriented transmission as it is only packet based. Therefore voice

services are supported on the packet network only. This leads to some disruption which we will

further investigate later.

The only major architectural difference is that the core network directly communicates with the

Node B instead of the RNC. Basically the functionalities of the Node B and the RNC are merged

into a common node called the Evolved Node B or the eNode B. This helps to reduce the

latency.



3.2.2. FEATURES

The main features of EPS to be mentioned are:

1. No circuit switched network

Voice call continuity between EPC and a CS-network is supported with the help of IMS.

2. Network sharing

LTE allows multiple PLMNs to share a radio access network. The eUTRAN broadcasts

multiple PLMN-IDs and the UE can choose the best one.

3. Evolution path for 3GPP and 3GPP2 networks EPC supports interworking between technologies like UMTS, CDMA2000,

1x/1xEV-DO, GPRS/EDGE and WiMAX.

4. Distributed architecture

If one node goes down, the other nodes can pick up the load from that faulty node.

Redundancy and load sharing are main benefits of the EPC's IP-based distributed

architecture.

3.2.3. ARCHITECTURE OVERVIEW

The overview architecture of the Evolved Packet System (LTE + EPC) is depicted in Figure 11.

S1-U

S1-MME S11

S5/S8

External

Packet Data

Network

(PDN)eNodeB

eUTRAN

LTE

S6a

EPC

PCRF

HSS

ePDG

MME

P-GWS-GW

Figure 11: EPS overview architecture

The EPS consists of a radio network (LTE) and the packet switched core network (EPC). The

radio network is also called an Evolved UTRAN or EUTRAN. The packet core network of EPC

is called the Evolved Packet Core network. The new entities in the Evolved Packet Core (EPC)

consist of the Mobility Management Entity (MME), the Serving Gateway (S-GW), the PDN Gateway (P-GW), Home Subscriber Server (HSS) and the evolved Packet Data Gateway

(ePDG).

The different nodes within the EPC are explained in the next chapters.



3.2.4. MOBILITY MANAGEMENT ENTITY (MME)

The MME is the prime node for all core signaling functionalities. It is responsible for:

- Managing and storing UE contexts information like IMSI (International Mobile Subscriber

Identity) and UE network capability

- Generating temporary identifiers for the UEs like Globally Unique Temporary Identity

- Mobility management functions such as coordinating the signaling for inter-S-GW handovers

- Session management function like coordinating the signaling to establish and to end bearers

for a service

- Distributing paging messages to eNode Bs

- Security/authentication control

The MME plays a key role in inter-technology handovers.

3.2.5. SERVING GATEWAY (S-GW) AND PDN-GATEWAY (P-GW)

Two gateways are used within the core network to handle the user traffic. The Serving Gateway communicates with the eNodeB and the PDN-Gateway communicates with the external packet

data network (PDN). The task-split between S-GW and P-GW is shown in Figure 12.

S1-U S5

External

Packet Data

Networks

(PDN)

eNodeB

SGi

Data Forwarding

Inter-3GPP mobility anchor

Gateway to PDNs

Inter-technology mobility anchor

IP address allocation

Data rate enforcment

S-GW P-GW

Figure 12: Task-split between S-GW and P-GW

The S-GW is defined to handle user data and is involved in the routing/forwarding of data

packets between the EUTRAN (eNodeB) and the P-GW. Packets are forwarded between the

S-GW and the P-GW over the S5 interface. The S-GW is connected to the EUTRAN via the S1-

U interface which provides user plane tunneling. The S-GW also performs mobility anchoring for inter-3GPP mobility like LTE to UMTS handovers. An S4 interface (not shown in above

figure) connects the S-GW with the legacy SGSN. A UE can be connected (via eNodeB) to

exactly one serving gateway at any instance.

The PDN Gateway is the node that connects the UE to external PDNs (Packet Data Networks)

and acts as the UE's default router. The world-wide Internet is an example of a PDN and IMS is

another example.



A UE may be connected to multiple PDNs through one or more PDN Gateways. The PDN-GW

is responsible for anchoring the user plane mobility between the 3GPP access networks and non-

3GPP access networks like 1xEV-DO.

The PDN Gateway is also responsible for the allocation of an IP address to the UE. Furthermore

it supports downlink data rate enforcement ensuring that a user does not exceed his subscribed

traffic rate.

In practical implementations both gateway functions may be included in a single network node (combined S-GW/P-GW).

3.2.6. HOME SUBSCRIBER SERVER (HSS)

The HSS (Home Subscriber Server) is a user database that stores subscription related

information to support call control and session management. It therefore communicates with the

MME and acts as a storehouse for user identification, numbering and the Service Profile.

The function of the HSS for the EPS may be combined with the HSS defined for IMS. Both

functional entities contain data for the subscribers within a domain.

3.2.7. EVOLVED PACKET DATA GATEWAY (EPDG)

The evolved Packet Data Gateway (ePDG) is used to interwork with non trusted non-3GPP IP

access systems.

untrusted

non-3GPP

access

network

EPC

P-GW

ePDG

Tunne

ling

Figure 13: Tunnel connection to ePDG

The ePDG secures the non trusted access by having a secured tunnel (IPsec tunnel) between the UE and the ePDG as shown in Figure 13. When a UE attaches to a non trusted non-3GPP access

network, first the UE discovers the IP address of the ePDG, then it sets up an IPSec tunnel to the

ePDG and finally it can access the services in the PDN. The UE cannot access any node in the

EPC until it authenticates and sets up an IPsec tunnel. The ePDG can also act as a local mobility

anchor within the non trusted non-3GPP access network.



3.2.8. POLICY CHARGING RULE FUNCTION (PCRF)

With the introduction of IMS the SIP session signaling (control plane) has already been

separated from user data (user plane). The same concept is continued with introduction of the

EPS. The EPS represents the user plain including a sophisticated mobility control while the

session signaling is done in IMS outside of the EPS.

The PCRF (Policy Charging Rule Function) is already known from the IMS architecture. It has the same role in EPS where it controls the data connections which are called “EPS bearers”. The

applications (which are represented by the P-CSCF in case of IMS, or e.g. by a video streaming

server) interact with the PCRF and the PCRF controls the P-GW and in some cases also the

S-GW and other access gateways to open and close gates and to enforce bandwidth and QoS. This interaction is also required to tie up the signaling and bearer for billing purposes.

S5/S8

PCRF

P-GW

External

Packet Data

Network

(IMS)

P-CSCF

SGi

Figure 14: Control of EPS bearers by PCRF

3.2.9. EUTRAN

The evolved NodeB (eNodeB) is part of the new 3GPP defined radio access network eUTRAN.

The eNodeBs are responsible for controlling the radio link and to care for data rates in excess of 100 Mbps. Figure 15 shoes the principle architecture of the eUTRAN.

LTE-Uu

S1 S1 S1

Evolved Packet

Core

X2

X2X2

Radio Resource Management

Radio Handover Management

Call Admission Control

Latency Reduction

Figure 15: eUTRAN architecture



Radio Resource Management (RRM), Radio Handover Management, and Call Admission

Control (CAC) are implemented at the eNode B. Having just eNode Bs in the eUTRAN

simplifies the architecture by having a reduced number of nodes and interfaces. This architecture reduces the cost for the operator and reduces the latency of the system. The UE and the eNodeB

communicate using OFDMA in downlink and SC-FDMA in uplink and uses advanced antenna

techniques.

The eNode Bs are interconnected by the X2 interface. The X2 interface between eNode Bs is

new and unique to LTE. This interface is primarily used for intra-LTE handover signaling and

data forwarding during handovers.

The S1 interface is the interface between the eUTRAN and the evolved packet core. The S1

interface within the EPC is split into two components:

The signaling part for mobility signaling (S1-MME) is handled by the MME

The data part (S1-U) is handled by S-GW

A many-to-many S1 interface is provided between the eNode Bs and EPC supports

redundancy/load sharing of network nodes and enables support for the Mobile Virtual Network

Operator (MVNO) model. A many-to-many S1 interface enables multiple service operators to share the same radio network.

3.2.10. TRACKING CONCEPT

During movement a mobile UE is covered by different eNodeBs. In active mode the network

always has full knowledge about the UE’s position, because it has to manage the resources

during handover. In contrast, in idle mode, as long as the UE is principally registered with the

network, it needs to know the position only on a level of some area. If this would not be the case, a search within the whole network would be required when terminating traffic for the UE arrives.

The concept of Tracking Areas (TA) has been developed for this purpose. This is similar to

Location Areas (LA) in GSM and Routing Areas (RA) in GPRS.

Basically the whole eUTRAN is divided into non-overlapping TAs as shown in Figure 16. Note

that cells of an eNodeB may belong to different TAs.

TA e

TA b

TA x

. . .eNB1

eNB2

Tracking Area 2

Tracking Area 1

TA x

TA y

TA z

. . .

UE1

UE2



Figure 16: Tracking Area concept

The actual TA identifier is broadcasted within the radio cell and the UE has to register its current

TA with the network (the MME). The MME allocates a list of TAs to a UE and the UE may

move freely within the list of TAs without updating its position. This reduces the idle traffic of TA update messages and in case of terminating traffic the MME has to search a UE only within

its assigned TA-list.

The TA-list are assigned individually to a UE and may follow e.g. usual moving patterns of a

user to further reduce and optimize the idle traffic of TA update messages.

3.2.11. REDUNDANCY AND DISTRIBUTED ARCHITECTURE

The many-to-many relationship is a further characteristic of the EPS. It enables redundancy and

load distribution on many levels as shown in Figure 17.

The EPS defines something called an MME pool area and S-GW service areas. An MME pool

area is defined as a group of tracking areas within which a UE will be able to stay attached to the

same MME. An MME pool area is served by one or more MMEs in parallel. When an MME attaches to the network, the eNodeB performs load balancing to choose an MME from the

associated MME pool. This redundant architecture also provides fault tolerance among MMEs in

an MME pool area.

A service area is defined as a group of tracking areas within which a UE will be able to stay

attached to the same S-GW. The S-GW service area is served by one or more S-GWs in parallel. The MME performs redundancy and fault tolerance support for S-GWs over the same service

area.

Tracking Area 1

Tracking Area 2

Tracking Area 3

Tracking Area 4

Tracking Area 5

MME 1 MME 2

MME Pool 1

S-GW 1

Service Area 1

S-GW 2

MME 3 MME 4

MME Pool 2

S-GW 3

Service Area 2

S-GW 4

Figure 17: Distributed architecture with areas and pools



3.3. EPS INTERFACES AND PROTOCOLS

The main interfaces of the EPS are shown in Figure 18.

SGi

LTE-Uu

S6a

S1-U

S5 PDN

eNodeB

UE

S1-MM

E

S11

X2

S1-U

S1-M

ME

S-GW

MMEHSS

P-GW

Figure 18: Main interfaces of EPS

The S1-MME is the interface defined between the eNode B and the MME. The S1-MME

interface is responsible for S1 bearer management. The interface between the MME and the

S-GW is called the S11 interface. The MME uses this interface to talk to the S-GW to manage the EPS bearers. The interface between the S-GW and the P-GW is called the S5 interface. The

S5 bearer can be either a GTP (GPRS Tunnelling Protocol) tunnel or a GRE (Generic Routing

Encapsulation) tunnel. GTP-C7 takes care of the S5 GTP-based bearer management. PMIPv68

takes care of S5 GRE-based bearer management9.

In the case where the S-GW is in a visited LTE network (roaming), the interface between S-GW and P-GW is referred to as the S8 interface. The only difference between the S5 and the S8

interface is the need for additional security. Otherwise, there is no difference between the S5 and

the S8 interface.

S6a is the interface defined between the MME and the HSS. DIAMETER is the chosen protocol

to interface with the HSS. X2 is the interface defined between the two eNodeBs. X2 plays a role in creating tunnels between source and target eNodeBs to forward packets during inter-eNodeB

mobility.

S1-U is the interface defined between the eNodeB and the S-GW. It is responsible for

forwarding the data packets across the eNodeB and the S-GW. S1-U is a user plane only

interface. The signaling required for establishing bearers between the eNodeB and the S-GW is

done using S1-MME and S11 interfaces.

S1-MME and the S11 interface are signaling only interfaces.

Between the UE and the MME the Non Access Stratum (NAS) protocol is defined. NAS messages are carried over the LTE-Uu between the UE and the eNodeB and are carried over the

7 GTP consists of a control part (GTP-C) and a user part (GTP-U). 8 PMIPv6: „Proxy Mobile IPv6“ is an IETF protocol to support mobility 9 GTP was proposed by mobile operators as an “all-in-one” protocol for bearer management, mobility and QoS

while PMIPv6 was proposed by IETF groups who always tend to re-use existing protocols.



S1-MME interface between the eNodeB and the MME. NAS takes care of mobility management

and session management of a UE. The mobility management procedure includes registration of

the UE to the network, intra-LTE mobility and inter-system mobility. The Registration process

ends up creating a mobility context for a UE at the MME. The session management procedure

includes creation, deletion and modification of EPS bearers.



4. REGISTRATION

This chapter explains the different operations that a UE performs from power on to power off. It

shows in detail the registration procedure and describes the different steps like authentication,

default bearer establishment and the IP address allocation procedure involved during the registration process.

4.1. AUTHENTICATION AND SECURITY

4.1.1. LIFE OF A MOBILE

To start with the big picture these are the major activities of a mobile between power on and

power off.

After powering on, a UE first completes frequency and time synchronization. Then it picks a preferred network and establishes an RRC (Radio Resource Control) connection with the

selected cell. Now it is ready to register with the network.

The Registration process makes the UE's presence known to the network. This enables the

network to deliver incoming calls or services to the UE. It also enables the UE to start any

services. During the Registration process a default EPS (Evolved Packet System) bearer is

established for a UE. Default EPS bearers provide kind of best effort QoS.

If some services require better QoS, then service addition procedures are used to establish these

better QoS bearers called dedicated bearers. Then the UE can perform different types of mobility

procedures based on its capabilities and serving network's capabilities.

Lastly, it releases all its bearers and deregisters from the network when it is powered off.

4.1.2. HIGH LEVEL VIEW OF NETWORK ATTACHMENT

Figure 19 depicts a high level view of the network attachment procedure which includes the

following steps:

After the UE has acquired the network and has established a signaling radio bearer between the

UE and the eNodeB, it performs the initial attach procedure where it selects an MME. An S1

signaling bearer between the eNodeB and the MME is also established.

An authentication is performed during registration to make sure the UE is the right mobile

connected to the right network. The authentication in EPS is a 2- way authentication: the UE and

the network both authenticate each other. Then the MME selects the S-GW and the P-GW for a

given UE.

The always-on IP connectivity for users of the EPS is enabled by establishing a default EPS bearer between the UE and the PDN-GW. An IP address can be allocated during or after default

bearer set up.



eNodeBUE

Network discovery

Access system selection

RRC connection establisment

Initial attach

S1 signalling bearer set up

MME selection

Authentication

S-GW and P-GW selection

Default bearer set up

IP address allocation

S-GWMME HSSP-GW

Figure 19: High level view of network attachment

4.1.3. INITIAL ATTACH REQUEST

Figure 20 shows some details of the initial attachment procedure.

The UE initiates the attachment procedure by the transmission of an Attach Request message which contains the IMSI (International Mobile Subscriber Identity), UE Network Capability,

PLMN (Public Land Mobile Network) ID and PDN Address Allocation to the eNodeB.

The UE Network Capability includes security information like NAS (Non-Access Stratum)

security algorithms. The PDN Address Allocation indicates whether the UE wants to perform IP

address allocation during the execution of the procedure and, when known, it indicates the UE IP version capability (IPv4, IPv4&IPv6, IPv6), which is the capability of the IP stack associated

with the UE. The IMSI uniquely identifies the subscriber. The PLMN ID indicates the selected

network of the subscriber.



eNodeBUE

Initial Attach Request

IMSI

UE network capability

PDN address allocation

PLMN ID

MME Pool

PLMN ID

eNodeB picks an

MME based on PLMN

ID and load

MME 1

MME 2

MME 3

Figure 20: Initial attachment and MME selection

The eNodeB then selects the MME based on the selected network and the load of MMEs in the

MME pool.

4.1.4. AUTHENTICATION

Figure 21 shows the details of the authentication procedure in EPS. It uses the EPS AKA

(Authentication and Key Agreement) procedure for mutual authentication of the UE and the

network. This procedure is derived from the UMTS Authentication and Key Agreement Protocol

as also the authentication procedure in IMS.

The network initiates authentication and the key agreement procedure (AKA) during the registration procedure. The MME contacts the HSS/AuC to request security parameters. The

HSS generates XRES, AUTN, KASME (Access Security Management Entity) using a random

number and a subscriber authentication key (K) with specific authentication algorithms. K is

known to the HSS and the UE. K is never transmitted out of either the HSS or the UE. However

during authentication both the UE and the HSS generate RES, AUTN and KASME. RES is used to

verify the authenticity of the UE and AUTN is used to verify the authenticity of the network.

The HSS sends the XRES, RAND, AUTN and KASME to the MME over a secured link. The

MME stores the XRES and the KASME and challenges the UE by forwarding AUTN and RAND

to the UE.

So at the UE side and the network side, RES and AUTN are generated using stored

authentication key and the random number. Then they are compared against each other. If it was a rogue network or if it was a rogue UE, it wouldn't have the authentication key. In such cases,

the values generated at both ends would not match.

KASME is the key parameter that is used to generate a set of parameters used for encryption and

integrity procedures.



UE

Authentication Request

(IMSI)

Authentication Response

(IMSI, RAND, XRES, AUTN

KASME)

Authentication Request

(RAND, AUTN)

Stores KASME and XRES

Generates KASME, AUTN

and RES using K and RAND

Checks if generated AUTN is

equal to received AUTN

Authentication Response

(RES)

Checks if received RES is

equal to XRES

MME HSS

Figure 21: Authentication procedure

4.1.5. SECURITY

The access security in EPS consists of integrity protection and encryption. Integrity protection

makes sure that packets in transit are not altered by anyone. Encryption ensures that only the

intended recipient knows what is being sent and unauthorized parties do not.

Within the access part of EPS we have to distinguish between the Access Stratum (messages between UE and eNodeB) and the Non Access Stratum (messages between UE and MME).

Messages of the Access Stratum (AS) are typically the RRC messages (Radio Resource Control)

whereas messages of the Non Access Stratum (NAS) are typically handover messages.

NAS and AS messages are both integrity protected and encrypted. User plane messages are just

encrypted. The EPS uses two types of security mechanisms, one for the NAS and another one for

the AS. The selection mechanisms are shown in Figure 22.



UE

NAS Security Mode CMD

(selected algorithms)

NAS Security Mode Complete

MME selects NAS integrity

and security algorithms

eNodeB

UE stores the

selected algorithms

eNodeB selects RRC and user

plane security algorithms

AS Security Mode CMD

(selected algorithms)

AS Security Mode Complete

UE stores the

selected algorithms

MME

Figure 22: Selection of security algorithms

For the selection of the NAS security algorithms the MME selects after authentication the NAS

integrity and encryption based on the prioritized list configured at the MME and also the security capabilities of the UE. The selected algorithms are communicated to the UE using the NAS

security mode command. This message and all the subsequent downlink NAS messages would

be integrity protected. The UE makes a note of the selected encryption and integrity algorithms.

It acknowledges the reception of the NAS security mode command by sending a NAS security

mode complete message. This message would be integrity protected. The NAS security

procedure enables encryption and integrity protection of the NAS messages.

For the selection of the AS security algorithms the eNodeB selects the RRC and user plane

integrity and the encryption algorithm to be used. Selection is done again based on the prioritized

list sent by the MME and also the security capabilities of the UE. The selected algorithms are

communicated to the UE using the AS security mode command. This message and all the

subsequent downlink AS messages would be integrity protected.

The UE makes a note of the selected encryption and integrity algorithms. It acknowledges the

reception of the AS security mode command by sending an AS security mode complete message.

This message would be integrity protected. The AS security procedure enables encryption and

integrity protection of the RRC messages and encryption on the user plane messages.



4.1.6. SUBSCRIPTION INFORMATION

The next step after the mutual authentication and the selection of integrity protection and

encryption algorithms is the update of location information and the download of subscription

data between MME and HSS. This is shown in Figure 23.

Update Location

Insert Subscriber Data

(List of all APNs, Default APN)

The MME selects the P-GW that serves the default APN

The MME selects the S-GW that supports all the services of the UE

Insert Subscriber Data Ack

MME HSS

Figure 23: Exchange of f Location and Subscription Information

After authentication, the MME lets the HSS know that it is currently serving the UE by sending

an Update Location message. The HSS sends an Insert Subscriber Data message to the MME.

The subscription data contains the list of all Access Point Names (APNs) that the UE is

permitted to access and the Default APN to be used for that UE.

The APNs correspond to the networks where the UE may request access to (e.g. the IMS for operator provided services or the Internet). Based on the APNs the P-GWs are selected. One of

the APNs is the default APN where a connection is setup after the registration. Connections to

other networks (based on the other APNs) are setup on demand only.

The MME proceeds now with establishing a default EPS beare. The MME establishes the default

bearer with a P-GW that serves the default APN. The MME also selects the S-GW that serves

the area where the UE is currently located.

4.2. DEFAULT BEARER SETUP

With default bearer establishment, the UE gets connected to the default PDN and can perform

best-effort type of services such as background downloading of e-mails. The default bearer

provides always on connectivity and accelerates setup of some services such as VoIP.

The set-up of the default EPS bearer between UE and P-GW is shown in Figure 24. We have to

distinguish three parts of the bearer:

the radio bearer between UE and eNodeB

the S1 bearer between eNodeB and S-GW

the S5-bearer between S-GW and P-GW



eNodeBUE

Store S-GW addr and TEID

Create default

bearer request Create default

bearer request

(S-GW TEID)

Create default

bearer response

(P-GW TEID,

IP-addr of UE)

Create default

bearer response

(S-GW TEID

towards eNodeB)

S5

S1 Control msg:

S-GW addr, TEID

(Attach accept,

GUTI, IP addr)

TEID: Tunnel Endpoint Identifier

GUTI: Globally Unique Temporary Identity

RRC Control msg:

default radio-b. info

(Attach accept,

GUTI, IP addr)

RRC Control msg:

(Attach complete)

Default radio bearer

S1 Control msg:

eNodeB addr, TEID

(Attach complete) Update bearer req.

(eNodeB addr.

and TEID)

Default S1-U bearer

Update bearer resp.

S5Default S1-U bearerDefault radio bearer

Default EPS-Bearer

MME S-GW P-GW

Figure 24: Default EPS Bearer Set-Up



The creation of the default S5 and S1 bearers is initiated by the MME by sending a create default

EPS bearer request to the S-GW. The S-GW and P-GW establish an S5 bearer by exchanging the

Create Default bearer request and response messages.

They both exchange their tunnel end point identifiers (TEID). The P-GW also generates an IP

address for the UE and forwards it to the S-GW. The S-GW indicates the successful creation of the default S5 bearer to the MME and also indicates its tunnel end point towards the S1 bearer.

Please note that the creation of the default bearers between the S-GW and the eNodeB is

coordinated by the MME.

The MME now sends an Attach Accept message to the eNodeB which contains a new GUTI

(Globally Unique Temporary Identity) and the IP address Information10. The Attach Accept

message is embedded in an Sl-MME control message and sent to the eNodeB. Along with the Attach Accept message the MME also forwards the S-GW tunnel endpoint ID (TEID) to the

eNodeB. The eNodeB makes a note of the S-GW tunnel ID and forwards the Attach Accept

message to the UE using an RRC message. The eNodeB sends then default radio bearer related

parameters in the RRC message. The UE now sends the Attach complete message embedded in an RRC Connection message to the eNodeB. This completes the establishment of the default

radio bearer.

The last step is to establish the default S1-U Bearer and to finish up with the Attach procedure.

The eNodeB forwards the Attach Complete message to the MME in an S1-MME control

message. This S1-MME control message includes the TEID of the eNodeB used for downlink

traffic on the S1-U interface. The MME sends the tunnel ID sent by the eNod B to the S-GW. This completes the establishment of the default S1 bearer between the eNodeB and the S-GW.

Now the end-to-end default EPS bearer is ready. The UE can use this default bearer for any

service that requires just default QoS treatment.

10 A UE is identified at the MME by its GUTI. It helps protect the user identity. The IMSI is rarely transmitted over

the air. Instead the GUTI is used to identify the UE.



5. SERVICE DATA FLOWS AND QOS

Besides offering a default bearer service with a default QoS a main target of EPS is to also offer

dedicated bearers with specific QoS. A main term in this area is the Service Data Flow (SDF).

The following chapters explain

how Service Data Flow are defined

how Service Data Flows are mapped to EPS bearers

how a dedicated bearer is created for a specific service

how QoS classes are defined within EPS

5.1. INTRODUCTION TO SERVICE DATA FLOW AND EPS BEARER

5.1.1. HIGH LEVEL VIEW

The Evolved Packet System (EPS) allows a user to connect to external IP networks. The public

Internet is the most common IP network, but there may also be a need for a user or group of

users to connect to private networks such as corporate networks or to an IMS services network.

In general, then the EPS provides a PDN connectivity service which allows a user to connect to

multiple external IP networks perhaps simultaneously. This situation is shown in Figure 25.

UE

EPS

PDN 1

PDN 2

PDN 3IP addresses

- A user may subscribe to multiple PDN connections

- PDNs are identified by an APN (e.g. „Internet“)

- a default APN is part of the user subscription information in HSS

P-GW 1

P-GW 2

Figure 25: Multiple PDN connections

A user subscription may allow the user to connect to two or more PDNs. A PDN can host one or more applications. For each of the subscribed PDNs, the user's profile in the HSS contains an

Access Point Name (APN) which identifies the PDN. This APN can be used to choose a suitable

PDN GW for the user's PDN connectivity service.

At the initial connection to the EPS, a default APN, identified in the subscription data, is used

during the Attach procedure for default PDN connection. Subsequently the user may establish

connectivity with another PDN by specifying the APN to which it wants to connect.

The UE may use different IP addresses per PDN connection.



5.1.2. SERVICES AND SERVICE DATA FLOW

A service in the context of EPS is an application that runs between the UE and the Application

Server. An application or a service is identified by the IP address of the UE and the IP Address

of the Application Server and also the UDP/TCP port numbers used by the UE and the

Application Server. Three networks can be distinguished which are responsible for the connectivity and the QoS of a service as shown in Figure 26.

UE

EPS

QoS ?

Backhaul

Network

QoS ?

PDN

QoS ? Application

Server

P-GW

Service Data Flow

Figure 26: Service Data Flow

The end-to-end logical connection between the UE and the Application Server is called the

Service Data Flow (SDF). A packet filter is used to identify a SDF. The Packet filter definition

includes the source IP address, the destination IP address and the ports used at the source and destination nodes. The packets for that service pass through different networks like EPS,

backhaul networks and the PDN which is hosting the service.

QoS (Quality of Service) is defined for an application or for a Service Data Flow. QoS require-

ments for a service must be met at all three networks to get an overall satisfaction. The EPS is

only responsible for meeting QoS goals within the EPS.

5.1.3. SERVICE DATA FLOW AND EPS BEARERS

A Service Data Flow (SDF) is an end-to-end application level packet flow between a UE and a

device in an external Packet Data Network (PDN). QoS is defined at the SDF level. It is however controlled or enforced at the EPS bearer level. This means that two SDFs with the same QoS

requirements can be supported by a single EPS bearer provided they are hosted on the same

PDN. This situation is shown in Figure 27 for SDF 1 and SDF 2.

Figure 27 further shows that the different service data flows are be mapped to certain EPS

bearers. Where is this mapping done?

The mapping of SDFs to the EPS bearer happens at the UE and the P-GW. One or more packet

filters are associated with an EPS bearer. The packet filters that define the association between

SDFs and an EPS bearer are called the Traffic Flow Template.



EPS

P-GW

UE

EPS bearer 1

SDF 2

SDF 1

EPS bearer 2

SDF 3

AS1

AS2

AS3

- A PDN connection may support more then one bearer

- A Service Data Flow has a defined QoS charcteristic

- An EPS bearer is the level where QoS is enforced

- SDFs may be aggregated into the same bearer

Figure 27: Service Data Flows an EPS bearers

5.1.4. EPS BEARER

As already mentioned in chapter 4.2 an EPS bearer consists of three traffic paths:

- the radio bearer

- the S2 bearer

- the S5 bearer

But this partition is only true if Generic Tunneling Protocol (GTP) is used. This is the case when

only 3GPP based access networks are used. In case of non 3GPP based access networks the EPS

bearer concept is different (see chapter 5.4).

In the case of GTP-based S5 connections, an EPS bearer is composed of the three traffic paths

mentioned above. The S5 interface carries the traffic of an EPS bearer in a GTP tunnel. Similarly

the S1 interface between the S-GW and eNodeB carries the traffic of an EPS bearer in another GTP tunnel. The third leg is the air interface component known as the radio bearer. There is a

one-to-one mapping between the EPS bearer and all three component paths.

On initial connection to the EPS, a default APN, identified in the subscription data, is used

during the Attach procedure for PDN connection. With the attach procedure a default EPS bearer

is established for a default APN. But after attachment further EPS bearers may be established. The user may e.g. wish to activate a service that requires a better QoS than what the default EPS

bearer is supporting. In that case a dedicated EPS bearer is established. Before establishing a

dedicated bearer to a new PDN other than the default APN, a default EPS bearer is established

for the new PDN.

5.1.5. DOWNSTREAM AND UPSTREAM PACKET CLASSIFICATION

Which network elements are responsible for classifying packets and mapping SDFs to EPS

bearer?

Taking into consideration the three parts of an EPS bearer we have the situation shown in Figure

28 which also explains what happens at each node.



UE

S1 BearerRadio Bearer S5 Bearer

Upstream:

UE maps SDF to

Radio Bearer

eNodeB maps

Radio Bearer to

S1 Bearer and

vice versa

S-GW maps

S1 Bearer to

S5 Bearer and

vice versa

Downstream:

P-GW maps SDF

to S5 Bearer

S-GW P-GWeNodeB

Figure 28: Downstream and Upstream packet classification

The PDN-GW is responsible for mapping IP packets in the downlink to the appropriate GTP

tunnel before it is forwarded to an S-GW. The packets arriving from an external Packet Data Network do not contain any explicit information on the service data flows to which they belong.

Therefore the PDN-GW must use IP network layer and transport layer header information to

classify incoming traffic and detect to which SDF it belongs. The PDN-GW will know which

EPS bearer the SDF is associated with and therefore the GTP tunnel.

Since there is a GTP tunnel for each EPS bearer, the PDN-GW must classify and map the

incoming packets from the PDN (downstream) to the correct tunnels.

The task at the S-GW is simpler. Since there is a one-to-one mapping between the S5 tunnel and the corresponding S1 tunnel the S-GW does not need to perform classification of packets. At the

eNodeB there is also a one-to-one mapping of the S1 tunnel with the radio bearer. So here there

is no need to perform a classification of packets.

In the upstream direction it is the UE which classifies upstream packets to s certain SDF and

maps theses packets to the corresponding radio bearers in the uplink direction.

5.2. QOS

Resources in networks are not available in abundance. This is in particular true for the radio part

of EPS. Imagine users clogging the network with viewing YouTube videos while others try to

setup an emergency call. Therefore the control of QoS is an important aspect of the EPS.

5.2.1. BEARER CLASSES

In EPS two bearer classes in respect to QoS are defined as shown in Figure 29.

EPS bearers are divided into two classes: Guaranteed Bit Rate (GBR) bearers and non-

Guaranteed Bit Rate (non-GBR) bearers. As the names suggests the GBR bearer, when

established, will be guaranteed a specific bit rate. For non-GBR bearers, there are no guarantees

under congestion conditions that any specific bit rate will be allotted to the service.



QCI

ARP

GBR

MBR AMBR

GBR bearers Non-GBR bearers

QCI QoS Class Identifier

ARP Allocation and Retention Priority

GBR Guaranteed Bitrate

MBR Maximum Bitrate

AMBR Aggregate Maximum Bitrate

Figure 29: Bearer classes in EPS

For these two bearer classes 9 QoS classes are defined. They are broadly divided into GBR and Non-GBR types. The key QoS parameters associated with a service data flow are QoS Class

Identifier (QCI) and Allocation and Retention Priority (ARP). QCI defines the general class of

the service. For GBR bearers Guaranteed Bit Rate (GBR) and Maximum Bit Rate (MBR)

parameters are specified. The GBR specifies the expected bit rate whereas the MBR puts a limit

on the maximum bit rate allowed.

For Non-GBR bearers the Aggregate Maximum Bit Rate (AMBR) parameter is specified. The

AMBR is the aggregate bit rate across all non-GBR bearers of a UE going towards the same

PDN. The ARP (Allocation and Retention Priority) parameter will be used in congestion

situations when not all users and their services can be accommodated. The ARP will be used by the admission control function in the eNodeB. In case of congestion the ARP bit indicates which

EPS bearer needs to be retained.

5.2.2. QCI CLASSES

Figure 30 shows the different QCI classes in EPS.

There are 9 different QCI classes in LTE. Each QCI value has a priority, which will be used by

EPS traffic nodes during congestion. If packet queues are close to overflow then Service Data

Flows (SDF) with lower priority than others will have their packets discarded first. Note that

IMS signaling packets have the highest priority while QCI 9 is equivalent to best effort service.

The Packet Delay Budget (PDB) associated with an SDF will be one of the inputs used by the

scheduler in the eNodeB. It is used to determine when to deliver packets of the SDF. The PDB values given in the table are a measure of the maximum time allowed for packet delivery from

the P-GW to the UE. The packet loss rate from the P-GW to the eNodeB is assumed to be zero in

non-congestion situations. The PLR (Packet Loss Rate) in the table refers to non-congestion

related losses over-the-air. It is used in an eNodeB to help decide how to set up the radio bearer

for a particular SDF.



QCI Bearer

Type

Application Example Packet

Delay

Packet

Loss

Prio

rity

1

GBR

Conversational VoIP 100 ms 10-2 2

2 Conversational Video (Life Streaming) 150 ms 10-3 4

3 Non-Conversational Video (Buffered Streaming) 300 ms 10-6 5

4 Real Time Gaming 50 ms 10-3 3

5

Non-

GBR

IMS Signalling 100 ms 10-6 1

6 Voice, Video, Interactive Games 100 ms 10-3 7

7 Video (Buffered Streaming)

TCP Apps (web, e-mail, FTP)

Platinum vs. Gold User

300 ms 10-6 6

8 8

9 9

Figure 30: QCI classes in EPS

5.2.3. DIFFERENTIATED SERVICE

Which mechanisms can be used to transport IP traffic of EPS bearers with different QoS

requirements? This is of particular importance for the backhaul network which is the part of the

network from the transceiver stations (eNodeB) back to the core network.

There are two approaches for QoS support in IP networks today, Integrated Service and

Differentiated Service (see Figure 31 ).

QoS in IP Networks

Integrated Service

(IntServ)

Differentiated Service

(DiffServ)

Reserve resources throughout the

network for every user

Used with RSVP signalling

Optional in EPS

Classify user packets into a small set

of classes

Mandatory in EPS

Figure 31: QoS methods in IP networks

With the Integrated Service solution, the user requests a particular QoS for a session and submits

this request to the network. The network carries signalling, typically Resource Reservation

Protocol signaling (RSVP) through the relevant nodes in the network that will carry the traffic. Each node in the network analyzes the request and sets aside the necessary resources to satisfy

the request.

On the other hand the Differentiated Service solution is a class-based approach and is a highly

scalable solution. Its name stems from the fact that traffic is treated differently in the network



based on its class. Each packet is individually marked with a DiffServ Code Point (DSCP) that

indicates how it should be treated relative to other traffic. DiffServ requires no state to be

maintained in the routers and no reservation is done in the routers. It is suitable for use in large

IP networks such as backbones.

Support of Differentiated Service in EPS nodes and the backhaul network is mandatory. Support

of Integrated Service is optional in EPS.

5.3. SERVICE ADDITION AND DEDICATED BEARER SETUP

Besides the default bearer which is setup during initial attachment the main focus is on adding

additional dedicated bearer channels on request of a service invoked by the user. This chapter gives an overview how this is done based on IMS.

5.3.1. SERVICE ADDITION EXAMPLE

From the view of the IMS the EPS is nothing more then an access network. This is shown in

EPS

P-GW

PDN

CSCFAS

INVITE INVITE

SDP negotiationSDP negotiation

EPS Bearer Creation

UE

Figure 32: Service based bearer creation

SIP signaling and the supporting IMS network are used initially to create an Service Data Flow

for a UE. Using SIP signaling, the UE and the session partner or in above figure an Application Server (AS) negotiate the media types, codecs, and QoS parameters of the service like a pay per

view movie hosted on the IMS network. The Call Session Control Function (CSCF) plays a key

role in the session establishment.

When both the UE and the Application Service have negotiated the bearer requirements of the

service, a dedicated EPS bearer needs to be created in LTE.



5.3.2. PCRF LINKS SDF AND EPS BEARERS

Before creating a dedicated EPS bearer for e.g. a pay per view application, EPS needs to know

the QoS and media related information. SIP signaling that took place was transparent to the EPS

nodes. EPS is the access network and IMS is the services network. The PCRF (Policy and Charging Rules Function) is the node that takes the media type and QoS negotiated for a service

and communicates it to the EPS access network. This is shown in Figure 33.

EPS

P-GW

PDN

CSCFVideo

streaming

server

PCRF

Rules required to create the

EPS bearers for the video

streaming service

Session information

based on SDP

UE

Figure 33: PCRF links SDF and EPS bearers

Once the session is established the P-CSCF in the IMS domain informs the PCRF. Based on the

information sent by the PCRF and the subscription information of the UE, the PCRF forms a set of rules. These rules are sent to the P-GW. The P-GW takes the responsibility of establishing the

dedicated EPS bearer.

5.3.3. DEDICATED BEARER CREATION

Figure 34 shows in an overview the creation of a dedicated EPS bearer which is very similar to

default bearer creation.

The P-GW initiates the establishment of the dedicated bearer setup procedure for e.g. the video

streaming application. It sends the QoS info, IMSI and the P-GW TEID for the S5/S8 bearer to

the S-GW. The S-GW in turn forwards it to the MME. The MME requests the eNodeB to create

a dedicated radio bearer towards the UE and also to send information to setup an S1-U bearer

between the eNodeB and the S-GW. The eNodeB does admission control and establishes a dedicated radio bearer. The eNodeB indicates the successful establishment of the radio bearer to

the MME. It also passes on the TEID required to establish the S1-U dedicated bearer.

The MME passes on this eNodeB tunnel ID information to the S-GW. The S-GW replies to the

P-GW with the S-GW TEID required for the S5/S8 dedicated bearer. Thus the whole EPS bearer

comprising three parts is finally (Radio, S1- and S5 - bearer) set up.



UE S-GW P-GW

AS

QoS policy

Application level signalling and media negotiation

eNodeB

MME

PCRF

Apply policies

Create new bearer request

Apply admission control

RRC procedures

Create new bearer response

S1 - bearerRadio bearer S5 - bearer

Figure 34: Creation of a dedicated bearer



5.4. PMIPV6-BASED EPS BEARER

As already mentioned in chapter 3.3 two solutions for signalling at the S5 interface have been

defined: GTP and PMIPv6.

If a PMIPv6 based S5-bearer is used the notion of EPS bearer is reduced to the sections between

UE and S-GW. The section between S-GW and P-GW is GRE-based and not bearer aware.

If a PMIPv6 based S5-bearer is used the scope of an EPS bearer stops at the S-GW. An EPS bearer here consists only of the two sections of the radio bearer and the S1 bearer. All the EPS

bearers for a UE going out to a PDN network use a common GRE S5 tunnel.

For a PMIPv6-based S5-bearer, only one GRE tunnel exists between the S-GW and a P-GW for

PDN connectivity of a UE.

Usually multiple services are hosted on the PDN and the UE is using all of them currently and

they all have different QoS requirements. Even in that case only a single GRE tunnel exits for a

UE for that PDN, but multiple EPS bearers exist between the UE and the S-GW.

For a packet in download direction the P-GW maps the external packet filters to the common

GRE tunnel. The detailed packet filtering and the SDF to EPS bearer mapping in this case is

done by the S-GW.

EPS bearerUE

S1 BearerRadio Bearer

S-GW P-GW

eNodeB

GRE tunnel

to P-GW

S-GW maps SDF

to/from S1-bearer P-GW maps SDF

to/from GRE tunnel

SDF

Figure 35: EPS bearer in case of PMIPv6 based signalling



6. EXERCISES AND QUESTIONS

After studying this part of the lecture you should be able to answer the following questions:

… to be added …



7. REFERENCES

7.1. BOOKS

Magnus Olsson: System Architecture Evolution (SAE):

Evolved Packet Core for LTE, Fixed and other Wireless Accesses

Gebundene Ausgabe: 464 Seiten

Verlag: Academic Press (24. August 2009) Sprache: Englisch

ISBN-10: 0123748267

ISBN-13: 978-0123748263

Gottfried Punz: Evolution of 3G Networks:

The Concept, Architecture and Realization of Mobile Networks Beyond

UMTS

Gebundene Ausgabe: 306 Seiten

Verlag: Springer, Wien; Auflage: 1., st Edition. (14. Februar 2010)

Sprache: Englisch

ISBN-10: 3211094393 ISBN13: 978-3211094396

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