gprs architecture

GPRSSYS

GPRS Architecture: Interfaces and Protocols Training Document

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GPRS Architecture: Interfaces and Protocols

The information in this document is subject to change without notice and describes only the product defined in the introduction of this documentation. This document is intended for the use of Nokia's customers only for the purposes of the agreement under which the document is submitted, and no part of it may be reproduced or transmitted in any form or means without the prior written permission of Nokia. The document has been prepared to be used by professional and properly trained personnel, and the customer assumes full responsibility when using it. Nokia welcomes customer comments as part of the process of continuous development and improvement of the documentation.

The information or statements given in this document concerning the suitability, capacity, or performance of the mentioned hardware or software products cannot be considered binding but shall be defined in the agreement made between Nokia and the customer. However, Nokia has made all reasonable efforts to ensure that the instructions contained in the document are adequate and free of material errors and omissions. Nokia will, if necessary, explain issues which may not be covered by the document.

Nokia's liability for any errors in the document is limited to the documentary correction of errors. NOKIA WILL NOT BE RESPONSIBLE IN ANY EVENT FOR ERRORS IN THIS DOCUMENT OR FOR ANY DAMAGES, INCIDENTAL OR CONSEQUENTIAL (INCLUDING MONETARY LOSSES), that might arise from the use of this document or the information in it.

This document and the product it describes are considered protected by copyright according to the applicable laws.

NOKIA logo is a registered trademark of Nokia Oyj.

Other product names mentioned in this document may be trademarks of their respective companies, and they are mentioned for identification purposes only.

Copyright © Nokia Oyj 2004. All rights reserved.

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Contents

Contents

1 Module objectives ..................................................................................4

2 Introduction ............................................................................................5

3 Network elements...................................................................................7 3.1 Packet Control Unit (PCU) .......................................................................8 3.2 Channel Codec Unit (CCU) ......................................................................8 3.3 Serving GPRS Support Node (SGSN) .....................................................8 3.4 Gateway GPRS Support Node (GGSN)...................................................9 3.5 GPRS MS...............................................................................................10 3.6 Domain Name Servers ...........................................................................12 3.7 Firewalls .................................................................................................12 3.8 Border Gateway .....................................................................................13 3.9 Charging Gateway..................................................................................13

4 GPRS interfaces ...................................................................................14

5 Transfer of packets between GSNs....................................................16

6 Nokia GPRS solution ...........................................................................19 6.1 Nokia GPRS functionality .......................................................................19 6.2 Nokia Base Station Subsystem (BSS) ...................................................22 6.3 Nokia Core Network Subsystem (CNS) .................................................25 6.4 Nokia Network Management System (NMS)..........................................32

7 Key points .............................................................................................35 7.1 GPRS architecture: key points ...............................................................35 7.2 Nokia GPRS solution: key points ...........................................................36

8 Review questions.................................................................................38

9 Appendix – GPRS transmission plane protocols..............................40

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1 Module objectives At the end of the module, the participant will be able to:

• Name the GPRS specific network elements and their most important functions

• Name and explain five important open interfaces in the GPRS network

• Explain the principle of the GPRS Tunnelling Protocol (GTP)

without using any references.

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Introduction

2 Introduction GPRS provides mobile users access to value-added WAP services and different external packet switched networks. These networks can be, for example, the Internet or corporate intranets. The GSM-BSS provides the radio interface, and the GPRS core network handles mobility and access to external packet networks and services. This is shown in Figure 1.

ExternalPacket

Networks

Value-AddedServices(WAP)

Radio Resource & Radio Link Management

NSS

Switching/routing, mobility & connection

management

GPRS (ps)Core Network

GSM (cs)Core Network

BSS

Figure 1. GPRS access to packet switched networks

The GPRS network acts in parallel with the GSM network, providing packet switched connections to the external networks. The requirements of a GPRS network are the following:

• The GPRS network must use as much of the existing GSM infrastructure with the smallest number of modifications to it.

• Since a GPRS user may be on more than one data session, GPRS should be able to support one or more packet switched connections.

• To support the budgets of various GPRS users, it must be able to support different Quality of Service (QoS) subscriptions of the user.

• The GPRS network architecture has to be compatible with future 3rd and 4th generation mobile communication systems.

• It should be able to support both point-to-point and point-to-multipoint data connections.

• It should provide secure access to external networks.

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A GPRS network must provide all of the functionality of a GSM network for packet switched networks and more. The GPRS is expected to perform the functions of a traditional mobile communication network and a traditional packet switched computer network. These functions are itemised below:

• Capability to separate circuit switched and packet switched traffic from mobile station (MS)

• Radio resource management, that is, allocation of radio resources to GPRS subscribers across the air interface

• Interfaces to Internet, intranets, Public Data Networks (PDN), and other Public Land Mobile Networks (PLMN)

• Authenticate subscriber requests for packet switched resources • Encrypt data transmitted on the air interface for security purposes • Data compression for data transmitted over the air interface • Interact with databases (HLR/VLR) containing subscriber information

such as IMSI, security data, and subscription information • Mobility management as in GSM • Location management as in GSM • Handover as a GPRS subscriber moves within a coverage area • Power control to minimise the transmitted power by the user • Network management that facilitates GPRS network management • Generation and collection of network performance statistics • Generation and collection of charging or billing information • Signalling links between the GPRS network elements • Routing of packets to appropriate destination • Protocol conversion between networks that may use different protocols • Buffering of data at GPRS nodes • Allocation of static or dynamic address for packets originating from MS • Protection of the GPRS network from security threats • Capability to monitor target subscriber by law enforcement agencies • Translation between logical names and IP addresses using Domain Name

System (DNS) • Facilitation of roaming subscribers so that they can connect to home

networks • Delivery of SMS messages through the GPRS network • Redundancy mechanisms if one or more network elements were to fail • Translation between private and public addresses using NAT and NAPT • Detection of faulty or stolen GPRS handsets

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Network elements

3 Network elements Figure 2 shows the architecture of a GPRS network. The GPRS system brings some new network elements to an existing GSM network. These elements are:

• Packet Control Unit (PCU)

• Serving GPRS Support Node (SGSN): the MSC of the GPRS network

• Gateway GPRS Support Node (GGSN): gateway to external networks

• Border Gateway (BG): a gateway to other PLMN

• Intra-PLMN backbone: an IP based network inter-connecting all the GPRS elements

• Charging Gateway (CG)

• Legal Interception Gateway (LIG)

• Domain Name System (DNS)

• Firewalls: used wherever a connection to an external network is required.

Not all of the network elements are compulsory for every GPRS network.

BSC

BTS

BTS

TRAU

BSC

BTS

BTS

TRAU

BSS

BSS

NSS

MSC/VLR GMSC

HLREIR AC

PSTN/ISDN

MS

GPRSMS

corp.network

WAP

PDNSGSNIP-

backbone

PCU

PCUCCU

CCU

CCU

CCU CG

BillingCentre

BG

Inter-PLMNNetwork

LIG

LEA

DNS

GGSN FW

Figure 2. GPRS architecture

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3.1 Packet Control Unit (PCU)

The PCU separates the circuit switched and packet switched traffic from the user and sends them to the GSM and GPRS networks respectively. It also performs most of the radio resource management functions of the GPRS network. The PCU can be either located in the BTS, BSC, or some other point between the MS and the MSC. There will be at least one PCU that serves a cell in which GPRS services will be available. Frame Relay technology is being used at present to interconnect the PCU to the GPRS core.

PCUDecides dynamically, which

resources are allocated to cs and ps usage, based on

• load situation• priority, and• operator set rules

BSCcs Radio Resource

Management

PCUps Radio Resource

Management

cs resources ps resources

Figure 3. PCU – its position within the BSS

3.2 Channel Codec Unit (CCU)

The CCU is realised in the BTS to perform the Channel Coding (including the coding scheme algorithms), power control and timing advance procedures.

3.3 Serving GPRS Support Node (SGSN)

The SGSN is the most important element of the GPRS network. The SGSN of the GPRS network is equivalent to the MSC of the GSM network. There must at least one SGSN in a GPRS network. There is a coverage area associated with a SGSN. As the network expands and the number of subscribers increases, there may be more than one SGSN in a network. The SGSN has the following functions:

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Network elements

• Protocol conversion (for example IP to FR)

• Ciphering of GPRS data between the MS and SGSN

• Data compression is used to minimise the size of transmitted data units

• Authentication of GPRS users

• Mobility management as the subscriber moves from one area to another, and possibly one SGSN to another

• Routing of data to the relevant GGSN when a connection to an external network is required

• Interaction with the NSS (that is, MSC/VLR, HLR, EIR) via the SS7 network in order to retrieve subscription information

• Collection of charging data pertaining to the use of GPRS users

• Traffic statistics collections for network management purposes.

3.4 Gateway GPRS Support Node (GGSN)

The GGSN is the gateway to external networks. Every connection to a fixed external data network has to go through a GGSN. The GGSN acts as the anchor point in a GPRS data connection even when the subscriber moves to another SGSN during roaming. The GGSN may accept connection request from SGSN that is in another PLMN. Hence, the concept of coverage area does not apply to GGSN. There are usually two or more GGSNs in a network for redundancy purposes, and they back up each other up in case of failure. The functions of a GGSN are given below:

• Routing mobile-destined packets coming from external networks to the relevant SGSN

• Routing packets originating from a mobile to the correct external network

• Interfaces to external IP networks and deals with security issues

• Collects charging data and traffic statistics

• Allocates dynamic or static IP addresses to mobiles either by itself or with the help of a DHCP or a RADIUS server

• Involved in the establishment of tunnels with the SGSN and with other external networks and VPN.

From the external network's point of view, the GGSN is simply a router to an IP sub-network. This is shown below. When the GGSN receives data addressed to a specific user in the mobile network, it first checks if the address is active. If it is, the GGSN forwards the data to the SGSN serving the mobile. If the address is inactive, the data is discarded. The GGSN also routes mobile originated packets to the correct external network.

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Host155.222.33.55

Corporate subnetwork131.44.15.xxx

GPRS subnetwork155.222.33.xxx

Host131.44.15.3

RouterRouter

LAN

Internet

Figure 4. GPRS network as seen by another data network

3.5 GPRS MS

Different GPRS MS classes were introduced to cope with the different needs of future subscribers. The mobiles differ in their capabilities.

classA

simultaneous• attach• activation• monitorno simultaneous traffic

simultaneous• attach• activation• monitor• invocation• trafficof GSM and GPRS

pure GPRS oralternative use of GSM and GPRS only

classB class

C

Figure 5. GPRS network as seen by another data network

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Network elements

Three GPRS MS classes were defined:

• Class A: With a class A mobile GSM circuit switched services and GSM GPRS services can be simultaneously activated. A subscriber can get data from an active GPRS link while simultaneously making a phone call. A class A mobile allows also a simultaneous attach, activation and monitor of the classical GSM and GPRS services.

• Class B: A class B mobile allows a simultaneous attach, activation and monitor of the circuit switched GSM and GPRS services. It does not allow a simultaneous transmission of user data on GSM and GPRS. For instance, a subscriber has established a GPRS data connection and receives data packets. A mobile terminating GSM circuit switched call is indicated. The subscriber accepts the call. While he is making the voice call, the GPRS virtual connection is “held or busy”, but no packet data transfer is possible. Having terminated the voice call, packet data can again be transmitted via the still existing GPRS virtual connection.

• Class C: A class C mobile is either a pure GPRS MS or it supports both GSM circuit switched services and GPRS. If it supports both then it can be used only in one of the two modes. If a subscriber switches his mobile into GPRS mode, he can originate or terminate GPRS calls, but he can no longer originate or terminate GSM circuit switched calls.

In GPRS and HSCSD, increased data rates can be achieved by channel bundling. Channel bundling is the allocation of several timeslots to a MS. In other words, the mobile stations have a multislot capability. In the specification 05.02, the individual GSM multislot MS classes are specified.

Maximum number of slots

Minimum number of slots

Type Multislotclass

Rx Tx Sum Tta Ttb Tra Trb 1 1 1 2 3 2 4 2 12 2 1 3 3 2 3 1 13 2 2 3 3 2 3 1 14 3 1 4 3 1 3 1 15 2 2 4 3 1 3 1 16 3 2 4 3 1 3 1 17 3 3 4 3 1 3 1 18 4 1 5 3 1 2 1 19 3 2 5 3 1 2 1 1

10 4 2 5 3 1 2 1 111 4 3 5 3 1 2 1 112 4 4 5 2 1 2 1 113 3 3 NA NA a) 3 a) 2

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14 4 4 NA NA a) 3 a) 215 5 5 NA NA a) 3 a) 216 6 6 NA NA a) 2 a) 217 7 7 NA NA a) 1 a) 218 8 8 NA NA 0 0 0 219 6 2 NA 3 b) 2 c) 119 6 3 NA 3 b) 2 c) 121 6 4 NA 3 b) 2 c) 122 6 4 NA 2 b) 2 c) 123 6 6 NA 2 b) 2 c) 124 8 2 NA 3 b) 2 c) 125 8 3 NA 3 b) 2 c) 126 8 4 NA 3 b) 2 c) 127 8 4 NA 2 b) 2 c) 128 8 6 NA 2 b) 2 c) 129 8 8 NA 2 b) 2 c) 1

a) = 1 with frequency hopping = 0 without frequency hopping.

b) = 1 with frequency hopping or change from Rx to Tx. = 0 without frequency hopping and no change from Rx to Tx.

c) = 1 with frequency hopping or change from Tx to Rx. = 0 without frequency hopping and no change from Tx to Rx.

3.6 Domain Name Servers

These devices convert IP names into IP addresses, for example, server.nokia.com to 133.44.15.5. There is a primary DNS server and a secondary DNS server. Details of DNS were described in Introduction to TCP/IP module, and information is also found in the IP CORE Course.

In the specifications, the DNS functionality is included in the SGSN. However, the main vendors have chosen to separate the DNS functions from the SGSN.

3.7 Firewalls

A firewall protects an IP network against external attack (for example, hackers from the mobile users or from the Internet). In the case of GPRS, the firewall might be configured to reject all packets that are not part of a GPRS subscriber-initiated connection. The firewall can also include NAT (Network Address Translation), see the Introduction to TCP/IP module.


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In the specifications for GPRS, the firewalls are not included. It is however included here due to the fact that operators usually need to implement firewalls in their GPRS network (for security reasons).

3.8 Border Gateway

The Border Gateway (BG) is a router that can provide a direct GPRS tunnel between different operators' GPRS networks. This is referred to as an inter-PLMN data network. It is more secure to transfer data between two operators' PLMN networks through a direct connection rather than via the public Internet. The Border Gateway will commence operation once the GPRS roaming agreements between various operators have been signed. It will essentially allow a roaming subscriber to connect to company intranet through the Home GGSN via the visiting PLMN network.

3.9 Charging Gateway

GPRS users have to be charged for the use of the network. In a GSM network, charging is based on the destination, duration, and time of call. However, GPRS offers connectionless service to users, so it not possible to charge subscribers on the connection duration. Charging has to be based on the volume, destination, QoS, and other parameters of a connectionless data transfer. These GPRS charging data are generated by all the SGSNs and GGSNs in the network. This data is referred to as Charging Data Records or CDRs. One data session may generate a number of CDRs, so these need to be collected and processed. The Charging Gateway (CG) collects all of these records, sorts them, processes it, and passes it on to the Billing System. Here the GPRS subscriber is billed for the data transaction. All CDRs contain unique subscriber and connection identifiers to distinguish it. A protocol called GTP' (pronounced GTP prime) is used for the transfer of data records between GSNs and the Charging Gateway.

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4 GPRS interfaces The GPRS system introduces new interfaces to the GSM network. Figure 6 illustrates the logical architecture with the interfaces and reference points of the combined GSM/GPRS network.

HLR

Air (Um)

Gb

GsGrGf

Gn

GpInter-PLMN

GPRSbackbone

Gp

Externalpacket network

GiSGSN

EIR

BSC

MSC/VLR

SMS-GMSC

Gd

GGSN

BG

Signalling and data

Signalling

GaGa

CG

SGSN

Gn

Gc

Figure 6. GPRS interfaces

Connections from the GPRS system to the NSS part of the GSM network are implemented through the SS7 network. The GPRS element interfacing with the NSS is SGSN. The important interfaces to the NSS are the SGSN-HLR (Gr), SGSN-EIR (Gf), and SGSN-MSC/VLR (Gs). The other interfaces are implemented through the intra-PLMN backbone network (Gn), the inter-PLMN backbone network (Gp), or the external networks (Gi).

The interfaces used by the GPRS system are described below:

• Um between an MS and the GPRS fixed network part. The Um is the access interface the MS uses to access the GPRS network. The radio interface to the BTS is the same interface used by the existing GSM network with some GPRS specific changes.

• Gb between a SGSN and a BSS. The Gb interface carries the GPRS traffic and signalling between the GSM radio network (BSS) and the GPRS network. Frame Relay based network services is used for this interface.


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GPRS interfaces

• Gn between two GSNs within the same PLMN. The Gn provides a data and signalling interface in the Intra-PLMN backbone. The GPRS Tunnelling Protocol (GTP) is used in the Gn (and in the Gp) interface over the IP based backbone network.

• Gp between two GSNs in various PLMNs. The Gp interface provides the same functionality as the Gn interface, but it also provides, together with the BG and the Firewall, all the functions needed for inter-PLMN networking, that is, security, routing, etc.

• Gr between an SGSN and the HLR. The Gr gives the SGSN access to subscriber information in the HLR. The HLR can be located in a different PLMN than the SGSN (MAP).

• Ga between the GSNs and the CG inside the same PLMN. The Ga provides a data and signalling interface. This interface is used for sending the charging data records generated by GSNs to the CG. The protocol used is GTP', an enhanced version of GTP.

• Gs between a SGSN and a MSC. The SGSN can send location data to the MSC or receive paging requests from the MSC via this optional interface. The Gs interface will greatly improve the effectiveness of the radio and network resources in the combined GSM/GPRS network. This interface uses BSSAP+ protocol.

• Gd between the SMS-GMSC and an SGSN, and between SMS-IWMSC and an SGSN. The Gd interface is available for more efficient use of the SMS services (MAP).

• Gf between an SGSN and the EIR. The Gf gives the SGSN access to GPRS user equipment information. The EIR maintains three different lists of mobile equipment: black list for stolen mobiles, grey list for mobiles under observation and white list for other mobiles (MAP).

• Gc between the GGSN and the HLR. The GGSN may request the location of an MS via this optional interface. The interface can be used if the GGSN needs to forward packets to an MS that is not active.

There are two different reference points in the GPRS network. The Gi is GPRS specific, but the R is common with the circuit switched GSM network:

• Gi between a GGSN and an external network. The GPRS network is connected to an external data networks via this interface. The GPRS system will support a variety of data networks. Because of that, the Gi is not a standard interface, but merely a reference point.

• R between terminal equipment and mobile termination. This reference point connects terminal equipment to mobile termination, thus allowing, for example, a laptop-PC to transmit data over the GSM-phone. The physical R interface follows, for example, the ITU-T V.24/V.28 or the PCMCIA PC-Card standards.

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5 Transfer of packets between GSNs User data packets are sent over the GPRS backbone in 'containers'. When a packet coming from an external packet network arrives at the GGSN, it is inserted in a container and sent to the SGSN. The stream of containers inside the GPRS backbone network is totally transparent to the user: To the user, it seems like he/she is connected directly via a router (the GGSN) to external networks. In data communications, this type of virtual stream of containers is called a tunnel. We say that the GSNs are performing tunnelling of user packets, see Figure 7.

UserpacketUser

packetUserpacket Userpacket

SGSN GGSN

The stream of containers forming a tunnel.

User

packet

Figure 7. User packets over the GPRS backbone in ‘containers’

The protocol that performs the tunnelling in GPRS is called GPRS Tunnelling Protocol (GTP). We can say that we transport GTP packets between the SGSN and the GGSN.

Over the GPRS backbone, IP packets are used to carry the GTP packets. The GTP packets then contain the actual user packets. This is shown in Figure 8. The user packet, for example, a TCP/IP packet that carries some part of an e-mail, is carried inside a GTP packet. The GTP packet is carried over the GPRS backbone using IP and TCP or UDP (in the example, UDP).

The GTP packet headers, including the tunnel ID (TID), will tell the receiving GSN who the user is. The tunnel ID includes the user IMSI (and another user specific number). The TID is a label that tells the SGSN and the GGSN, whose packets are inside the container.


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Transfer of packets between GSNs

User packetTunnel ID:

IMSI…

THE GTP PACKETIP (+TCP/UDP)

Who is the user?

To which GSN?

GSN IP-address

E.g. a TCP/IP packet carrying e-mail

Figure 8. GTP container

From the point of view of the user and the external network, the GTP packets that contain the user packets could be transferred between the GSNs using any technology, for example, ATM, X.25, or Frame Relay. The chosen technology for the GPRS backbone is IP.

All the network elements (the GSNs, the charging gateway, etc.) connected to the GPRS backbone must have an IP address. IP addresses used in the backbone are invisible to the MS and to the external networks. They are what we call private IP addresses. That is, the user packets are carried in the GPRS core between the SGSN and the GGSN using the private IP addresses of the GPRSbackbone.

This concept of tunnelling and hiding backbone addresses ('private') to the user level is illustrated in the following figures. Figure 9 shows a close-up of the user and backbone IP address levels. Figure 10 shows the GTP tunnel related to the user payload, and the relationship between the protocol stacks in the Gi and Gn interfaces.

GGSN

GTP

IP IP

IP backbone data using private IP addresses

SGSNMS

IP

GTPTunnel

user data using 'public' IP addresses

Figure 9. Transfer of packets between the GGSN and the MS

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BTS BSC

SGSN

GGSN Internet

GPRS Core

Network

SS7

HLRMSC/VLR

L1L2IP

GTP

USERPAYLOAD

UDP

TCP/UDPIP

APP

L1L2

Tunnelledpayload

Server

Figure 10. GTP tunnelling and user payload

Note For additional information on the GPRS transmission protocols, see the Appendix – GPRS transmission plane protocols.


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6 Nokia GPRS solution

6.1 Nokia GPRS functionality

This section describes the capabilities of the Nokia GPRS solution.

6.1.1 SGSN to MSC/VLR interface (Gs)

The Nokia solution includes the SGSN to MSC/VLR interface (Gs). For class-A and class-B mobile terminals, an association is made in the VLR and SGSN to indicate that they refer to the same physical mobile. This will give the following functions:

• Class-A and B GPRS mobiles can receive paging for circuit switched calls via GPRS channels. The GPRS is then suspended and the mobile moves to circuit switched mode to start the call.

• Combined routing area and location area attaches and updates.

• Combined detaches from GPRS and circuit switched services.

6.1.2 Cell reselection

There are no handovers as such in GPRS. Instead, there is cell reselection, which is made autonomously by the mobile. The parameters used by the mobile for cell reselection are sent from the network and can be different for each cell.

There is an option in the GPRS specifications to enable network controlled cell reselection. This requires GPRS terminals to send measurement results to the BSS.

6.1.3 SMS through GPRS

Nokia GPRS supports the sending of SMSs via the GPRS network. This is more efficient for GPRS mobiles using SMS since it frees up the GSM signalling channels for other purposes. SMS through GPRS is achieved by the Gd interface (SGSN to Gateway MSC). For each subscriber, a parameter in the HLR indicates if the SMS will be delivered via GPRS or via GSM channels.

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6.1.4 Charging

Charging information is collected by the SGSN and GGSN, and is then delivered to the Nokia Charging Gateway, where it is processed and forwarded to the billing system.

6.1.5 Roaming

A Border Gateway (see Figure 11) enables users to use a secure GPRS-tunnelled connection to their home network when roaming (via an inter-operator backbone network), rather than connecting via the public Internet.

Inter PLMNbackbonenetwork

GPRSBackboneIP Network

BorderGateway

GPRSbackboneIP network

GGSNGGSN Internet

Operator A(Home network)

Operator B

Roaming user Border Gateway

Secure GPRS Tunnelled Connection

Figure 11. Roaming via Border Gateway and Inter-PLMN network'

6.1.6 Quality of Service

In each cell, the total circuit switched capacity is available for use by GPRS dynamically. Circuit switched calls always have priority over GPRS traffic, so implementing GPRS will not reduce the quality of service (QoS) given to speech- and circuit switched data subscribers.

If a guaranteed minimum quality of service for GPRS users is required, it is possible to reserve a number of timeslots per cell that can only be used for GPRS traffic.

6.1.7 Access to Internet

Direct Internet Connection (transparent access) access is the simplest way to connect to the Internet, because all services are provided by the GGSN itself, or


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to be more exact, the access point (AP), which is associated with the physical interface to the Internet. This is illustrated in Figure 12.

GGSNGPRS

backboneIP Network

MS BSCBTS SGSNISPBackbone

AP

Ethernet

Figure 12. Transparent access - Direct Internet connection

The address and netmask of the AP will be dictated by the network configuration of the operator's ISP backbone. The Internet ISP backbone sees the AP as a router, and the mobile stations (MSs) as nodes in the AP's subnet.

6.1.8 Corporate access solutions

A key difference for an operator will be the ability to provide secure connections to corporate networks (and hence capture the corporate's complete mobile telecommunications business). The Nokia GPRS solution includes features that allow offering of secure and reliable corporate access solutions.

Multiple access points One Nokia GGSN can connect to many corporate intranets. For each GPRS subscriber, the access point (AP) used (that is, which external network to connect the user to), is defined in the subscriber data in the HLR.

DHCP and RADIUS server access The Nokia solution supports both static IP address allocation (defined in the HLR) and dynamic IP address allocation, either from:

• RADIUS/DHCP servers within the operators network, external ISP or corporate network, or from

• The GGSN's internal address pool.

The RADIUS and DHCP clients included in the Nokia GGSN can retrieve IP addresses from corresponding servers located, for example, in corporate networks. This also allows the subscriber to be authenticated against the corporate intranet server (non-transparent access). Small corporate intranets may not have a RADIUS server. In this case, the GGSN internal pool can be used to allocate IP addresses.

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Secure connections: Non-transparent access A dedicated and Virtual Private Network (VPN) connection or a non-transparent access to an intranet covers both corporate nets and independent ISPs. In this case, part of the service is provided outside the GPRS operator’s network. The AP must provide secure access to that external part, and co-operate with its infrastructure (for example authentication, address allocation, and accounting).

GGSNGPRS

backboneIP Network

Intranet

Internet

Service

Infra servers - RADIUS - DNS

MS BSCBTS SGSN

ISPBackbone

AP

AP

GTP

Figure 13. Non-transparent access - Dedicated and VPN connections

Whether the AP is connected to a VPN device or an intranet, it is regarded as a router and the MSs are part of its subnet. When the GPRS and intranet operators sign an interworking agreement, they should also define RADIUS addresses and (if necessary) DHCP server addresses. In addition, netmask and the default route can be exchanged when the agreement is made.

6.2 Nokia Base Station Subsystem (BSS)

The functions performed by the Nokia BSS elements to support GPRS include:

• Communication with GPRS MS using CS 1-4 and MCS 1-9 (EGPRS)

• Separation of circuit switched (CS) and packet switched (PS) traffic and sending of the PS traffic to SGSN

• Support of GPRS protocols such as RLC, MAC, BSSGP, and FR


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• Allocation of channels and radio blocks to MS using the USF flag

• Multiplexing and demultiplexing of data transmission on a TS

• Controlled by the NMS (network management system).

Most operators will have an existing GSM network that may have to be upgraded for GPRS. These changes are discussed in this section. The features available in the Nokia GSM/EDGE BSS10 are:

• Adaptive Multirate Codec (AMR), which can significantly improve speech quality and provide more capacity even in low C/I conditions.

• Enhanced Data Rates for Global Evolution (EDGE), which should increase the air interface throughput by a factor of 3. New EDGE-capable TRX will be introduced for the UltraSite and MetroSite.

• Tri Band - Common BCCH for GSM 900 and GSM 1800 TRX in the same cell

• GSM - WCDMA networking for inter-system handover and hence seamless services to end-users

• MS location services, which will enable location-based services to be offered.

• Automated radio network tuning with NetAct' OSS 3.1.

6.2.1 BTS

All Nokia 2nd-generation, Talk-family, Nokia PrimeSite and Nokia MetroSite BTSs support GPRS coding schemes CS1 and CS2 without any hardware changes.

6.2.2 BSC

Implementing GPRS has no effect on the capacity of the Nokia BSC to handle circuit switched calls. To support GPRS, new plug-in units are required in the BSC. These units are stated below:

• One Packet Control Unit (PCU unit) must be installed into each BCSU signalling unit of the BSC (one slot is vacant).

• One Switching Unit (SW64) must be installed into each Group Switch (GSW), since there are new internal PCM links with GPRS, and their numbering is fixed.

• If needed, new ET cartridges (ET5C) could also be installed. In a fully configured BSC, these might already be installed.

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Patipan Saengcharoen

Inserted Text

, E-GSM 900

Patipan Saengcharoen

Inserted Text

/BSS10.5


Figure 14. New plug-in units in BSC for GPRS

Nokia PCU unit

The PCU unit is a new plug-in unit of the BCSU. It takes care of switching GPRS traffic between Abis interface (TRAU frames) and Gb interfaces (FR) using the Group Switch (GSW). The PCU also supports the new protocols (RLC, MAC, FR) required for GPRS operation and handles GPRS channel allocation and radio resource management functions.

The following specifications apply to the PCU:

• BSC contains eight active and one redundant PCU in a n+1 configuration.

• One PCU can handle 256 GPRS channels, that is, TCHs of 32 TRXs.

• One PCU can be connected to a maximum of 64 BTSs and/or 64 cells.

• The data processing capacity of one PCU is 2 Mbit/s.

• A BSC must be fully (hardware) equipped, that is, each BCSU must have one PCU, but the number of active PCUs is determined by software licence.


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• Each PCU has a separate frame relay interface to the SGSN (Gb interface). This can be configured in 64 kbit/s steps, from two to 31 PCM timeslots.

6.3 Nokia Core Network Subsystem (CNS)

6.3.1 MSC/VLR

The MSC/VLR is not involved in GPRS data transfer, but supports signalling for class-A and class-B mobiles as well as SMS delivery.

To allow support for terminals attached to both GSM and GPRS services (A and B type), Nokia has implemented the Gs interface between the MSC/VLR and the SGSN. This interface supports paging and combined LA and RA update procedures for class-A and B mobiles.

6.3.2 HLR and EIR

The Nokia-combined Home Location Register (HLRi), Authentication Centre (AC), and Equipment Identity Registry (EIR) is based on the DX200 switching platform.

As for circuit switched services, subscriber information for GPRS is stored in the Home Location Register (HLR). The HLR supports procedures such as GPRS attach/detach and authentication. Nokia has implemented interfaces between the HLR and SGSN (Gr) and the Equipment Identity Register (EIR) and SGSN (Gf). The interface is implemented using MAP v.3.

For SMS support, one new parameter per subscriber has been added in the HLR to indicate whether short messages should be delivered via the MSC or the SGSN.

6.3.3 Nokia Serving GPRS Support Nodes

The Serving GPRS Support Node (SGSN) of a GPRS network is equivalent to the MSC of a GSM network. It combines the functions of a digital switching platform with the functions necessary for interfacing between the GSM Base Station Subsystem (BSS) and IP backbone networks.

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Figure 15. The Nokia SGSN

The SGSN is connected to one or several BSCs (Base Station Controllers) by the Gb interface. The number of BSCs connected to a SGSN depends on the amount of data traffic expected. One SGSN can support BSCs working under several different MSCs. The SGSN can be physically located at the MSC or BSC site. There must at least one SGSN in a GPRS network.

The SGSN is connected to the GPRS network via the GPRS backbone. The GPRS backbone is a private network that provides IP connectivity between the GPRS network elements in order to carry signalling, traffic and charging data. This network is not directly accessible from the public Internet.

The primary function of the Nokia SGSN is to convert the IP network protocol to the protocols used in the BSS and the mobile terminal. It then passes the data to the relevant GGSN when a connection to an external data network is required. Additional functions performed by the Nokia SGSN include mobile terminal authentication and mobility management, as well as user data compression to and from the terminal. Finally, it handles signalling interfaces with the MSC/VLR and HLR, collects charging and statistical information, and provides flexible network management interfaces.

The Nokia SGSN consists of a number of functional units, each with its own processor and back-up facility carrying out a number of tasks. These functional units have independent tasks, but communicate when and as necessary using a common message bus. It is not the intention of this section to give a complete explanation of all the units in this section, but only a simple overview of them to give an understanding.

The SGSN consists


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• Packet Processing Unit (PAPU) The main purpose of the PAPU is to process user data and protocol conversion between the BSS and the GPRS backbone network and vice versa. It handles the GPRS mobility management (GMM) procedures such as GPRS attach, location management, and GPRS detach. It is responsible for session management (SM) including PDP context activation, modification, and deactivation, as well as SMS delivery. It takes care of ciphering and compression between the MS and SGSN. Up to 16 + 1 PAPUs can be found in a Nokia SGSN

• Signalling and Mobility Management Unit (SMMU) The main purpose of the SMMU is to support subscriber mobility management using SS7-based interfaces Gr, Gd, Gs, and Gf. It has a temporary storage for GPRS subscriber data called visiting GPRS subscriber database, similar to the VLR in a MSC. If Gs interface is implemented, the SMMU handles all the combined GPRS/GSM mobility management for those GPRS and IMSI attached subscribers that have class A or B capabilities. It supports the SMS delivering procedure forwarding the short messages either to SMS-GMSC through Gd interface or to the MS after checking related subscriber information on the visiting GPRS subscriber database. Up to 4 + 1 SMMU can be within a Nokia SGSN.

• Operation and Maintenance Unit IOMU) The OMU acts as an interface between the user and the exchange and takes automatic recovery measures, as needed, based on its collected fault data. The tasks of the Operation and Maintenance Unit include traffic control functions, maintenance, system configuration administration, and system management.

• Marker and Charging Unit (MCHU) The Charging Unit within the MCHUcollects and stores charging information, generates call detail records (CDRs) and transfers them to Charging Gateway (CG) through the Ga interface. If CG is not present in the GPRS network, the CDRs are saved and transferred off-line to Billing and Customer Care System (BCCS). The Marker within the MCHU controls and supervises the group switch (GSW), hunts free circuits, and is responsible for establishing and releasing all connections.

• a Group Switch (GSW) used for semi-permanent connections between the SGSN and the BSS, and the SGSN and the NSS

• units to connect transmission systems to the GSW (Exchange Terminal, ET), and

• a high-speed Message Bus (MB) for interconnecting computer units

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ExchangeTerminal

(ET)

Gr,Gs, Gd(to NSS) Group

Switch(GSW)

Clock andSync. Unit

(CLS)ExchangeTerminal

(ET)

GroupSwitch

(GSWB)

Clock andSync. Unit

(CLS)

PacketProcessing

Unit(PAPU)

Signalling &Mobility

Man. Unit(SMMU)

Operation &Maintain.

Unit(OMU)

Marker &Charging

Unit(MCHU)

WDU

N+1 N+1 2N 2NGn (to GGSN)

Charging

Gb(to BSC)

WDU

Figure 16. Logical architecture of the Nokia SGSN

The main units supporting GPRS are PAPU and SMMU. Each of the functional units is described in the following chapters. The SMMU and PAPU have N+1 redundancy, whereas the MCHU, OMU, MB, GSW and CLS have 2N redundancy.

6.3.4 Nokia Gateway GPRS Support Nodes

The Gateway GPRS Support Node (GGSN) provides the interconnection between the GPRS network and external packet data networks such as the Internet. The main functions of the GGSN are:

• Interfacing the GPRS backbone to external data networks

• GTP tunnelling to the SGSN

• Gathering of charging and statistics information

• Network management interfaces

• Dynamic IP address allocation to mobile station (MS)

• Protecting the GPRS backbone from external attacks.

Charging in GGSN Various charging information is provided, for example, data volume uplink or downlink, PDP context active time, tariff change, and access point name. This relates to the network or services used, static or dynamic IP address usage, PDP IP address, and cause for record close.


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GGSN interworking services The allocation of IP addresses can be carried out by one of the following:

• GGSN allocates the address from an internal pool.

• Internal DHCP client can be used to access a DHCP server in the intranet or ISP network.

• Internal RADIUS client can be used to access a RADIUS server in the intranet or ISP network.

The exact functions provided by interconnecting networks depend on which of the following three connection options is required (see Figure 17):

• Direct Internet connection to an existing ISP infrastructure owned by the operator

• Dedicated connection to an intranet outside the operator's network.

• Indirect connection to an intranet employing a Virtual Private Network (VPN). The virtual connections run between the VPN software in the GGSN and the VPN software in the accessed intranet.

Figure 17. Various connections between GPRS and external networks

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6.3.5 Nokia Charging Gateway (CG)

CG and CDRs In the GPRS standardisation, ETSI gives the following two possibilities to implement the CG functionality:

• Stand-alone

• Integrated in GSNs (SGSN and GGSN).

In the Nokia GPRS solution, the CG is a stand-alone network element, as shown in Figure 18. Why is that?

• Better service for the operator: A separate mediation device is not needed. If CG is in GSNs, a mediation device is needed because GSNs are not able to send Charging Detailed Records (CDRs) to the Customer Care and Billing System (CCB). If GSNs send CDRs to the CCB, there are as many access points to the CCB as there are GSNs. This means more workload.

• Each CG has one access point to the CCB.

• An ETSI-specified real-time transfer protocol, GTP' (enhanced GPRS Tunnel Protocol), is used to transfer CDRs to the CCB.

When a subscriber switches the terminal on, the PDP context is activated and the GGSN starts to send CDRs to the CG. The GGSN informs SGSN to which CG unit it sends CDRs.

SGSN

OperatorIP backbone

GGSNBorder Gateway

Inter operatorIP network Internet

Billing SystemCharging gatewayGTP'

GTP'

Figure 18. Implementation of the Nokia CG in the GPRS backbone

GGSN has only one type of CDRs, that is the G-CDR,

The SGSN sends four CDR types:


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1. S-CDR – The content is like G-CDR plus some other fields.

2. M-CDR is for mobility management.

3&4. SMS-CDR

The GPRS Charging Gateway receives CDRs from GGSNs and SGSNs using a real-time transfer protocol (GTP’). The CG forms an intermediate storage for the CDRs. Therefore, no long-term storing capabilities are required at GSNs.

In normal conditions all CDRs from the same PDP context are transferred to the same CG. If one CG is down, a redundancy procedure built in the GTP' protocol allows the transfer of CDRs from the GSNs towards other CGs.

CG functionality The Nokia Charging Gateway will consolidate and pre-process the CDRs before passing them to the billing system. Standard billing system interfaces FTAM, FTP over TCP/IP and NFS are implemented in CG. The CG implementation enables the auditing and tracing of every phase of CDR processing. The CG validates and consolidates CDRs, and produces them in a format suitable for the Customer Care and Billing System (CCBS).

Nokia CG benefits CG offers the following major benefits:

• Reliable storage capability for CDRs, eliminating the need for costly and failure risk storage capabilities at the GSNs

• Fast transfer of CDRs from the GSNs and CG hot billing internal architecture are future-proof for hot billing and prepaid, enabling credit control and fraud detection in an early phase.

• Automated validation makes it easier to detect errors.

• Reduction of the CCB processing load by pre-processing and consolidating

• The audit trail function enables tracing of CDR-processing phases and CDR contents, for example, in case of customer complaints.

An overview of the CG functions and management is shown in Figure 19.

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SGSN

GGSN

BillingSystem

CG

• Receiving CDRs from GSNs• Real-time transfer protocol• Fixed CDR format (Rel 1)

• Intermediate CDR storage• CDR consolidation• Pre-processing• Handling erroneous CDRs

• Interfaces towards Billing System / CCB

• CDR transfer towards BS

• Configuration management• CDR transfer settings• backup/restore• tariff changes

• Fault management• current alarms• alarm history

• Performance management• KPIs

• Time management• synchronising with other NEs

• Security management• password settings

• Management methods• SNMP

Charging Gateway Management

Figure 19. CG functions

6.4 Nokia Network Management System (NMS)

The Nokia solution to support the operation of a network is known as the Nokia NetAct. The structure of the solution is not built around the technology but the functions and processes that an operator must perform to ensure the operation of the business.

Nokia has launched the Nokia NetAct Framework in order to support the transition from 2G to 3G. It also extends the multivendor integration capability of the Network Management System (NMS).

NetAct provides a full-scale management capability for both packet data and traditional voice traffic, independent of technology. Thanks to this, it is possible to deploy new technologies with the same system that manages the current infrastructure.

The following figure illustrates the different functions that the NetAct supports. All the functions are brought together in a common framework and are connected to the physical network elements through a UMA (Unified Mediation and Adaptation) object, which allows NetAct to talk to Nokia elements and 3rd party systems.


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Workflow ManagerWorkflow Manager

Reporter Reporter

Administrator Administrator

3rd Party Tools 3rd Party Tools

Rating & Charging

Rating & Charging

Planner Planner

Configurator & Provisioning

Configurator & Provisioning

Monitor Monitor

Service QualityManager

Service QualityManager

Unified Mediation and Adaptation

Unified Mediation and Adaptation

Network Network

Common networktypology

Common WEB GUI

Figure 20. Nokia NetAct Framework

The Nokia network elements (e.g. RNC, AXC, BTS) themselves provide the necessary functions on commissioning, setting up, or troubleshooting the individual equipment. Nokia NMS − sitting on top of the managed network elements − provides tools for making large-scale modifications at the network level.

BusinessManagementSystems

ServiceManagementSystems

NetworkManagementSystems

ElementManagementSystems

Functions of Service management systems:• take care of subcriber data• provision services and subscribers• collect and rate, bill offered services• create, promote and monitor services

Network management system (NMS):• collect information from the underlying networks

and pre/post-process the raw data• analyse and distribute information• optimise network capacity and quality

Element management systems (EM):• EMs are part of the NE (RNC, BTS, AXC,etc.)

functionality• monitor the functioning of the equipment• collect raw data (performance indicators)• local GUI provided for site engineers• mediate towards the NMS system

MIS

PlanningSystem

BTS

BTS AXCRNC NE x

NE y

CCB

NE = network element, CCB = customer care and billing, MIS = management information system

NMS

Figure 21. How the TMN is visualised in the Nokia GSM/UMTS solution

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6.4.1 IP configuration management

In the Nokia NMS solution, the technology differences between IP and other subnetworks have been scaled down, so shared management methods can be used. This results in major savings, both in time and O&M resources. IP core network planning can be integrated into the operator's general network planning process. The tasks include capacity planning and IP routing set-up in order to ensure proper functionality of this transport network. In the element management layer, access to the actual element can be supported by web-based applications, so the needed configurations of the IP elements are always visible to O&M personnel from any O&M screen. Special emphasis has been placed on security management applications.

Internet / Intranet

Management Data

IP Core DevelopmentIP Core Development• IP Core Planning• IP Core Implementation• IP Core Analysis

Nokia NMS for UMTSNokia NMS for UMTS• Shared Applications• Common Resources• Shared Processes

Element Management

Figure 22. Management of the IP backbone in the Nokia NetAct


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Key points

7 Key points

7.1 GPRS architecture: key points

• A GPRS network is expected to perform the functions of GSM network and data network.

• The new elements of the GPRS network are the PCU, SGSN, GGSN, CG, BG, DNS, and Firewalls.

• The functions of the Gateway GPRS Support Node (GGSN)are the following:

− Routing mobile-destined packets from external networks to relevant SGSN

− Routing packets originating from an MS to the correct external network

− Interfacing to external IP networks − Collecting charging data and traffic statistics − Allocating dynamic IP addresses to mobiles either by itself or with

the help of a DHCP or a RADIUS server.

• The functions of the Serving GPRS Support Node (SGSN) are the following:

− Converting protocols used in IP backbone to protocols used in the BSS and MS

− Handling of authentication and mobility management − Routing data to relevant GGSN when connecting to an external

network − Collecting charging data and traffic statistics − Handling of ciphering and data compression.

• The interfaces in the GPRS network are the following:

− Gb SGSN to BSS − Gn between GSNs (GTP) − Gr between SGSN and HLR (MAP) − Gs SGSN to MSC (BSSAP+)

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− Gi GGSN to external data networks − Gf SGSN and the EIR (MAP) − Gd between SGSN and the GMSC (SMSC) − Ga between GSNs and CG.

• Tunnelling is the process by which user packets are transported encapsulated in containers and transported through a network.

• The tunnelling protocol in GPRS is called the GPRS Tunnelling Protocol (GTP) over the GPRS backbone. The backbone is an IP network.

• Tunnelling is used when:

a. the packets with private IP addresses have to transmitted through a public network

b. packets of one protocols have to be sent through a network that does not understand it

c. for security reasons.

7.2 Nokia GPRS solution: key points

• The components of the Nokia GPRS solution are: the Nokia SGSN, the Nokia GGSN, the Nokia LIG, Nokia Charging Gateway (CG), Nokia BSC, the Nokia NMS for GPRS and the Nokia BTSs.

• The features available in the Nokia GSM/EDGE BSS10 are adaptive multirate codec (AMR), Enhanced Data Rates for Global Evolution (EDGE), Tri Band - Common BCCH, GSM-WCDMA networking, MS location services, Automated radio network tuning.

• The capacity of the Nokia SGSN:

− Maximum attached subscriber capacity 120 000 (SG1)/ 240 000 (SG2)

− Up to 300 000 short messages in the busy hour

− Mean switching capacity of 48 Mbit/s (SG1) and 100 Mbit/sec (SG2)

− Capacity can be configured in steps of 25% of maximum capacity.

• The Nokia CG is a stand-alone element, which collects CDRs from GSNs. The CG validates and consolidated CDRs, and produces them in a format suitable for the Customer Care and Billing System (CCBS).

• The new features in T12 are Capacity Indication Tool, Capacity Estimation Tool, NMS backup, Service Access Control, Multivendor Integration ( Corba Integration Kit, ASCII Alarm Forwarding feature), Radio Network Management (Automatic Picocell Planning, Channel


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Finder), GPRS backbone management (Configuration, Firewall and VPN Management, Backbone Name and Address Management), GPRS Performance Management (ASCII Interface for GPRS Measurement Data, Basic Report Set) and Routing Area IP manager.

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8 Review questions 1. Draw the GPRS architecture including the main GPRS network elements.

2. What are the functions of SGSN?

3. What are the functions of GGSN?

4. Which interfaces exist in GPRS (draw into sketch in Question 1)?

5. Which protocol is used on each of the interfaces?

6. What is GTP?

7. Why is GTP used?

8. What modifications need to be performed to Nokia BTS?


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Review questions

9. What are the important specifications for the Nokia PCU solution?

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9 Appendix – GPRS transmission plane protocols

1. Overview of protocols used in GPRS

A GPRS network introduces many new protocols designed to convey user data in a reliable and secure way. Information is passed between the existing GSM network and the GPRS network by employing protocols on two separate planes:

• Transmission plane protocols are used for the transmission of user data and control functions.

• Signalling plane protocols are used to convey signalling information that controls and supports the transmission plane functions.

The transmission plane protocols convey user data in the form of IP datagrams from the mobile station to external networks, such as the Internet or corporate data networks.

The signalling plane contains many protocols that are already employed in existing GSM network elements.

2 GPRS transmission plane protocols

When looking at the transmission protocols, we can assume, that a connection between the MS and the external PDN has been established. The network elements GGSN, SGSN, PCU/BSC, BTS/CCU are now responsible for the user data transfer. The user data transfer can be decomposed into several steps:

• The user data has to be transmitted from the external PDN to the GGSN; and it has to be transmitted from the GGSN to the external PDN. The user data transfer is organised via the interface Gi.

• The subscriber’s packets (PDU) have to be exchanged between the GGSN, which is interfacing the external PDN, and the SGSN, which is interfacing the BSS and locally serving the GPRS Ms. The user data transfer between the two network entities is organised based on the Gn-interface specification.

• User data has to be transmitted in a well defined way between the SGSN and the MS. The transmission is organised on a logical level peer-to-peer between the SGSN and GPRS MS. This is done with the protocols SNDCP and LLC (see figure below).

• The SNDCP and LLC protocol are used to specify the user data exchange between the GPRS MS and the SGSN on a logical, peer-to-peer level. Lower layer protocols define, how user data is exchanged


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Appendix – GPRS transmission plane protocols

between the BSS network elements and SGSN and GPRS MS. The protocols BSSAP and NS are responsible for user data transmission between SGSN and PCU, while

• the protocols RLC, MAC, and GSM RF (Radio Frequency Layer and Physical Link Layer) organise the user data transport between PCU + CCU and the GPRS MS. (see figure below).

(Note: the subject is easier to understand for a beginner, if he assumes the PCU and CCU to be realised at the BSC site.)

MAC

GSMRF

RLC

LLC

SNDCP

IP/X.25

MAC

GSMRF

RLC

NS

L1bis

BSSGP

Relay

NS

L1bis

BSSGP

LLC

SNDCP

L1

L2 L2

IP IP

UDP /TCP

UDP /TCP

GTP GTPRelay

IP/ X.25

MS BSS SGSN GGSNUm Gb Gn Gi

L1

BSSGP: BSS GPRS Protocol LLC: Logical Link Control SNDCP: SubNetwork DependentNS: Network Service MAC: Medium Access Control Convergence ProtocolGTP: GPRS Tunnelling Protocol RLC: Radio Link Control TCP: Transmission Control Protocol IP: Internet Protocol UDP: User Datagram Protocol

L2‘

IP/X.25

L1‘

Relay

L2‘

IP/X.25

L1‘

L2‘

IP/X.25

L1‘

Relay

Application/ Higher level protocols

Rou-ter

Figure 23. Transmission plane protocols

2.1 Transmission protocols in the Gn interface

The Gn interface forms the GPRS backbone network. The protocol layers L1, L2, IP, and TCP/UDP form a transmission network solution, based on standard Internet protocols or standard layer 1 and 2 link protocols. Only the highest layer protocol, the GPRS Tunnelling Protocol (GTP) is GPRS specific.

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2.1.1. Transport

The L1 and the L2 protocols are vendor dependent OSI layer 1 and 2 protocols that carry the IP datagrams for the GPRS backbone network between the SGSN and the GGSN.

The Internet Protocol (IP) datagram in the Gn interface is internally used in the GPRS backbone network. The GPRS backbone (core) network and the GPRS subscribers use different IP addresses. This makes the GPRS backbone IP network invisible to the subscribers and vice versa. The GPRS backbone network carries the subscriber IP or X.25 traffic in a secure GPRS tunnel.

All data from the mobile subscribers or external networks is tunnelled in the GPRS backbone.

TCP or UDP are used to carry the GPRS Tunnelling Protocol (GTP) PDUs across the GPRS backbone network. TCP is used for user X.25 data and UDP is used for user IP data and signalling in the Gn interface.

GGSNSGSNPDN

(z.B. X.25, IP)Gi

IP/X.25

L2Link Layer

IPInternetProtocol

UDPUser

DatagramProtocol

TCPTransmission

ControlProtocol

GTPGPRS

TunnellingProtocol

L1Physical

Layer

L2‘Link Layer

L1‘Physical

Layer

IP / X.25

Relay

L2‘Link Layer

L1‘Physical

Layer

IP / X.25

L2Link Layer

IPInternetProtocol

UDPUser

DatagramProtocol

TCPTransmission

ControlProtocol

GTP

L1Physical

Layer

Gn

• outside thescope of the Rec.

• depends on agreement

• PDU en-/de-capsulation

• Tunnelling protocol between GSNs

• Signalling between GSNs

• TCP: reliable• UDP: unreliable but fast

• UDP = minimum solution in NSS

• IPv4 or IPv6• path selection (next hop)

• datagram format adjustment

• outside the scope of the Rec.

• operator dependent

Figure 24. . GPRS Tunnelling Protocol principle

2.1.2 GPRS Tunnelling Protocol (GTP)

The GPRS Tunnelling Protocol (GTP) allows multi-protocol packets to be tunnelled through the GPRS backbone between GPRS Support Nodes (GSNs). This is illustrated above.

The GTP can have proprietary extensions to allow proprietary features. The relay function in the SGSN relays the user PDP (Packet Data Protocol) PDUs (IP or X.25) between the Gb and the Gn interfaces.


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GTP is defined both for the Gn interface, that is, the interface between GSNs within the same PLMN, and the Gp interface between GSNs in different PLMNs.

The UDP/IP and TCP/IP are examples of paths that may be used to multiplex GTP tunnels. The choice of path is dependent on whether the user data to be tunnelled requires a reliable link or not. Two modes of operation of the GTP layer are therefore supported for information transfer between the GGSN and SGSN.

• unacknowledged (UDP/IP)

• acknowledged (TCP/IP).

A UDP/IP path is used when the user data is based on connectionless protocols, such as IP. A TCP/IP path is used when the user data is based on connection-oriented protocols, such as X.25.

The GTP layer supports both modes simultaneously.

In the transmission plane, the tunnel created by the signalling plane is used to carry user data packets between network elements connected to the GPRS backbone network, such as the SGSNs and GGSNs. No other systems need to be aware of GTP, for example, the MSs are connected to a SGSN without being aware of GTP.

UDPHeaderUDP

Header

GTPHeaderGTP

Header

user IP Data

GPRSBackbone IP

Header

GPRSBackbone IP

Header

User Data Payload (T-PDU)(user IP Data)

User Data Payload (T-PDU)(user IP Data)

GTPHeaderGTP

HeaderUser Data Payload (T-PDU)

(user IP Datagram)User Data Payload (T-PDU)

(user IP Datagram)

UDPHeaderUDP

HeaderGTP

HeaderGTP

HeaderUser Data Payload (T-PDU)

(user IP Datagram)User Data Payload (T-PDU)

(user IP Datagram)

UDP Layer

GTP Layer

BackboneIP Layer

Figure 25. The GTP protocol header being added to user data

A GTP tunnel is defined by two associated PDP contexts in different GSN nodes and is identified by a Tunnel ID (TID). A GTP tunnel is necessary for forwarding packets between an external packet data network and an MS. The Tunnel ID identifies the MM and PDP contexts (MM Context ID and a NSAPI).

The NSAPI (Network Service Access Point Identifier) is a fixed value between 0 and 15 that identifies a certain PDP context. It identifies a PDP context belonging to a specific MM context ID.

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The GTP header

The GTP header contains 16 octets and is used for all GTP messages.

The information contained in the GTP header includes the following:

• The type of GTP message (signalling messages = 1-52, but when used for data transmission the GTP message type = 255).

• The length of the GTP message (G-PDU) in octets.

• A Sequence Number to provide a transaction identity for signalling messages and a growing sequence number for tunnelled T-PDUs. (A T-PDU is an IP datagram from an MS or a network node in an external packet data network. The T-PDU is the payload that is tunnelled in the GTP tunnel).

• A flag to indicate whether an LLC frame number is included or not.

• An LLC frame number that is used for the Inter SGSN Routing Update procedure to co-ordinate the data transmission on the link layer between the MS and the SGSN.

• A TID (Tunnel Identifier) that points out MM and PDP contexts.

The content of the GTP header differs depending on whether the header is used for signalling messages or user data (T-PDUs).

Tunnel ID (TID) format

The Tunnel Identifier (TID) consists of the following:

• Mobile Country Code (MCC)

• Mobile Network Code (MNC)

• Mobile Subscriber Identification Number (MSIN)

• Network Service Access Point Identifier (NSAPI)

These represent the MM and PDP contexts.

2.2. Higher layer peer-to-peer transmission between SGSN and MS The protocols are responsible for a peer-to-peer user data transmission between MS and SGSN:

• Subnetwork Dependent Convergence Protocol Layer (SNDCP)

• Logical Link Control Layer (LLC).


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SGSN

SNDCPSubNetworkDependent

ConvergenceProtocol

FRFrame Relay

L1bisPhysical

Layer

BSSGPBSS

GPRSProtocol

LLCLogical Link

Control

MS

SNDCP

RLCRadio Link

Control

MACMedium Access

Control

GSM RF

IP / X.25

LLC

Um Gb

•compression•multiplexing/de-multiplexing •segmentation & re-assembly

• logical connection• acknowledge/ unacknowledged peer-to-peer operation

• ciphering• SAPs to higher layer (SNDCP, GMM, SMS)

MAC

GSMRF

RLC

FR

L1bis

BSSGP

Relay

Figure 26. Higher Layer peer-to-peer user data transport

2.2.1 Logical Link Control (LLC)

The Logical Link Control (LLC) layer offers a secure and reliable logical link between the MS and the SGSN for upper layer protocols. It is independent of lower layers.

The LLC layer is responsible to transmit

• signalling and control information,

• SMS, and

• Subnetwork Dependent Convergence Protocol (SNDCP) packets. SNDCP provides a mapping and compression function between the network layer (IP or X.25 packets) and the lower layers. It also performs segmentation, reassembly, and multiplexing.

Two modes of operation of the LLC layer are defined for information transfer: unacknowledged and acknowledged. The LLC layer can support both modes simultaneously.

In acknowledged mode, the receipt of LLC-PDUs is confirmed. The LLC layer retransmits LLC-PDUs if confirmation has not been received within a certain timeout period.

In unacknowledged mode, there is no confirmation required for LLU-PDUs.

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Signalling and SMS is transferred in unacknowledged mode.

In unacknowledged mode, the LLC layer offers he following two options:

• Transport of "protected" information means that if errors occur within the LLC information field, the frame will be discarded.

• Transport of "unprotected" information means that if errors occur within the LLC information field, the frame will not be discarded.

The LLC layer supports several different QoS delay classes with different transfer delay characteristics.

The network layer protocols for signalling, SMS, and user data are multiplexed to the lower layers in the following way :

• SAPI is the Service Access Point Identifier, which is used to identify the points where the LLC provides a service to a higher layer. SAPIs have different priorities.

• TLLI is the Temporary Logical Link Identity, which unambiguously identifies the logical link between the MS and SGSN. TLLI is used for addressing at the LLC layer.

Figure 27. Multiplexing of network protocols

LLC provides the services necessary to maintain a ciphered data link between an MS and an SGSN.


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The LLC connection is maintained as the MS moves between cells served by the same SGSN. When the MS moves to a cell being served by a different SGSN, the existing connection is released and a new logical connection is established with the new SGSN.

LLC is independent of the underlying radio interface protocols. In order to allow LLC to operate with a variety of different radio interface protocols, and to ensure optimum performance, it may be necessary to adjust, for example, the maximum LLC PDU length and the LLC protocol timer values. Such adjustments can be made through negotiation between the MS and the SGSN. The maximum length of an LLC PDU shall not be greater than 1600 octets minus the BSSGP protocol control information.

A logical communication pipe is established between the GGSN and the MS through a SGSN. The LLC protocol link is established between the MS and the SGSN upon GPRS attach. The GPRS Tunnelling Protocol (GTP) establishes a tunnel between the SGSN and the GGSN at PDP context activation. In the LLC header, the NSAPI (Network layer Service Access Point Identifier) identifies which application inside the MS the packet belongs to.

2.2.2 SNDCP (Subnetwork Dependent Convergence Protocol)

Network layer protocols are intended to be capable of operating over a wide variety of subnetworks and data links. GPRS supports several network layer protocols providing protocol transparency for the users of the service.

To enable the introduction of new network layer protocols to be transferred over GPRS without any changes to GPRS, all functions related to the transfer of Network layer Protocol Data Units (N-PDUs) are carried out in a transparent way by the GPRS network. This is one of the requirements of SNDCP.

Another requirement of the SNDCP is to provide functions that help to improve channel efficiency. This is achieved by means of compression techniques.

The set of protocol entities above the SNDCP consists of commonly used network protocols. They all use the same SNDCP entity, which then performs multiplexing of data coming from different sources to be transferred using the service provided by the LLC layer. The Network Service Access Point Identifier (NSAPI) is an index to the PDP context of the PDP that is using the services provided by the SNDCP (see Figure 4). One subscriber may have several PDP contexts and NSAPIs.

Each active NSAPI uses the services provided by the Service Access Point Identifier (SAPI) in the LLC layer. More than one NSAPIs may be associated with the same SAPI.

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Figure 28. SNDCP used to multiplex different protocols

2.3 Transmission protocols in the Gb interface (BSS layers)

SGSN

SNDCPSubNetworkDependent

ConvergenceProtocol

FRFrame Relay

L1bisPhysical

Layer

BSSGPBSS

GPRSProtocol

LLCLogical Link

Control

BSS (PCU)

BSSGPBSS

GPRSProtocol

FRFrame Relay

L1bisPhysical

Layer

Gb

•LLC frame transmission (transparent)

• routing information•QoS information•no error correction

unreliable BSSGPPDU transport

E1/T1 (PCM30/PCM24)

Figure 29. BSSAP, NS, and L1bis


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The Gb interface allows many users to be multiplexed over the same physical link using Frame Relay (FR). Bandwidth is allocated to a user upon activity (when data is sent or received) and is reallocated immediately thereafter. This is, in contrast to the A interface, where a single user has the exclusive use of a dedicated physical resource throughout the lifetime of a call irrespective of activity.

GPRS signalling and user data are sent in the same transmission plane and therefore no separate dedicated physical resources are required to be allocated for signalling purposes.

Data rates over the Frame Relay Gb interface may vary for each user without restriction, from zero data to the maximum possible line rate (for example 1984 kbit/s, which is the maximum available bit rate of a 2Mbit/s (E1) link).

2.3.1 Physical Layer Protocol

Several physical layer configurations and protocols are possible at the Gb interface and the physical resources are allocated by Operation & Maintenance (O&M) procedures. Normally a G703/704 2Mbit/s connection is provided.

2.3.2 Network Services layer

The Gb interface Network Services layer is based on Frame Relay. Frame Relay virtual circuits are established between the SGSN and BSS. LLC PDUs from many users are statistically multiplexed onto these virtual circuits. These virtual circuits may traverse a network of Frame Relay switching nodes, or may just be provided on a point to point link between the PCU and the SGSN (if the PCU and SGSN are co-located). Frame Relay is used for signalling and data transmission over the Gb interface.

The following characteristics apply for the Frame Relay connection:

• The maximum Frame Relay information field size is 1600 octets.

• The Frame Relay address length is two octets.

• Frame Relay Permanent Virtual Circuits (PVC) are used.

• The Frame Relay layer offers detection of errors, but no recovery from errors.

• One or more Frame Relay PVCs are used between an SGSN and a BSS to transport BSSGP PDUs.

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2.3.3 Base Station System GPRS Protocol (BSSGP)

The Base Station System GPRS Protocol (BSSGP) transfers control and signalling information and user data between a BSS and the SGSN over the Gb interface.

The primary function of BSSGP is to provide Quality of Service (QoS), and routing information that is required to transmit user data between a BSS and an SGSN.

A secondary function is to enable two physically distinct nodes, the SGSN and PCU, to operate node management control functions.

There is a one-to-one relationship between the BSSGP protocol in the SGSN and in the BSS/PCU. If one SGSN handles multiple BSSs/PCUs, the SGSN has to have one BSSGP protocol device for each BSS/PCU.

The main functions for the BSSGP protocol are to:

• provide a connectionless link between the SGSN and the BSS

• transfer data in an unconfirmed way between the SGSN and the BSS

• provide for bi-directional control of the data flow between the SGSN and the BSS

• handle paging requests from the SGSN to the BSS

• give support for deleting old messages in the BSS, for example when an MS changes BSSs

• support multiple layer 2 links between the SGSN and the BSS.

2.4 Transmission protocols in the Um interface (BSS protocols) BSS (PCU, CCU)MS

SNDCP

RLCRadio Link

Control

MACMedium Access

Control

GSM RFphy. Link & RF

IP / X.25

LLC

Um

RLCRadio Link

Control

MACMedium Access

Control

GSM RFphy. Link & RF

•LLC segmentation/ re-assembly•acknowledged/ unacknowledged mode

•Backward Error Correction BEC

•Access signalling procedures•physical channel bundling•sub-multiplexing•physical channel organisation•channel coding•GSMK

Figure 30. RLC, MAC, and physical layers


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2.4.1 Physical layer

The physical layer can be divided into the Radio Frequency (RF) layer and the Physical Link layer.

The Radio Frequency (RF) is the normal GSM physical radio layer. Among other things the RF layer specifies:

• the carrier frequency characteristics and GSM radio channel structures • the radio modulation scheme used for the data • the radio transmitter and receiver characteristics as well as performance

requirements. The GSM RF physical layer is used for GPRS with the possibility for future modifications.

The Physical Link layer supports multiple MSs sharing a single physical channel and provides communication between the MSs and the network.

Network controlled handovers are not used in the GPRS service. Instead, routing area updates and cell updates are used.

The Physical Link layer is responsible for:

• Forward Error Correction (FEC) coding, allowing the detection and correction of transmitted code words and the indication of incorrectable code words

• the interleaving of one RLC Radio Block over four bursts in consecutive TDMA frames.

2.4.2 Medium Access Control (MAC)

The Medium Access Control (MAC) protocol handles the channel allocation and the multiplexing, that is, the use of physical layer functions.

The GPRS MAC function is responsible for:

• Providing efficient multiplexing of data and control signalling on both the uplink and downlink. This process is controlled by the network. On the downlink, multiplexing is controlled by a scheduling mechanism. On the uplink, multiplexing is controlled by medium allocation to individual users (for example, in response to a service request).

• Mobile originated channel access, contention resolution between channel access attempts, including collision detection and recovery.

• Mobile terminated channel access, scheduling of access attempts, including queuing of packet accesses.

• Priority handling.

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2.4.3 The Radio Link Control (RLC)

The Radio Link Control (RLC) protocol offers a reliable radio link to the upper layers. Two modes of operation of the RLC layer are defined for information transfer: unacknowledged and acknowledged. The RLC layer can support both modes simultaneously.

The RLC function is responsible for:

• Providing transfer of Logical Link Control layer PDUs (LLC-PDU) between the LLC layer and the MAC function.

• Segmentation and reassembly of LLC-PDUs into RLC Data Blocks. See Figure 2.

• Backward Error Correction (BEC) procedures enabling the selective retransmission of uncorrectable code words. This process is generally known as Automatic Request for Retransmission (ARQ).

Note The Block Check Sequence for error detection is provided by the Physical Link layer.

Information FieldFH FCS

Information FieldBH BCS



Normal TDMABurst

RLC Block

LLC Frame

LLCLayer

Normal TDMABurst

Normal TDMABurst

Normal TDMABurst

RLC/MACLayer

PhysicalLayer

FH = Frame HeaderFCS = Frame Check SequenceBH = Block HeaderBCS = Block Check Sequence (When SDCCH coding is used, BCS corresponds to the Fire code)

Figure 31. Segmentation of LLC-PDUs into RLC data blocks


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3. Signalling plane

In the signalling plane, the GTP specifies a tunnel control and management protocol which allows the SGSN to provide GPRS network access for an MS. The signalling plane also handles path management and location management. Signalling is used to create, modify and delete tunnels. The underlying protocols TCP/UDP, IP, L2, L1 are used as transport solution for GTP signalling transport.

Between the MS and the SGSN, GPRS Mobility Management and Session Management Information are exchanged. The protocols below MM in the BSS, SGSN and MS are used for signalling transport.

SGSN

GMM/SMGPRS MobilityManagement

andSession

Management

NSNetworkService

L1bisPhysical

Layer

BSSGPBSS

GPRSProtocol

LLCLogical Link

Control

MS

GMM/SMGPRS MobilityManagement

andSession

Management

RLCRadio Link

Control

MACMedium Access

Control

GSM RF

IP / X.25

LLCLogical Link

Control

Um Gb

•GPRS attach / detach•security• routing area update, location update

•PDP context activation, modification & deactivation

• logical connection• acknowledge/ unacknowledged peer-to-peer operation

• ciphering• SAPs to higher layer (SNDCP, GMM, SMS)

MAC

GSMRF

RLC

NS

L1bis

BSSGP

Relay

Figure 32. Signalling plane over BSS

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Signalling via the interfaces Gc, Gf, Gd, and Gr is based on the modified mobile specific SS7 protocols MAP and BSSAP.

L1

SCCPSignalling Conne-ction Control Part

TCAPTransactionCapabilities

Application Part

MAPMobile

ApplicationPart

MTP L2

MTP L3

L1

SCCP

TCAP

MAPMobile

ApplicationPart

MTP L2

MTP L3

L1

SCCP

MTP L2

MTP L3

L1

SCCP

MTP L2

MTP L3

BSSAP+BSSAP+

BSSApplication

Part +

SGSN Gr,f,d

SGSN MSC/VLRGsHLRGcGGSN

GPRS-specific

MAPextension

Subset ofBSSAPfunction-alities

Figure 33. Signalling to registers and SMSC


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References Nokia DX200 SGSN Product Description

Nokia GPRS Charging Gateway Product Description

Nokia GGSN Product Description

Nokia GPRS Solution Description

Nokia GPRS System Description

3GPP Specification 23.060















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gprs architecture

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