Gprs Architecture 1

SYSTEM TRAINING

GPRS Architecture: Interfaces and Protocols

Training Document

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© Nokia Networks Oy 1 (73)

GPRS Architecture

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 Networks' 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 Networks. The document has been prepared to be used by professional and properly trained personnel, and the customer assumes full responsibility when using it. Nokia Networks 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 Networks and the customer. However, Nokia Networks has made all reasonable efforts to ensure that the instructions contained in the document are adequate and free of material errors and omissions. Nokia Networks will, if necessary, explain issues which may not be covered by the document.

Nokia Networks' liability for any errors in the document is limited to the documentary correction of errors. Nokia Networks 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.

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Copyright © Nokia Networks Oy 2005. All rights reserved.

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Contents

Contents

1 Module objectives....................................................................5

2 Introduction...............................................................................6

3 Network elements.....................................................................83.1 Packet Control Unit (PCU)..........................................................93.2 Channel Codec Unit (CCU)........................................................93.3 Serving GPRS Support Node (SGSN).......................................93.4 Gateway GPRS Support Node (GGSN)...................................103.5 GPRS MS.................................................................................113.6 Domain Name Servers.............................................................133.7 Firewalls...................................................................................133.8 Border Gateway........................................................................143.9 Charging Gateway....................................................................14

4 GPRS interfaces.....................................................................154.1 IP Multimedia Subsystem.........................................................174.1.1 PoC System architecture..........................................................18

5 Transfer of packets between GSNs......................................20

6 Nokia GPRS solution..............................................................236.1 Nokia GPRS Release functionality...........................................236.1.1 GPRS IP service.......................................................................236.1.2 SGSN to MSC/VLR interface (Gs)............................................236.1.3 Cell reselection.........................................................................246.1.4 SMS through GPRS.................................................................246.1.5 Charging...................................................................................24

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6.1.6 Roaming...................................................................................256.1.7 Quality of Service.....................................................................256.1.8 Access to Internet.....................................................................266.1.9 Corporate access solutions......................................................266.2 Nokia Base Station Subsystem (BSS)......................................286.2.1 BTS...........................................................................................296.2.2 BSC..........................................................................................296.3 Nokia Core Network Subsystem (CNS)....................................316.3.1 MSC/VLR..................................................................................316.3.2 HLR and EIR............................................................................316.3.3 Nokia Serving GPRS Support Nodes.......................................326.3.4 Nokia Gateway GPRS Support Nodes.....................................366.3.5 Nokia Charging Gateway (CG).................................................386.3.5.1 Intelligent Content Delivery.......................................................436.4 Nokia Network Management System (NMS)............................456.4.1 GPRS NMS platform T11, T11.5 and T12................................466.4.2 Nokia NMS T11.5 functionality.................................................476.4.3 Nokia NMS T12 features..........................................................476.4.4 Evolution of Nokia network management solution - Nokia

NetAct.......................................................................................48IP configuration management.......................................................................49

7 Key points...............................................................................517.1 GPRS architecture: key points.................................................517.2 Nokia GPRS solution: key points..............................................52

8 Review questions...................................................................54

9 Appendix – GPRS transmission plane protocols................58

References................................................................................................73


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2 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)

Describe the Nokia GPRS Release 1 and Release 2 functionality

Describe the PoC service

without using any references.

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3 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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4 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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4.1 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 csand psusage, based on

• load situation• priority, and• operator set rules

BSCcsRadio Resource

Management

PCUpsRadio Resource

Management

csresources psresources

Figure 3. PCU – its position within the BSS

4.2 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.

4.3 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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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.

4.4 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

Router

Router

LAN

Internet

Figure 4. GPRS network as seen by another data network

4.5 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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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.

Multislotclass

Maximum number of slots

Minimum number of slots

Type

Rx Tx Sum Tta Ttb Tra Trb1 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.

4.6 Domain Name Servers

These devices convert IP host names into IP addresses. The backbone DNS is not used by mobile stations. Instead network element convert for example Access Point Names to IP addresses (e.g. APN “Internet”= 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 TCP IP for Mobile Packet Core.

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.

4.7 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).

4.8 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.

4.9 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. The CDR must 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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5 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.

28 ©NOKIA CTXX 3220_3.0en.PPT/ 19.12.2002 / BWi

GPRS interfacesSCP

Air (Um)

Gb

GsGr

Gf

Gn

GpInter-PLMN

GPRSbackbone

Gp

Externalpacket network

GiSGSN

EIR

BSC

MSC/VLR

SMS-GMSC

Gd

GGSN

BG

Signalling and data

Signalling

Ga

GaCG

SGSN

Gn

GcGe

HLR

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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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.

Ge between the SGSN and the service control point SCP. AN example use of this interface is SGSN IN Based Prepaid. Trough the Ge interface it is possible to receive subscriber's prepaid information from the HLR and thus control duration and volume of a Packet Data Protocol (PDP) context. In prepaid it also possible to control the MO-SMS sending. The prepaid account is stored in a Service Control Point (SCP). Information about the subscriber's prepaid account is created in the HLR. The SGSN receives this information at GPRS attach when it fetches subscriber information from the HLR.

SCP controls the account balance during the PDP context or during the MO-SMS sending. SCP allocates data volume or/and time in certain units to SGSN for continuing processing of the PDP context.


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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.

5.1 IP Multimedia Subsystem

GPRS will continue to evolve and since the core network is shared with UMTS improvements to UMTS will also add capabilities to GSM/GPRS. One of the major future changes is the IP Multimedia Subsystem/IP Multimedia Core network.

The IP Multimedia CN subsystem contains a set of signalling and bearer related network elements for provisioning of multimedia services. IP multimedia services are based on an IETF defined session control capability which, along with multimedia bearers, utilises the IP-Connectivity Access Network

The IP multimedia core network (IM CN) subsystem enables PLMN operators to offer their subscribers multimedia services based on and built upon Internet applications, services and protocols. The intention is that such services will be developed by PLMN operators and other third party suppliers including those in the Internet space using the mechanisms provided by the Internet and the IM CN subsystem. The IM CN subsystem will enable the convergence of, and access to, voice, video, messaging, data and web-based technologies for the wireless user, and combine the growth of the Internet with the growth in mobile communications.

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29 ©NOKIA

Nokia IP Multimedia Core Solution (simplified)

HLRMSC/ MSS

SGSN

SGSNGGSN

GGSN

CG

LIG

BG

DNS

IMR CPS

GERAN/UTRAN

MPC

MGW

IP Multimedia core

Figure 7 IMS example (concept figure)

The complete solution for the support of IP multimedia applications consists of terminals, IP-Connectivity Access Networks, and the specific functional elements of the IM CN subsystem. An example of IP-Connectivity Access Network is the GPRS core network with GERAN and/or UTRAN radio access networks. The actual IMS consist of a large number of functions enabling session control for IP connections. New elements described in figure 7 are:

CPS Connection Processing Server for controlling multimedia sessions using SIP (Session Initiation Protocol), and for centralized registration and charging

IP Multimedia Registers for storing subscriber and service information

Note further information on the IMS is available in

3GPP TS 23.228

3GPP TS 22.228

www.nokia.com

5.1.1 PoC System architecture

Push to talk one-to-one and group calls are enabled over GPRS and EGPRS networks. User logon to the network which means they logon to the PoC service. This logon is a SIP registration. Group sessions are created with SIP


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messages between the PoC Core Network and terminals. To join a group call the user creates a SIP session. SIP is only a signalling protocol the actual voice is relayed by the user plane of the PoC Call Processor as RTP packets. If group connections are wanted they are created by multiplying voice packets to all members of a group session. Thus PoC creates a radio group based on the end user choice of members.

30 ©NOKIA

Trafficon PoC Network

SGSNPAPU units

BSCPCU units

BTS

GGSNWith PoC APN

GPRSbackbonenetwork

Customer Care and Billing

ProvisioningNetwork Management

WEB based• Management• Provisioning• Self provisioning

PoCCallProcessors

PoCRegister

LDAP parametermanagement

GPRS

Green elements required for PoC

IP layer

Service managementRadio access layer architecture

Consumers

Access Control& EnterpriseConnectivity

HTTP human interfaceCommunicationParameters(timers)

CDRparameters

SUN

Operator Data Center

Enterprises

SystemParameters

Sub SystemServerParameters

Radio interface management

Packet Core management

Figure 8 PoC architecture.

In order to create a sufficient QoS in loaded networks GPRS transmits the PoC voice using 3GPP release 99 streaming and header compression features and release 4 feature package 1. This is necessary to counter the unpredictable nature of the GRPS packet switched interface

It is necessary to create two PDP contexts per PoC service logon. One is a streaming context and is used for mainly the speech packets (and some control packets, for example for starting a one-to-one call session and talking party identification for group talkbursts). The other context is a best effort context, and is used for the SIP signalling and text chat within a talkgroup. The SIP signalling is used for service logon/logoff (including authentication), joining/detaching a group session, and information request types of functions.

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6 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 9.

SGSN GGSN

The stream of containers forming a tunnel.

Figure 9. 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 10. 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 endpoint Identifier (TEID), will tell the receiving GSN who the user. The TEID is a 32 bit value allocated at PDP-context activation. The TEID is a label that tells the SGSN and the GGSN, whose packets are inside the container and uniquely, identifies a context.

Note that there are two version of GTP. What is described here is the 3GPP specified GTPv1. There is an older version called GTPv0. The GTP protocols have a different structure but many of the functions are identical. One of the most visible differences is that GTPv0 uses a TID value instead of the TEID. The TID is built of IMSI+NSAPI and is a tunnel identifier just like TEID. The


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GTPv0 is fixed in its format and can thus not be extended in the future. The GTPv1 is extendable. Most GPRS networks run the GTPv1.


Tunnelling

User packetTunnel End

Point ID:

THE GTP PACKETIP (+TCP/UDP)

Identifier that identifiesa tunnel endpoint in thereceiving GSN? Answers:

What user?Which connection?

To which GSN?

GSN IP-address

E.g. a TCP/IP packetcarrying e-mail

Figure 10. 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. The user packets are carried in the GPRS core between the SGSN and the GGSN using a set of IP addresses which are only used in the GPRS backbone. The backbone addresses are hidden by the tunnel and by firewall functionality

This concept of tunnelling and hiding backbone addresses to the user level is illustrated in the following figures. Figure 11 shows a close-up of the user and backbone IP address levels. Figure 12 shows the GTP tunnel related to the user payload, and the relationship between the protocol stacks in the Gi and Gn interfaces.

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Tunnelling

GGSN

GTP

IP IP

IP backbone data using hidden IP addresses

SGSNMS

IP

GTPTunnel

user data using 'public' IP addresses

Figure 11. Transfer of packets between the GGSN and the MS

BTS BSC

SGSN

GGSN Internet

GPRS Core

Network

SS7

HLRMSC/VLR

L1

L2

IP

GTP

USERPAYLOAD

UDP

TCP/UDP

IP

APP

L1

L2

Tunnelledpayload

Server

Figure 12. 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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7 Nokia GPRS solution

7.1 Nokia GPRS Release functionality

This section describes the capabilities of the Nokia GPRS solution.

7.1.1 GPRS IP service

Nokia GPRS Release 1 supports Internet Protocol version 4 (IPv4) point-to-point connections. Each user can activate up to two PDP contexts simultaneously. Both static and dynamic IP addressing is supported (defined per subscriber in the HLR). Nokia GPRS Release 1 does not include network-initiated PDP context activation, anonymous access, point-to-multipoint services, or X.25 network interface functions. These will be considered for later releases.

Nokia Release 2 supports IPv4 and IPv6 on the user plane. Each user can activate up to four PDP contexts simultaneously. In Release 2, the data throughput capacity, number of subscribers, and number of PDP contexts per user has been doubled compared to Release 1.

In both releases, capacity can be increased in steps of 25 %, 50 %, 75 %, and 100 % of maximum capacity. This will be discussed in detail later.

7.1.2 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.

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7.1.3 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. This is not implemented in Nokia GPRS Release 1.

7.1.4 SMS through GPRS

Nokia GPRS Release 1 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. The Nokia SMS implementation is still based on MAP v.2, so no changes to the Gateway MSC (SMS-GMSC) should be needed. Release 2 supports SMS for prepaid GPRS subscribers.

7.1.5 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. Nokia GPRS Release 1 supports charging based on:

Data volume transferred on the uplink and downlink direction

Length of time for which with PDP context is active

Access point used to external networks (for example, Internet vs. corporate intranet).

Nokia Release 2 supports prepaid GPRS subscribers, for which new charging mechanisms are needed:

Charging based on location

Duplicate CDR prevention mechanism. If a CDR is resent to a new CG, it is labelled as 'possible duplicate'.

GPRS Server Node based prepaid: SGSN receives subscriber's prepaid status from the HLR and sends prepaid CDRs as soon as possible to Charging Gateway. CG sends them to Charging Centre which controls subscriber's account state. Charging Centre sends information to the HLR when the account limit has been reached. HLR in turn asks SGSN to disconnect PDP context.


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IN-based prepaid based on Phase 3 CAMEL (Customised Applications for Mobile Network Enhanced Logic), which is a network feature providing a possibility to offer Operator Specific Services (OSS) for the subscriber, such as Intelligent Network services. SGSN has interface to SCP, which has information on the subscriber's account, and grants permission to activate PDP context or sending short messages. SCP also monitors the account limit and asks SGSN to deactivate PDP context when the limit is reached.

7.1.6 Roaming

A Border Gateway (see Figure 13) 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. Nokia Release 1 does not support roaming between PLMN. Release 2 has multiple PLMN support.

D O C U M E N T T Y P E

T y p e U n it O r D e p a r t m e n t H e r eT y p e Y o u r N a m e H e r e T y p e D a t e H e r e

I n t e r P L M Nb a c k b o n e

n e t w o r k

G P R SB a c k b o n eI P N e t w o r k

B o r d e rG a t e w a y

G P R Sb a c k b o n eI P n e t w o r k

G G S NG G S N I n t e r n e t

O p e r a t o r A( H o m e n e t w o r k )

O p e r a t o r B

R o a m in g u s e r

B o r d e r G a t e w a y

S e c u r e G P R S T u n n e l le d C o n n e c t io n

Figure 13. Roaming via Border Gateway and Inter-PLMN network'

7.1.7 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.

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The ETSI GPRS specifications define several QoS classes, which are associated with each PDP context, covering priority, reliability, delay and throughput. Nokia GPRS system Release 1 does not support this QoS functionality. The GPRS QoS can be considered the 'best effort'.

Nokia GPRS system Release 2 supports the following QoS functionality:

Priority-based QoS

Interactive and background classes supported

Diffserv applied for uplink packets towards the GPRS backbone

Prioritisation for downlink packet towards BSS.

7.1.8 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 to be more exact, the access point (AP), which is associated with the physical interface to the Internet. This is illustrated in Error: Reference source not found.

GGSN

GPRSbackboneIP Network

MS BSCBTS SGSNISPBackbone

AP

Ethernet

Figure 14. 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.

7.1.9 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.


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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.

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).

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GGSN

GPRSbackboneIP Network

Intranet

Internet

Service

Infra servers - RADIUS - DNS

MS BSCBTS SGSN

ISPBackbone

AP

AP

GTP

Figure 15. 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.

Nokia Release 2 also supports the following security features:

Home subscriber trace based on IMSI/MSISDN

Visiting subscriber trace based on IMSI/MSISDN

Equipment traces (IMEI).

7.2 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

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).


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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.

7.2.1 BTS

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

Coding schemes CS3 and CS4 are under consideration for future releases. CS3 and CS4 channel coding require more than one 16 kbit/s Abis traffic channel and can be supported in the existing BTS products only with hardware upgrades. Currently, Nokia plans to introduce new CS3- and CS4-capable TRXs for the Talk-family and Nokia MetroSite base stations. CS3 and CS4 will not be implemented in Nokia 2nd-generation BTS and Nokia PrimeSite.

7.2.2 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.

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

Figure 16. 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.


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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.

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.

7.3 Nokia Core Network Subsystem (CNS)

7.3.1 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.

7.3.2 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. It can be expanded from a capacity of 120 000 to 1 200 000 subscribers. The Nokia DX200 HLR supports up to 300 000 subscribers. The EIR supports 200 000 entries in the white list and 4 million entries in the grey and black lists.

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.

The optional GGSN to HLR interface (Gc) is under study for inclusion in future Nokia releases.

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7.3.3 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.

Figure 17. The Nokia SGSN Release 1

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.


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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

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.

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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

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+12N 2N

Gn (to GGSN)

Charging

Gb(to BSC)

WDU

Figure 18. 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.

The SGSN capacity

The capacity of the Nokia SGSN can be expressed in terms of subscriber capacity, packet switching capacity, buffering capacity, interface capacity and the number of location and routing areas served. Table 1 compares SGSN Releases 1 and 2.


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Table 1. Comparison of SGSN Release 1 and 2

SGSN Release 1 SGSN Release 2

Standards Based on ETSI Rel 97 Based on ETSI/3GPP

IP connections IPv4 IPv4 and IPv6

QoS Best Effort Priority, Interactive & Background classes, diffserv.

Active PDP context per subscriber 1 – 2 1 - 4

25 % capacity (minimum configuration)

4 active PAPU units

1 active SMMU unit

- 30 000 subscribers

- 12 Mbps


- 25 Mbps

50 % capacity

8 active PAPU units

2 active SMMU units


- 24 Mbps


- 50 Mbps

75 % capacity

12 active PAPU units

3 active SMMU units


- 36 Mbps


- 75 Mbps

100 % capacity (maximum configuration)

16 active PAPU units

4 active SMMU units


- 48 Mbps


- 100 Mbps

Subscriber capacity

The maximum GPRS-attached subscriber capacity and the number of PDP contexts is tabulated above. Subscriber capacity is modularly added with steps of 30 000 (Release 1) or 60 000 (Release 2) subscribers by adding new functional units (4 PAPUs and 1 SMMU).

Routing areas and location areas

SGSN supports 16 000 routing areas with a maximum of 1000 per PAPU. SGSN supports 2000 location areas. One routing area is always served by one PAPU. Location areas can freely cross PAPU/SGSN boundaries.

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7.3.4 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.

Nokia GGSN consists of three primary components: hardware, software, and other modules such as firewalls. The GGSN has not changed much when migrating from Release 1 to 2.

Hardware platform

Nokia GGSN Release 1 is called GN2500 (see Figure 19). GN2500 products use Intel Pentium II 650 MHz processors, high-availability IP routing.

Figure 19. Nokia GGSN Release 1

Nokia GGSN Release 2 is based on the Nokia IP 740 series of internet routers. When configured as a GGSN, the IP 740 is called the GN3500. The processor is an Intel Pentium III 866 MHz.

GN2500 and GN3500 both support various network interface cards (NICs), such as Quad port Ethernet 10/100 adapter is used when SGSN(s) and GGSN(s) are located on the same site or connected to a LAN or VLAN.

Nokia GGSN Release 1 (GN2500)

The essential features of the Nokia GGSN Release 1 (GN2500) solution are:

GPRS interfaces supported are Gn (backbone), Gp (Inter PLMN), Ga (CG), and Gi (External).


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Protocols supported include GTP, GTP', Ethernet, SNMP, NTP, FTP, HTTP, NTP, UDP, TCP, IP, IP in IP, VRRP, DHCP, and RADIUS.

Support of up to 100 access points per GGSN. For each user, the access point to be used is defined in the HLR subscriber data. The access points can even share the same physical interface if required.

GGSN can allocate IP address from a pool of IP addresses or using a DHCP or RADIUS client.

50 000 GTP tunnels which can be encrypted.

integrated GGSN / BG / Router / Firewall platform.

Nokia GGSN Release 2 (GN3500)

The key features of the Nokia GGSN Release 2 (GN3500) are the following:

GPRS interfaces supported are Gn, Gp, Ga, and Gi.

Protocols supported include GTP, GTP', Ethernet, SNMP, NTP, FTP, HTTP, NTP, UDP, TCP, IP, IP in IP, VRRP, DHCP, and RADIUS.

Support for IPv4 and IPv6

Up to one million active PDP contexts

Multiple PDP contexts per IP using different tunnel end-point identifiers

Support for 2G and UMTS networks using Release 99 QoS

Support for up to 250 access points per GGSN

Integrated GGSN / BG / Router / Firewall platform

GGSN can allocate IP address by itself or using a DHCP/RADIUS.

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.

Nokia GGSN Release 2 supports prepaid, hot billing, and flat rate users.

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.

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The Nokia GGSN Release 1 and 2 provides direct Internet access in addition to access to intranets and ISPs. Multiple access points are provided. The exact functions provided by interconnecting networks depend on which of the following three connection options is required (see Figure 20):

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 20. Various connections between GPRS and external networks

7.3.5 Nokia Charging Gateway (CG)

CG and CDRs

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

Stand-alone


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Integrated in GSNs (SGSN and GGSN).

In the Nokia GPRS solution, the CG (Release 1 and 2) is a stand-alone network element, as shown in Figure 21. 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 21. 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:

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.

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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 Charging Gateway Release 1 provides an access point to the billing system from each Charging Gateway unit. Several GSNs can send the CDRs to the same Charging Gateway. This reduces the amount of nodes from which the billing system has to fetch the consolidated CDRs.

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 22.


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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 22. CG functions

Scalability

Scalability is achieved by adding server capacity (D3 server, about 1 000 000 CDRs/day and K8 server, about 10 000 000 CDRs / day ) and by internally distributing processes between computers (not in Release 1).

GSNs are configured to send CDRs to a regional CG. The minimum CG configuration consists of two CG units that share the overall CDR load from the GSNs.

Nokia Charging Gateway Release 2

The new features in CG Release 2 are the following:

Improved charging architecture including new CDR creation triggers to support new services

Duplicate CDR prevention mechanism: if a CDR is resent to a new CG, it is labelled as 'possible duplicate'.

Improved performance and throughput from CG 1.0: 30 million CDRs / day in the L-class servers.

Capacity increase

Support for prepaid, hot billing, flat rate

GPRS Server Node based prepaid: SGSN receives subscriber's prepaid status from HLR and sends prepaid CDRs as soon as possible to Charging Gateway that sends them to Charging Centre, which controls subscriber's account state. Charging Centre sends information to the HLR

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when account limit has been reached which in turn asks SGSN to disconnect PDP context.

IN-based prepaid based on Phase 3 CAMEL (Customised Applications for Mobile Network Enhanced Logic), which is a network feature providing a possibility to offer Operator Specific Services (OSS) for the subscriber, such as Intelligent Network services. SGSN has interface to SCP, which has information on subscriber's account and grants permission to activate PDP context or sending short messages. SCP also monitors account's limit and asks SGSN to deactivate PDP context when limit is reached.

Nokia Charging Gateway Release 3

The list below shows some of the features in CG Release 3:

The following CDRs and CDR formats are supported:

G-CDR, S-CDR, M-CDR, S-SMO-CDR (SMS, Mobile Originated) and S-SMT-CDR (SMS, Mobile Terminated).

CDR releases GPRS Rel 1, GPRS Rel 2 and 3G Rel 1.

Duplicate CDR prevention delegation: Through the Graphical User Interface, administrators can decide whether duplicate prevention takes place in CG 3.0 or whether it is delegated to a Billing System/Charging Center. By default, duplicate prevention is on in CG 3.0.

Configurable output interface: CG 3.0 can process three different CDR versions and supports three standard output data interfaces (one at the time). Administrators can select the output interface through the GUI and also include/exclude CDR fields in/from the CG 3.0 output format.

GTP’ message logging: CG 3.0 enables writing of GTP’ messages into a file in a text format. By default, GTP' message logging is off in CG 3.0.

Online maintenance of network element information. You can add, modify or remove NE information online.

Audit log: CG 3.0 enables logging of all administrative tasks that are performed through the GUI.

Online backup of the CG 3.0 system: Based on a shell script that takes an online backup of the CG 3.0 system configuration.

Improved performance and throughput from CG 2.0: 50 million CDRs / day in the L-class servers.

Calling Party Number: There is a new field in the S-SMT-CDR (SMS Mobile Terminated CDRs) sent by 2G SGSN SG2. The field is called Calling Party Number and it is presented as a 9-byte long ISDN number.


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7.3.5.1 Intelligent Content Delivery

The Nokia ICD (Intelligent Content Delivery) solution offers mobile terminal users easy access to all their services from a single access point. With the Nokia ICD solution, operators can offer easily accessible content services with simple pricing that reflects the value of those services popular with subscribers. The ICD solution provides a rich set of service control functions that enhance the user experience and fulfil the ethical and regulatory requirements of mobile content delivery.

59 ©NOKIA

Nokia ICD System Release 1, ED 1

ISN 1.1

Gi

Contentproviders

Gn

Serviceaccess

CG 4.0BSS

GGSN 4.0

TA 1.2

OSC 1.0

NSM 1.0

CA 1.1

ICDNetAct

3.1 ED2 CD 9/03

Prepaid System

OSS

CCBS

GGSNfunctions

TrafficAnalyser

Contentfunctions

Figure 23 The ICD components. The (ISN) consist of GGSN and two data analyzers TA and CA.

The Traffic Analyzer handles HTTP traffic with high granularity. For example, decisions can be based on URL. All layers of the IP stack are visible to the TA.

The CA operates only at layer 7, thus it can recognize very detailed information in the traffic flow; content type, headers, and so on

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One of the major functions of the Nokia ICD is to bring intelligence to the packet core network. The ICD system is service aware. ICD classifies traffic by IP address, protocol, and destination URL, allowing traffic to be priced according to how much users will benefit from particular types of data.

The ICD integrates subscription, customer care, and billing solutions into a seamless functionality, including online charging data and online subscriber and services management throughout the network. It also provides simple, standard interfaces to operators' existing billing and customer care systems, while well-defined interfaces with external systems minimize the required integration work for new services.

60 ©NOKIA

Single Click Access

Nokia ICD System

Single PDP Context

Single PDP Context

Operator MMS Service

Operator Email Service

OperatorWAP

Service

Service Provider

WAP Service

GGSN

ap1.operator.com

APAP

Service switchingService switching

Single APN configured in terminal

Single APN configured in terminal

PDP Context

Nokia ISN

Access settings screen

Figure 24 Single access point use for multiple services

The ICD solution also provides a rich set of service control functions that both improve the user experience and meet the ethical and regulatory requirements of mobile content delivery. One of the main features of ICD is the ability to allow user access to multiple services via one single access point. This makes the use of data services considerably easier for end users and thus more attractive.

Charging options in ICD are more flexible than earlier charging.

• Volume-based charging

• Transferred data volume

• Per service, not limited to PDP Context

• Bytes of transferred data

• Event-based charging

• Event (transaction/hit to a specific WAP/HTTP URL)


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• A request/response pair

• Number of events

• Time-based charging

• Distinguish active/inactive usage time)

• Subscription (free access during subscription period

• Seconds of data transfer

• Subscription-based charging

• Free access during content service subscription period

• May be combined with event-based charging

7.4 Nokia Network Management System (NMS)

The Nokia NMS for GPRS builds on the Nokia NMS/2000 solution (based on HP UNIX workstations and servers). Several new services have been added for managing the GPRS-based BSS and NSS parameters. For management of the GPRS backbone, a new separate SNMP application server is used.

GSMIP

Q3

SNMP

Nokia NMS for GPRS

Figure 25. Nokia NMS for GPRS

The Nokia NMS for GPRS provides an integrated solution for GPRS management. This means:

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Integrated access: all applications and tools can be seamlessly accessed from the same terminal.

Combined GSM and GPRS management, allowing seamless co-operation between applications over organisational boundaries.

Integrated fault management. Both SNMP traps from the GPRS network and Q3 alarms from the GSM network are displayed and managed in the same alarm applications.

Integrated performance management, allowing the collection and monitoring of both GSM and GPRS counters.

Support for 3rd party element managers through the HP OpenView Network Node Manager-based SNMP server.

7.4.1 GPRS NMS platform T11, T11.5 and T12

D O C U M E N T T Y P E 1 (

T y p e U n i t O r D e p a r t m e n t H e r eT y p e Y o u r N a m e H e r e T y p e D a t e H e r e

N M S / 2 0 0 0D C N

N e t w o r k

S w i t c h # 1 S w i t c h # 2

R o u t e r # 1 R o u t e r # 2

N M S - S Y S T E M

C S D S S S

H u b # 1

1 0 M 1 0 M 1 0 M

L A NL A N

C u s t o m e rO f f i c e L A N

S w i t c h # 3

1 0 / 1 0 0 M

G P R SB BS e r v e r

R o u t e r # 3F i r e w a l l

G P R S B B

N M S / 2 0 0 0 T 1 1 . 1C o m m .& D BS e r v e r

L A N # 0 H e a r t b e a tL A N # 1 P r i m a r y L A NL A N # 2 S e c o n d a r y L A NH S R P ( H o t S t a n d b yR o u t i n g P r o t o c o l ) L A NC o n n e c t i o n t o O f f i c e L A N

L A N / W A N

L A N

L A N

N M S / 2 0 0 0 T 1 1 . 5

Figure 26. T11.1 and T11.5 platforms at the NMS site


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NMS/2000 T11.1 was used for GSM network management and it consisted of1 - 3 NMS/2000 servers that interacted with the GSM elements through a set of routers and switches and a data communication network (DCN) as shown in the left-hand part of Figure 26.

NMS T11.5 was an enhancement to T11.1 with additional hardware for GPRS management was added. This consisted of a communication and database server and GPRS BB server on a workstation. These servers are connected through a set of switches and routers to the GPRS backbone (BB) network.

In the NMS site concept of both T11.1 and T11.5 NMS platforms are integrated. This is shown in Figure 26.

7.4.2 Nokia NMS T11.5 functionality

The main functionality of the NMS/2000 T11.5:

Topology sharingWith the topology sharing application we can modify existing GPRS topology and download it into the network.

Fault monitoringFault monitoring (FM) in GPRS follows the same concept as in GSM. The applications included in GPRS FM are Alarm Monitoring, Alarm Viewer, Alarm History, Alarm Manual, Alarm Forwarder, FM Rule Editor and Correlation Rule Editor. E.g. SNMP traps are sent by the network elements to the GPRS BB server and finally collected at the communications and database server.

GPRS BB managementIts features include Network Element time management (it consists of centralised time management for IP network elements), Performance monitoring (incl. traffic volume, QoS, and service availability), Configuration management, Element management, and Name and address (DNS) management

Security management (SM)

Fault monitoring

7.4.3 Nokia NMS T12 features

The list of new features in T12 are itemised below:

NMS System ManagementMaintenance Mode (optional)

Multivendor Integration

CORBA Integration Toolkit (optional)

ASCII Alarm Forwarding (optional)

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GPRS Performance Management. GPRS Backbone Firewall and VPN Management (optional, T11.5)

GPRS Backbone Name and Address (DNS) Management (optional, T11.5)

ASCII Interface for GPRS Measurement Data (optional)GPRS Basic Report Set (basic for GPRS customers)

GPRS backbone management Capacity indication tool (basic)capacity estimation tool (basic)NMS backup (optional)service access control (optional)Radio Network ManagementAutomatic picocell planning and channel finder(optional)

Routing area IP manager which is used for IP routing area configuration. Direct access to the DNS is used to read and update the routing area ID information in the DNS database

7.4.4 Evolution of Nokia network management solution - Nokia NetAct

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 27. 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 AXC

RNC NE x

NE y

CCB

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

NMS

Figure 28. How the TMN is visualised in the Nokia 3G/UMTS solution

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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 29. Management of the IP backbone in the Nokia NetAct


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8 Key points

8.1 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.

8.2 Nokia GPRS solution: key points

Nokia GPRS Release 1 supports: GPRS point-to-point IPv4 service, two PDP contexts per user, SGSN to MSC/VLR interface (Gs), MS-initiated cell reselection, SMS through GPRS, Charging based on transferred data, length of time with PDP context active, location of MS and used access point, Secure GPRS tunnelled connection when roaming, QoS - dynamic capacity for GPRS in a cell, guaranteed minimum capacity for GPRS traffic is a cell, 'best effort' QoS for individual PDP contexts, Multiple access points per GGSN, DHCP and RADIUS server access, Secure connections via the Internet.

Nokia Release 2 supports all of the above and IPv6 connections, up to four PDP context per user, IN prepaid, inter-PLMN support, QoS, subscriber and equipment tracing, etc.

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)


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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 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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9 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)?


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

10. Compare Nokia SGSN Release 1 and 2.

SGSN Release 1 SGSN Release 2

Standards

IP Connections

QoS

SGSN Subscribers

SGSN Data Rates

Active PDP context per sub

Max # PDP contexts per SGSN


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11. Compare Nokia GPRS Release 1 and 2.

Features GN2500 (GGSN Rel. 1) GN3500 (GGSN Rel. 2)

IP versions

# of AP per GGSN

# of active PDP context

Address Allocation

Networks

QoS supported

Multiple PDP per IP Address

Charging

12. Compare Nokia CG Release 1 and 2.

Features CG Release 1 CG Release. 2

Networks

CDR

Cell ID

Pre-paid support

Hot billing support

Flat Rate User support

Dup. CDR Prevention Mech (DCPM)

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10 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 between the BSS network elements and SGSN and GPRS MS.


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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 GTP

Relay

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 30. 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 31. . 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.

D O C U M E N T T Y P E

T y p e U n i t O r D e p a r t m e n t H e r eT y p e Y o u r N a m e H e r e T y p e D a t e H e r e

U D PH e a d e r

U D PH e a d e r

G T PH e a d e r

G T PH e a d e r

u s e r I P D a t a

G P R SB a c k b o n e I P

H e a d e r

G P R SB a c k b o n e I P

H e a d e r

U s e r D a t a P a y l o a d ( T - P D U )( u s e r I P D a t a )

U s e r D a t a P a y l o a d ( T - P D U )( u s e r I P D a t a )

G T PH e a d e r

G T PH e a d e r

U s e r D a t a P a y l o a d ( T - P D U )( u s e r I P D a t a g r a m )


U D PH e a d e r

U D PH e a d e r

G T PH e a d e r

G T PH e a d e r



U D P L a y e r

G T P L a y e r

B a c k b o n eI P L a y e r

Figure 32. 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 MSThe 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 33. 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 34. 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 35. 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 36. 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 37. 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 38. 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 39. 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 40. Signalling to registers and SMSC


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References

Nokia DX200 SGSN Product Description

Nokia GPRS Charging Gateway Product Description

Nokia GN2500 GGSN Product Description

Nokia GPRS Solution Description

Nokia GPRS System Description

GSM Specification 03.60 (GPRS Service Description R.97)

GSM Specification 03.64 (Overall Desc. GPRS Radio Interf. R.97)

GSM Specification 04.08














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