System Architecture of GPRS

Group R1:

Xiong Guangyu

Nik A. Salleh

System Architecture

� To understand GPRS system architecture it is helpful to first understand the architecture of GSM system.

� GPRS is an enhancement over the GSM and adds some nodes in the network to provide the packet switched services. These network nodes are called GSNs (GPRS Support Nodes) and are responsible for the routing and delivery of the data packets to and from the MS and external packet data networks (PDN).

Network Architecture

Fig. 3-1 The SGSN and GGSN additions

Addition of two network elements:

�Serving GPRS Support Node(SGSN)�Gateway GPRS Support Node(GGSN)

GGSNSGSNBSC

Network Architecture

�GPRS subscriber terminals� Only GPRS terminals (TEs) can access GPRS

servers� GPRS enabled phones, PDAs with embedded GSM, PC

cards for laptops

� These terminals will be backward compatible with GSM for voice calls, because existing GSM phones:� cannot handle the enhanced air interface� cannot packetize data directly

Network Architecture

� GPRS BSS� A software upgrade is required in the existing Base

Transceiver Site (BTS).� The Base Station Controller (BSC) also requires a software

upgrade, and the installation of a new piece of hardware called a packet control unit (PCU).

� The PCU directs the data traffic to the GPRS network and can be a separate hardware element associated with BSC.

� The PCU provides a physical and logical data interface out of BSS for packet data traffic.

Network Architecture

� GPRS NetworkAdd new core network elements� Serving GPRS Support

Node (SGSN)� Gateway GPRS

Support Node (GGSN)

Fig 3.2 The overlay network interworks between public and private network

GGSN

SGSN

Network Architecture

�GPRS can be thought of as an overlay network onto the GSM network.

�The data overlay network provides packet data transport from 9.6 to 171 Kbps.

�Multiple users can share the same air-interface resources.�GPRS uses most of existing GSM network elements, plus

new network elements, interface, and protocols for building a packet-based mobile cellular network.

Network Architecture

Fig 3-3 The network reference model for GSM

� Databases (VLR and HLR)� All the databases involved in

the network requires software upgrades to handle the new call models and functions introduced by GPRS.

� The home Location Register (HLR) and Visitor Location Register (VLR) especially require upgrades to functionally service GPRS.

HLR

VLR

SGSN

Data Routing

� A main issues in the GPRS network is the routing of data packets to/from a mobile user, which is divided into two areas:� Data packet routing � Mobility management

Data Routing-- Data Packet Routing

� Data Packet Routing� GGSN

� handles interaction with the external data network.� routes external data packets to the SGSN

� There are the use of the various tools in a GPRS network (Fig. 3-5).

� Three different routing schemes are possible:1) Mobile-originated message.2) Network-initiated messages when the MS is in its home network.3) Network-initiated messages when the MS has roamed to another

BPRS operator’s network.

Data Routing-- Data Packet Routing

Fig. 3-5 The various components for data routing

Data Routing-- Data Packet Routing

� GPRS operators will allow roaming through an interoperator backbone network

� GPRS operators connect to the interoperator network by a border Gateway (BG)

� The main benefits of the architecture are its flexibility, scalablity, interoperability and roaming

� The GPRS network encapsulates all data network protocols into its own encapsulation protocol, called the GPRS Tunning Procotocol (GTP).

Data Routing-- GPRS Mobility Management

� The operation of GPRS is partly independent of the GSM network. However,some procedures share the network elements with current GSM functions.

� A mobile station has three states in the GPRS system:� Idle� Standby� Active

GPRS Mobility Management

Fig. 3-6 The GPRS traffic protocol stack

GPRS Mobility Management

� Data is transmitted between a mobile station and the GPRS network only when the mobile station is in the active state.

� In the active state, the SGSN knows the cell location of the mobile station.

� In the standby state, the location of the station is known only as to which routing area it is in.

� In the idle state, the mobile station does not have a logical GPRS context activated or any Packet-Switched Public Data Network (PSPDZ) addresses allocated, The MS can receive only those multicast messages that can be received by any GPRS mobile station.

Network Architecture-New Interfaces

Fig. 3-7 The new interfaces in GPRS

Network Architecture-New Interfaces

� Gb --Between the PCUSN and SGSN, using Frame Relay.� Gr -- Between SGSN and HLR, extension of the Mobile

Application Part (MAP).� Gn -- Between SGSN and GGSN, using the GTP(tunneling)

protocol.� Gi -- Between GGSN and PDNs (X.25 and Internet Protocol

[IP]).� Gs -- Between SGSN and MSC/VLR, for some simultaneous

GPRS and GSM operations.� Gd -- Delivers SMS messages via GPRS (same as MAP from

GSM).� Gc -- Between GGSN and HLR.

The Different Backbone Used

Fig. 3-8 The different backbones used

SGSN

The Different Backbone Used

� Each SGSN is linked to Packet Control Unit Switching Nodes (PCUSN) with a Frame Relay network:� The only protocol possible, with ETSI specifications� Simpler than X.25� Capable of supporting data rates upto 2 Mbps

� The SGSN and GGSN are linked together within the GPRS backbone based on IP routing.

� GPRS tunnels the protocol data unit (PDU) using the GPRS Tunneling Protocol (GTP).

� GPRS will support interworking of mobile stations with IP first and X.25 later. Further, it will transmit the corresponding PDU transparently by encapsulation and decapsulation.

Initial Implementations

� The first releases of GPRS products must support IP and interworking with the Internet and intranets.� Domain Name Server (DNS)� Dynamic Host Configuration Protocol (DHCP)

TDMA-GPRS Physical Channel Capacity

Fig. 3-10 Physical Channel Capacity

� The Time Division Multiple Access (TDMA frame structure for GPRS is the same as for GSM (Fig. 3-10).

� GPRS provides flexible allocation of physical channels to GPRS service.

� The GPRS traffic load in a given cell varies as a function of time.

GPRS Logical Channels

Fig. 3-11 GPRS Logical Channels

� a logical channel refers to a flow of information between entities for a particular purpose.

� Logical channels are carried within the physical channels.

GPRS Logical Channels

� Packet Broadcast Control Channel(PBCCH) � A downlink function used for broadcast of

system information to the mobile station in a cell� Packet Common Control Channel(PCCCH)

� A control channel service for signaling for the packet data:� Packet Random Access Channel (PRACH) � Packet Paging Channel (PPCH)� Packet Access Grant Channel (PAGCH)� Packet Notification Channel (PNCH)

GPRS Logical Channels

� Packet Data Traffic Channel (PDTCH)� The traffic channel is an up and downlink function used

for user data traffic transfer.� PDTCH is temporarily dedicated to a user or group of

users.� PDTCH for uplink and PDTCH for downlink are

unidirectional and assigned separately to support asymmetric user traffic flow.

GPRS Logical Channels

� Packet-Dedicated Control Channel (PDCCH)� Packet Associated Control Channel (PACCH)

� An uplink and downlink function used to carry signaling information to and from the mobile station

� Packet Timing Advance Control Channel/Uplink (PTCCH/UL)� Used for estimation of timing advance of one mobile station

� Packet Timing Advance Control Channel/Downlink (PTCCH/DL)� Used to transmit timing advance information to several mobile

stations

Mapping Logical Channels onto Physical Channels

� Multiple logical channels are mapped onto the same physical channel in a timesharing manner.

Fig. 3-12 Mapping Logical Channels onto Physical Channels