gprs.pdf

Communication Networks IV.Borhanuddin Mohd Ali

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EVOLUTION OF THE GSM

The data capability of GSM is rather limited (9.6kb/s). Enhancements and annexes to GSM has beendeveloped in order to extend this capability.

They are namely High Speed Circuit Switched Data (HSCSD), Enhanced Data for the Evolution of GSM(EDGE), and General Packet Rate Service (GPRS).

General Packet Radio System (GPRS)

This is a packet-switched network based on GSM.

The radio and network resources are only accessed when data actually needs to be transmitted betweenthe mobile user and the network.

In between alternating transmissions, no network resources need to be allocated. Therefore, packet-switching saves resources, especially in the case of bursty transactions.

GPRS utilises the same principle as the HSCSD, bundling of timeslots to achieve higher data rates.

Additionally, new channel coding schemes have been defined. With 8 timeslots and coding scheme 4,GPRS is able to provide a net throughput rate of 160 kbit/s.

The channel coding schemes provides for higher data transmission rates per timeslot sacrifice dataprotection for speed. Therefore, the new coding schemes 2, 3 and 4 make less provision for forward errorcorrection and can only be applied when radio conditions are good to excellent.

1 Timeslot 2 Timeslots 8 TimeslotsCS-1: 8.0 kbit/s 16.0 kbit/s 64.0 kbit/sCS-2: 12.0 kbit/s 24.0 kbit/s 96.0 kbit/sCS-3: 14.4 kbit/s 28.8 kbit/s 115.2 kbit/sCS-4: 20.0 kbit/s 40.0 kbit/s 160.0 kbit/s

GPRS The Logical ArchitectureAn entirely new core network is required.

However, the base station subsystem or BSS can be used for both circuit-switched services and GPRSpacket-switched services. BSS needs to be upgraded with the so-called Packet Control Unit or PCU.

The new network elements within the GPRS core network are theServing GPRS Support Node SGSB.Gateway GPRS Support Node or GGSN,Border Gateway or BGCharging Gateway or CG.


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The SGSN is part of the GPRS core network. Together with the GGSN, the SGSN forms the GPRS corenetwork .SGSN and GGSN can coexist in the same node.

The PCU interfaces the new GPRS core network to the existing GSM BSS.This is done by converting packet data coming from the SGSN into the so called PCU-frames that havethe same format as TRAU-frames.These PCU-frames are transparently routed through the BSC towards the BTS.The BTS needs to determine the respective coding scheme and other options before processing a PCU-frame.

• Additionally, the PCU takes over all GPRS radio related control functions from the BSC. Thismeans that the PCU is in charge of all functions that are related to the Radio Link Control (RLC)and Medium Access Control (MAC) Protocols (RLC/MAC) on the network side.

• The PCU can be located anywhere in the network. It depends on the location of the TRAU(Transcoder/rate adapter unit) where a network operator needs to place the PCU, too.

The GGSN or Gateway GPRS Support Node

GGSN is one of the new network elements that need to be introduced for GPRS. The GGSN is part of theGPRS core network and interconnects the PLMN towards external packet data networks via the GInterface.

• GGSN is a packet switch with some extra functionality required for its additional functions in amobile network.

• From the perspective of an external network like the internet, the PLMN looks like a normal IP-subnetwork, and the GGSN appears to be an IP-router.

• GPRS supports IP, PPP and IHOSS as packet data protocols (PDP). However, which PDP's aresupported depends on the capabilities of the different GGSN(s). Not every GGSN supports allPDPs. When activating a PDP-context, a MS needs to access the correct GGSN that may servethat request. Another option is that the SGSN selects the correct GGSN based on subscriptiondetails or APN.

• Different GGSNs within a PLMN may support different packet data protocols or they may havedifferent capabilities. For instance:=> GGSN A can represents the operator's ISP-GGSN=> GGSN B interconnects the PLMN towards the internet=> GGSN C is a specialized GGSN that interconnects only to the IP-based intranet of onespecific volume customer (e.g. large corporation).

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Routing and Mobility Management

From MS, the SGSN decapsulates the packet, examine the address information and route them to theright GGSN, which then route them to the PDN.

The network specific routing procedure are used by the PDN for routing the packets to the destination.

The packets of the destination are routed through the PDN to the GGSN after the destination address hasbeen evaluated.


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The GGSN evaluates the destination address for the routing context and queries the correspondingSGSN and associated tunnel information. The packet is then encapsulated and tunneled to the MS.

Two packet-encapsulating schemes:• Between GGSN, packets are routed using GPRS Tunnel Protocol. This allows any standard

protocol to be used, even if they are not supported by all SGSN.

• Between SGSN and MS, separates the layer 2 protocols from the network layer protocol.

Mobility Management

Before an MS can establish a connection it must establish a logical connection context (so calledattachment procedure), resulting in an assignment of a temporary logical connection identity.

After this, one or more routing context for one or more PDP, could be negotiated with the GGSN.

A ciphering key sequence indicates how the user data is encoded.

The GPRS register is consulted to check whether an MS has access to the respective PDN. If access isallowed the GPRS is asked to update its routing context.

Agreements are constantly updated during a GPRS session. The location of an MS is monitored on thebasis of the state diagram.

The MS informs the SGSN when it changes the cell while in state Ready. In state Standby an update isdone only when a change in Routing Area is experienced.

Protocol Architecture

All data packets from and to external networks within the GPRS backbone network is being transmitted inGPRS Tunnel Protocol (GTP). The relevant signalling information and the data packets are transmittedbetween SGSN and GGSN.

This implements the IWF between GSM and the external network protocol.

The fixed network protocol between SGSN and GGSN (Gn interface) will use standard protocols.

Between MS and SGSN, the lowest sublayer of the GSM layer 3 is used. It can multiplex severalnetwork layer connections on a virtual data link layer connection, and offers coding and datacompression algorithm.

Base Station Subsystem GPRS Application Protocol (BSSGP) at the Gb interface performs the samefunction as the GSM function.

In order to perform multiplexing of various traffic at the physical channel, the Data Link layer at the Uminterface is divided into 2 layers: radio link control (RLC) and medium access control (MAC).


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LLC Sublayer:Uses the LAPDm, called LAPG (LAP on GSM).Some modifications made are:

Variable frame lengths: A variable frame lengths are applicable, hence frame delimiters and bitstuffings are not necessary. Length Indicator (LI) are required at the header.

Variable address types: New address types are used hence address field must be variable,controlled by the Address Extension (EA) bit in the header.

Each Data Link Control Identifier (DLCI) contains a SAPI and TEI. Can contain addresses suchas TLLI, TMSI, IMSI and IMGI.The address are assigned during the check in process.

Prioritized SAPIs Services are differentiated using SAPI, as in GSM.

The MAC Layer:

Allows large number of users to access the network. Contains contention resolution, multiplexing, andreservation strategy with QoS consideration.

Here several TCH can be allocated to an MS at the same time (multislot assignment), and the data ratecan be dynamically changed to n 9.6kb/s.

The Physical layer: Divided into physical link layer (PLL) and physical radio frequency layer(RF). The RF is for modulation and demodulation of radio waves, the PLL is for coding andinterleaving and synchronization.

RLC/MAC Protocol

It controls access to the radio channel through the normal process of contention resolution, multiplexingand channel reservation

It works in multislot mode in which the MS can be allocated several slots at the same time.

Channel access is based on Slotted Aloha.

LLC Frame is first segmented distributed over the RLC/MAC PDU. An RLC/MAC PDU is interleaved into4 normal bursts, which will be transmitted in consecutive TDMA frames, in the same time slots.

The ARQ/LLC protocol controls the selective requests for errored frames.

The LLC


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The mapping of PDCH to the physical channels follows the current GSM principle.

PDCH is given a time slot and TDMA number.

The same PDCH data can be sent to different MS at the same time.

PDCH is used as a bidirectional signalling channel. All the necessary signalling takes place on the Master PDCH.The MPDCH is divided into:

Packet BCCHPRACHPPCHPAGCH

Channel Structure

A multiframe structure is required for mapping the logical channel to the physical GSM channels.

Along with the IMSI, an MS operating in packet data mode must be able to receive the signalling channels of GSM.

A 52 Multiframe, from 2 26 frame is defined for the RLC/MAC protocol.

Multiframe structure consists of 4 52-frame multiframes.

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gprs.pdf

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