Subject : Mobile Communication

Topic : 3G Communication System

Difficulty : Medium

• General Packet Radio Services is a new bearer service for GSM that greatly improves and simplifies wireless access to packet data networks.

  • GPRS applies packet radio principal to transfer user data packets in an efficient way between MS and external packet data network.

  • Modulation technique used in GPRS technology is GMSK. It is an important step on the path to 3G. It is a support to leading internet communication protocols.

  • GPRS uses radio channel i.e. 200 kHz wide. Radio channel carries digital data stream of 270.833 kbps. This rate is divided into 8 time slots each carrying 33.55 kbps per time slot. Data rate is 14.4 kbps per time slot and GPRS can combine upto 8 time slots giving data rate of 115 kbps.

  • The general packet radio service (GPRS) enhances GSM data services significantly by providing end-to-end packet switched data connections. This is particularly efficient in Internet/intranet traffic, where short bursts of intense datacommunications are actively interspersed with relatively long periods of inactivity.

  • As there is no real end-to-end connection to be established, setting up aGPRS call is almost instantaneous and users can be continuously on-line.

  • Usershave the additional benefits of paying for the actual data transmitted, rather thanfor connection time.

  • Because GPRS does not require any dedicated end-to-endconnection, it only uses network resources and bandwidth when data is actuallybeing transmitted. This means that a given amount of radio bandwidth can be shared efficiently among many users simultaneously.

  • Services of GPRS have been developed toreduce connection set-up time and allow an optimum usage of radio resources.

  • GPRS provides a packet data service for GSM where time slots on the air interfacecan be assigned to GPRS over which packet data from several mobile stations ismultiplexed.

  • GPRS is mainly used to send and receive data such as e-mail and web browsing. GPRS is relatively cheap service as compared to circuit switched data.

  • GPRS terminals:

  • Class A: MS supports simultaneous operation of GPRS and GPS services.

  • Class B: MS can able to register with the network for both GPRS and GSM services but can use only one of the two services at a given time.

  • Class C: MS can attach for either GPRS or GSM services.

  • Benefits of GPRS technology:

  • Higher data rates (14.4-115 kbps)

  • Circuit switching and Packet switching can be used in parallel.

  • Constant connectivity

  • Easy billing

  • Efficient use of radio bandwidth.

  • Limitations of GPRS technology:

  • It has limited cell capacity.

  • Data rate is lower than mentioned

  • It does not provide store and forward service therefore if the MS is not available the data get lost.

  • GPRS Architecture: It is same as GSM architecture with some modifications as shown in figure. There is addition of new packet data switching and gatewaynodes, and an upgrade to existing nodes to provide a routing path for packetdata between the wireless terminal and a gateway node. The gateway node providesinterworking with external packet data networks for access to the Internet,intranet, and databases.

A. Mobile station (MS): Mobile station communicates with BSS by using radio air interface. To connect with SGSN the GPRS facility should be added in MS. It consists of User Equipment and SIM. SIM gives GM subscribers their identity.

B. Base Station Subsystem (BSS): It consists of many BSC (Base Station Controller) and BTS (Base Transceiver Station). BSCs connect to single MSC and each BSC controls several hundred of BTSs. The BTS and BSC interface is called as Abis interface. BSC reduces burden on MSC in case of handover from one BTS to another connected to same BSC.

C. Core Network (CN):

• MSC (Mobile Switching Controller) is the central unit of the network. It controls mobility management, call set up, location updating, routing, basic switching and supplementary services.

• HLR (Home Location Register) consists of subscriber information and location of each user who reside in same city as MSC. It gives IMSI.

• VLR (Visitor Location Register) temporarily stores the IMSI of each roaming subscriber.

• AUC (Authentication Centre) is strongly protected database which handles authentication and encryption key for every single subscriber in HLR and VLR. It consists of EIR (Equipment Identity Register) to check for stolen phones.

• It also consists of SGSN and GGSN.

D. Serving GPRS Support Node (SGSN): It is functionally connected with BSC and physically can be at MSC or BSC site. One SGSN can support BSCs of several MSC sites. Functions of SGSN are:

• Authentication of GPRS mobiles.

• Mobile registration in GPRS network.

• Mobile mobility management.

• Handling MO and MT traffic.

• Collection of charging information of air interface usage

• TCP/IP header compression

E. Gateway GPRS Support Node:

• It interfaces GPRS backbone network and external packet data networks.

• Converts the GPRS packets from SGSN to the PDP format

• Converts PDP addresses to GSM addresses of the destination user.

• It stores the current SGSN address and profile of the user in its location register.

• There is many-to-many relation among SGSNs and GGSNs.