GPRS stands for General Packet Radio Service.
- It can offer speeds of 14.4KBps to 171.2 KBps.
- It allows short bursty traffic such as emails and web browsing as well as large volumes of data.
- It is less costly as compared to circuit switched data and SMS.
- It is referred to as always connected because of fast connection setup mechanism to offer perception of being fast.
- It is called 2.5G because of high speed transmission over GSM and TDMA.
Quality of Service Parameters in GPRS
The quality of service in GPRS is based on service precedence, reliability, delay, throughput.
- Service Precedence: Priority of service in relation to another service, like high, medium, low.
- Reliability: indicates transmission characteristics required by application, such as probability for loss, duplication, missequencing and corruption (undetected errors) of packets.
- Delay: maximum values for main delay and 95 percentile delay. Delay is defined as end-to-end transfer time between two mobile stations or between mobile stations and signaling interface to external data packet network.
- Throughput: specifies maximum or peak bit rate and mean bit rate.
GPRS Network Architecture
GPRS uses GSM like architecture for voice. The GSNs, or GPRS Support Nodes are the network nodes in a GPRS network. They are responsible for delivery and routing packets between mobile stations and external packet data networks (PDNs).
There are two types of GSNs -
- Serving GSN (SGSN): There tasks include packet switching, routing, transfer, mobility management, authentication and charging functions. It processes and records new mobile subscriber's location. It sends queries to HLR (Home Location Register) for profile data of GPRS subscribers. It maintains location register to store location information and user profiles of all GPRS users registered with SGSN.
- Gateway GSN (GGSN): This is an interface between the GPRS backbone network and external packet data networks. It maintains routing information for tunnelling PDUs (Protocol Data Units) and uses PDPs (Packet Data Protocol) to convert GPRS packets for data networks like Internet. It also performs authentication and charging functions of data transfer.
GPRS Network Enhacements
The GPRS is different from GSM as it has SGSN and GGSN along with upgraded GSM elements:
- Base Station Subsystem: needs enhancements to recognise and send packet data. BTS (Base Transceiver Station) upgrade for transportation of user data to SGSN and supporting packet data transportation between BTS and Mobile Station.
- Home Location Register: To register GPRS user profiles and respond to queries frm SGSN.
- Mobile Station: It is different from mobile station of GSM.
- SMS Nodes: To support SMS transmissions via SGSN.
Channel Coding
Channel coding is done to protect transmitted data packets against errors. The technique used is similar to GSM's. Reliable Coding Scheme is used, wherein many redundant bits are added for burst errors. This scheme is used under bad channel conditions, which gives a data rate of 9.05 KBps per time slot. In good channel conditions, however, no scheme is used, and the data rate is 21.4KBps per time slot.
Transmission Plane Protocol Architecture
- Signaling Plane: it comprises of protocols for control and support of transmission plane. It includes GPRS attach and detach, PDP context activation, control of routing paths, allocation of network resources. Enhanced MAP (Mobile Application Part) is employed between SGSN and HLR, and between SGSN and EIR (Equipment Identity Register). It is a mobile network specific extension of signaling system SS#7 used in GSM. It transports signaling information related to location updates, routing information, user profiles and handovers. Exchange of MAP messages is accomplished over TCAP (Transmission Capabilities Application Part) and SCCP (Signaling Connection Control Part).
- GPRS Backbone: It includes the transmission plane between SGSN and GGSN. The GTP (GPRS Tunneling Protocol) is used to encapsulate data packets and signaling information, and tunneling then through GPRS backbone by adding GPRS specific routing information.
- BSS-SGSN Interface: It is divided into several layers. The SNDCP (Sub-Network Dependent Convergence Protocol) is used to transfer data packets between SGSN and Mobile Station, multiplexing of several connections of network layer onto one virtual logical connection of underlying LLC layer, segmentation, compression and decompression of data. Logical Link Control (LLC) is a data link layer protocol for GPRS, assures reliable transfer of user data across a wireless network. BSSGP (Base Station Subsystem GPRS Protocol) delivers routing and QoS related information between BSS and SGSN. Network Service manages convergence sublayer that operates between BSSGP and Frame Relay Core by mapping service requests to services.
- Air Interface: Physical layer and Data Link Layer.
- Data Link Layer: It has three sublayers. Logical Link Control (LLC) provides reliable logical link between mobile station and its SGSN. It provides sequence control, in-order delivery, flow control, detection of transmission errors, retransmission, encryption for data confidentiality and variable lengths. The Radio Link Control (RLC) is used to establish reliable link between mobile station and Base Station Subsystem. It is responsible for segmentation and reassembly of LLC frames into RLC data blocks and ARQ of uncorrectable data. Medium Access Control (MAC) controls the access attempt of mobile station on radio channel shared by multiple mobile stations. It has algorithms for contention resolution, multiuser multiplexing on packet data traffic channel (PDTCH), scheduling and prioritizing based on negotiated QoS.
- Physical Layer: It has two sublayers. The Physical Link Layer (PLL) provides services for data transfer over physical channel between mobile station and network. It provides data unit framing, data coding, detection and correction of physical medium transmission errors. It uses services of the radio frequency layer. The Physical RF Layer (RFL) performs modulation of physical waveforms based on sequences of bits received from PLL. It demodulates received waveform into sequence of bits for PLL to intercept.
- Multiple Access Radio Resource Management: GPRS uses a combination of FDMA and TDMA. It has two frequency bands each 45 MHz apart, with uplink between 890-915 MHz, and downlink between 935-960 MHz. Each band of 25 MHz width is divided into 124 single carrier channels of 200 KHz width further divided into 8 time slots. The logical channels are for signaling, broadcast of general system information, synchronisation, channel assignment, paging or payload transport. There are two types of channels - traffic channels and signaling channels. Traffic channels are only allocated when data packets are sent or received, and are released after transmission.
