GSM - Architecture, Protocols and Services, Third Edition

Chapter 8. Improved data services in GSM: GPRS, HSCSD and EDGE

The main data service used in modern GSM systems is the GPRS. Therefore, the major part of this chapter is devoted to GPRS. However, HSCSD is also of importance. HSCSD was available earlier than GPRS and was therefore chosen by some network providers in order to provide higher data rate services in GSM as fast as possible and it is still used in many networks. A third important aspect of providing improved data rates in GSM systems is EDGE, which helps to increase the data rate of both GPRS and HSCSD by allowing for higher-order modulation schemes when the signal strength is sufficiently high. Let us now first focus on GPRS in the following.

GPRS

Packet data transmission has already been standardized in GSM phase 2, offering access to the Packet Switched Public Data Network (PSPDN); see Appendix A. However, on the air interface such access occupies a complete circuit switched traffic channel for the entire call period. In the case of bursty traffic (e.g. Internet traffic), such access leads to a highly inefficient resource utilization. It is obvious that in this case, packet switched bearer services result in a much better utilization of the traffic channels. This is because a packet channel will only be allocated when needed and will be released after the transmission of the packets. With this principle, multiple users can share one physical channel (statistical multiplexing).

In order to address these inefficiencies, ...