on, chan- nel coding can be reduced to improve the user throughput from 9.6 to 14.4 Kbps while keeping the same bit error rate. If the user moves and the quality of the channel decreases, channel coding is changed back to the original rate of 9.6 Kbps, which tolerates higher interference. HSCSD is sometimes called multislot service and it increases data throughput by combining one to four time slots on one carrier frequency into a single data channel. The maximum user data rate of HSCSD is 4 X 14.4 Kbps = 57.6 Kbps. HSCSD is a circuit-switched service and its user fee is based on the connection duration and the number of time slots used. EDGE implements a new modulation scheme in the GSM air inter- face. Originally GSM used binary modulation that was very noise tolerant. In good propagation conditions we can use a less noise tolerant nonbinary modulation scheme and increase the user data rate via the same channel. EDGE triples the user data rate with the help of 8-PSK modulation, which transmits three bits in each symbol instead of one. EDGE defines several coding schemes and by selecting a suitable modulation and coding scheme the system can adapt its operation to channel conditions. EDGE will increase the user data rate from the original 9.6/14.4 Kbps per time slot to 59.2 Kbps (no error control) per time slot. The most important application for EDGE will be GPRS, which is then called EGPRS (GPRS with EDGE). We have used GSM here as an example of a digital cellular radio system and illustrated the functionality of the network with a few simplified exam- ples. A more detailed study of cellular network functionality is beyond the scope of this book. More comprehensive descriptions about the operation of GSM are given in [1, 2]. 5.7 GPRS GPRS will surpass HSCSD because it provides optimum service for data users. It is a genuine packet-switched system in which the radio channel is Mobile Communications reserved only for the time during which data transmission takes place. It sup- ports asymmetrical transmission and