d the ciphering sequence. An encryption algorithm uses the ciphering key that is calculated by the authentication center and by the SIM. The ciphering key depends on the subscriber-specific key together with the random number that is given to the mobile station at the beginning of each call. 5.6.4.4 Mobile Subscriber Identity The MS is normally addressed over the air interface by using a temporary mobile subscriber identity (TMSI), which is allocated for each mobile located inside an LA. The global identity of the mobile, international mobile sub- scriber identity (IMSI), which is stored into SIM, is very seldom sent over the air interface to prevent eavesdropping devices from using it as trigger infor- mation. A new TMSI is allocated for the next call when communication is in ciphered mode. IMSI is a global subscriber identification but it is not the same as the telephone number. A subscriber may have several telephone numbers, for example, one for telephone and one for fax, connected to one IMSI in the HLR. He or she may also change SIM (IMSI is changed) but keep the same telephone number. 5.6.5 GSM Enhanced Data Services The business of mobile data services is expected to grow fast when mobile telephone business becomes mature. The original second generation cellular systems provided quite a low data rate (9.6 Kbps) and they utilized a Introduction to Telecommunications Network Engineering symmetrical circuit-switched operation principle that is not optimum for data applications. Most data services have the traffic characteristics of asym- metric high-data-rate "bursty" data transfer. To meet increased demand for better data services, standards have been developed for a new channel coding (14.4 Kbps), high-speed circuit-switched data (HSCSD), GPRS, and enhanced data rate in GSM evolution (EDGE). One physical channel of GSM carries a 22.8-Kbps data rate and trans- mission of user data at only 9.6 Kbps seems wasteful. Most of the overhead is used for error control. For those users who are close to the base stati