pt with packet data access functions. GPRS is introduced both in GSM and TDMA/136 networks. A cellular network that includes GPRS functionality can connect mobile terminal systems to packet- data networks of type IP or X.25. For the reader who is not familiar with the tech- nology of mobile networks, some basic information might be needed: Networks that provide communication services to moving terminal systems in a cellular access area are generally called public land mobile network, PLMN. Terminal Systems are called mobile systems, MS. The network interface between MS and PLMN is called the radio interface. 4.4 General Packet Radio Service, a Case Study There are basically two types of radio interfaces: interfaces based on time divi- sion multiple access (TDMA) and interfaces based on code division multiple access (CDMA). The difference between the methods is how the capacity of radio channels is divided. Both GSM and TDMA/136 are TDMA systems. In TDMA systems the capacity of a radio channel is divided into equal and fixed time slices, called timeslots. The capacity of a timeslot is such that it must be able to carry compressed digi- tised voice signals with acceptable quality. A radio channel in GSM is 200 kHz and is divided into eight or sixteen timeslots, depending on the degree of voice compression that will be used. The radio channel in TDMA/136 is 30 kHz and is divided into three or six timeslots, depending on the degree of compression. Channels from MS to PLMN (the uplink) and from PLMN to MS (the down- link) have exactly the same structure of timeslots. Both GSM and TDMA/136 provide communication service based on TDMA and on circuit-switched connections inside the PLMN. In the radio interface, a timeslot must be dedicated for each call for its duration. A timeslot is regarded as the (temporary) physical media connection between MS and PLMN during a call. The TDMA schema works fine for isochronous services like voice communica- tion, but provides low rates for data communication (10 kbps). The TDMA schema