tems is based on the transmission of digitally coded information. The radio transmission itself is analogue - there are no square-type radio waves. GSM (global system for mobile communication) systems were launched on the market in 1992. In 1993/94 commercial PDC (personal digital communication) systems were implemented in Japan and D-AMPS (digital AMPS), later called TDMA systems, were implemented in the USA. In second-generation systems speech is still dominating, but fax, short message service (SMS) and data transmission are also available. Several supplementary services compa- rable to the fixed network can be used, and fraud prevention and ciphering of user data are possible. The GSM network is divided into two systems, the switching system and the base station system. The switching system is responsible for performing call processing and subscriber- related functions. The base station system performs all the radio-related functions. A cell is the basic unit of a cellular system and is defined as the area of radio coverage given by one base station antenna system. See Figure 13.21. As GSM has grown worldwide, it has expanded to operate at three frequency bands: 900, 1800 and 1900 MHz. Various frequencies are used by mobile networks in the first and second generation. See Figure 13.22. In GSM each radio frequency can be used by maximum of eight subscribers. Each subscriber is allocated a time slot or channel and uses this channel for speech (13 kbit/s) or data transmission (9.6 kbit/s). 13.11.2 GPRS The GPRS system provides a basic solution for IP communication between mobile stations and Internet service hosts or a corporate LAN. Figure 13.21 The cell is covered by one antenna system Understanding Changing Telecommunications - Building a Successful Telecom Business MHz 0 NMT 450 GSM 900 PDC 1500 GSM 1800 D-AMPS 1900 D-AMPS 800 NMT 900 GSM 1900 PDC 800 Figure 13.22 Frequency allocation. D-AMPS was the digital evolution of the US AMPS analogue mobile telephony system, now known as TDMA, and also known as IS-13