European radio mes- saging system (ERMES). However, in many countries use of paging systems has decreased because many cellular systems provide a similar or even better bidirectional messaging service. 5.4.4 Analog Cellular Systems Section 5.3 we introduced the operation of cellular networks in general. The first cellular technologies were analog and they became available in the first half of the 1980s. These systems are often referred to as first generation cellular systems and these are the most important analog cellular systems: Advanced Mobile Phone System (AMPS) in the United States; Nordic Mobile Telephone (NMT) used in Nordic countries; Total Access Communications System (TACS) in the United Kingdom. These systems are quite similar but incompatible. They use a frequency band in the range of 800 to 900 MHz (NMT uses 450 MHz as well) and fre- quency modulation. The frequency band is divided into channels and one of these is allocated for each call. We call this radio access principle frequency- division multiple access. 5.4.5 Digital Second Generation Cellular Systems In this section we review the most important digital cellular networks that came into use in the first half of the 1990s. We often refer to these systems as second generation cellular systems. 5.4.5.1 GSM GSM operates at the 900-MHz frequency band and it became the most widely used second generation cellular technology. The structure and opera- tion of the GSM network are explained in Section 5.5. In GSM the subscrip- tion information is stored on a smart card and a subscriber can change his or her mobile telephone any time. When he or she inserts his or her card into the new telephone, he or she has access to exactly the same service as previ- ously. The access method used in GSM is TDMA, in which each frequency channel is divided into time slots for multiple users. Introduction to Telecommunications Network Engineering 5.4.5.2 Digital Cellular System at 1,800 MHz Digital cellular system at 1,800 MHz (DCS-1800) is also known as GSM- 1800. It i