r and application: The fixed terminals are terminals installed in cars. The GSM portable terminals can also be installed in vehicles. Figure 1-11 The SIM. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction to GSM Introduction to GSM The hand-held terminals have experienced the biggest success thanks to their weight and volume, which are continuously decreasing. The mobile station also provides the receptor for SMS messages, enabling the user to toggle between the voice and data use. Moreover, the mobile facilitates access to voice-messaging systems. The mobile station also provides access to the various data services available in a GSM net- work. These data services include X.25 packet switching through a synchronous or asynchronous dial-up connection to the PAD at speeds typically at 9.6 Kbps General Packet Radio Services using either an X.25- or IP-based data transfer method at speeds up to 115 Kbps High-speed circuit-switched data at speeds up to 64 Kbps The data speeds will vary by application and other conditions such as air interfaces across a hostile link. The Base Transceiver Station (BTS) The Base Transceiver Station (BTS) (Figure 1-12) houses the radio trans- ceivers that define a cell and handles the radio link protocols with the mobile station. In a large urban area, a large number of BTSs may be deployed. The requirements for a BTS are Ruggedness Reliability Portability Minimum cost The BTS corresponds to the transceivers and antennas used in each cell of the network. A BTS is usually placed in the center of a cell. Its transmit- ting power defines the size of a cell. Each BTS has between 1 and 16 trans- ceivers depending on the density of users in the cell. Each BTS serves a single cell. It also includes the following functions: Encoding, encrypting, multiplexing, modulating, and feeding the RF signals to the antenna 