Global System for Mobile Communications
The GSM standard (Global System for Mobile Communications) for mobile telephony was introduced in the mid-1980s and is the European initiative for creating a new cellular radio interface. The GSM system uses a TDMA radio access system employed in 135 countries, operating in 200 KHz channels with eight users per channel. It is the most widely deployed digital network in the world today, used by 10.5 million people in more than 200 countries.
1.1 GSM Bandwidth Allocation
GSM can operate four distinct frequency bands:
GSM 450: GSM 450 supports very large cells in the 450 MHz band. It was designed for countries with a low user density such as in Africa. It may also replace the original 1981 NMT 450 (Nordic Mobile Telephone) analog networks used in the 450 MHz band. NMT is a ﬁrstgeneration wireless technology. GSM 900: When speaking of GSM, the original GSM system was called GSM 900 because the original frequency band was represented by 900 MHz. To provide additional capacity and to enable higher subscriber densities, two other systems were added afterward: GSM 1800: GSM 1800 (or DCS 1800) is an adapted version of GSM 900 operating in the 1800 MHz frequency range. Any GSM system operating in a higher frequency band requires a large number of base stations than for an original GSM system. The availability of a wider band of spectrum and a reduction in cell size will enable GSM 1800 to handle more subscribers than GSM 900. The smaller cells, in fact, give improved indoor coverage and low power requirements. GSM 1900 (or PCS 1900): PCS 1900 (Personal Communications System) is a GSM 1800 variation designed for use on the North American Continent, which uses the 1900 MHz band. Since 1993, phase 2 of the speciﬁcations has included both the GSM 900 and DCS 1800 (Digital Cellular System) in common documents. The GSM 1900 system has been added to
Mobile Communication Systems and Security Man Young Rhee Ó 2009 John Wiley & Sons (Asia) Pte Ltd
Mobile Communication Systems and Security
the IS-136 D-AMPS (Digital Advanced Mobile Phone System) and IS-95 Code Division Multiple Access (CDMA) system, both operated at the 1900 MHz band. The ITU (International Telecommunication Union) has allocated the GSM radio spectrum with the following bands: .
GSM 900: Uplink: 890–915 MHz Downlink: 935–960 MHz GSM 1800: Uplink: 1710–1785 MHz Downlink: 1805–1880 MHz GSM 1900: Uplink: 1850–1910 MHz Downlink: 1930–1990 MHz
In the above, uplink designates connection from the mobile station to the base station and downlink denotes connection from the base station to the mobile station.
1.2 GSM System Architecture
A cell containing a Mobile Station (MS) is formed by the radio coverage area of a Base Transceiver Station (BTS). Several BTSs together are controlled by one Base Station Controller (BSC). The BTS and BSC form the Base Station Subsystem (BSS). The combined trafﬁc of the MSs in their respective cells is routed through the Mobile Switching Center (MSC). Several databases are required for call control and network management: the Home Location Register (HLR), the Visitor Location Register (VLR), the Authentication Center (AuC), and the Equipment Identity Register (EIR). The GSM system architecture comprised with a set of essential components is illustrated in Figure 1.1. The GSM system network can be divided into three subgroups that are interconnected using standardized interfaces: Mobile Station (MS), Base Station Subsystem (BSS), and Network SubSystem (NSS). These subgroups are further comprised of the components in the following sections.
1.2.1 Mobile Station (SIM þ ME)
The Mobile Station (MS) can refer to a handset or mobile equipment (ME). The Subscriber Identity Module (SIM) card in a GSM handset is a microprocessor smart card that securely stores various critical information such as the subscribers identity as well as the authentication and...