em for mobile communication. It is a cellular (user COV- erage area defined in the shape of cells) communication network. It was developed Fundamentals of EMS, NMS and OSS/BSS Core network Figure 9.2 GSM network simplified architecture. as an attempt to standardize the European mobile telephone network and is now one of the most popular standards for mobile phones in the world. GSM networks usually operate in the 900 MHz or 1800 MHz bands and in some countries they use the 850 MHz and 1900 MHz bands. The modulation schema used in GSM is Gaussian minimum shift keying (GMSK) and bandwidth is distributed among many users using a combination of time and frequency division multiple access (TDMA/FDMA). The GSM network consists of (see Figure 9.2): Mobile station (MS): This is the user/mobile equipment that helps the user to make calls and receive subscribed services. Base station system (BSS): It is linked to the radio interface functions and has two components. Base transceiver station (BTS): The BTS is the signal handling node between an MS and the BSC. Its main function is transmitting/receiving of radio signals and encrypting/decrypting communications with the base station controller (BSC). Base station controller (BSC): The BSC is the controller for multiple BTS and its main functionality is to handle handover (user moving from one cell to another) and allocate channels. Switching system/network switching subsystem/core network: It has switch- ing functions and a main center for call processing. The components in the core network are: - Home location register (HLR): This is a database for storing subscriber information. The information includes subscriber profile, location infor- mation, services subscribed to, and the activity status. A subscriber is first registered with the HLR of the operator before he or she can start enjoying the services offered by the operator. Communication Networks Mobile services switching center/mobile switching center/call server (MSC): This is the main switching node of the GSM netwo