GSM (Global System for Mobile Communication)

GSM is a TDMA-based wireless communications system. Work on the GSM specifications started in the 1980s in Europe as a result of the capacity limits being experienced by analog networks such as NMT.The GSM 900 system uses two 25-MHz bands for the uplink and downlink, and within this spectrum 200-KHz channels are allocated. The uplink and downlink are separated by a 45-MHz spacing. GSM 1800 uses two 75-MHz bands for the uplink and downlink. Again 200-KHz channels are allocated within those bands and are separated by a 95-MHz spacing. The 1900-MHz systems use two 60-MHz bands for the uplink and downlink using 200-MHz channels within those bands and separated by 80-MHz spacing.

Europe felt the need for a common mobile telephony standard since different countries had differing analog networks, and as a result roaming of subscribers between these networks was not possible. CEPT (Conference European des Postes et Telecommunications) is a standardization arena in Europe. A new group called GSM (Groupe Special Mobile) was formed within CEPT in 1982 whose task was to specify a unique radio communication system for Europe at 900 MHz. The mobile station (MS) is the terminal (phone, PDA mobile unit) provided to the subscriber. It is essentially a GSM two-way radio that conforms to the air interface specifications.

The base station subsystem is functionally subdivided into the base station controller (BSC) and the base transceiver station (BTS). A single BSC normally controls a large number of BTSs. BTSs contain the radio equipment and are connected to cell site antennas. The BTS is essentially a layer two bridge if viewed from a high-level perspective. It provides an entry point for the subscribers who are present in the cell, allowing them to make or receive calls. Some of the base station functions are radio transmission in GSM format, use of frequency hopping techniques, coding and decoding of radio channels, and measurement of quality and received power on traffic channels.

GSM uses two databases, called the home location register (HLR) and the visitor location register (VLR). The HLR contains the subscriber's profile information (which is static) as well as the current location of the subscriber (i.e., it knows the reachability information of the subscriber). The VLR stores the current location or point of attachment to the network and the state of the mobile terminal. For mobile terminated calls, the HLR is the initial signaling contact point in the mobile network, whereas the VLR is the initial signaling contact when the call originates from the mobile