A schematic overview of the GSM system is shown in the figure given below . The system is composed of three main elements ; the switching subsystem, the base station subsystem, and the mobile. The switching part makes the connection between the two users, the base station part controls the communication across the radio interface, and the mobile acts as the transmitter receiver for the user.

A schematic overview of the GSM system is shown in the figure given below . The system is composed of three main elements ; the switching subsystem, the base station subsystem, and the mobile. The switching part makes the connection between the two users, the base station part controls the communication across the radio interface, and the mobile acts as the transmitter receiver for the user.

The best known part of the cellular network is certainly the mobile stations. Different types of mobile stations are distinguished by power and application. The mobile station ( MS) types include not only vehicle mounted portable equipment but also handheld stations popularly known as mobile handsets. A significant architectural aspect of the MS relates to the concept of Subscriber Identity Module ( SIM). The SIM card contains a unique International Mobile Subscriber Identity ( IMSI) used to identify the subscriber to the system. The SIM is basically a smart card, containing all the subscriber -related information stored on the users side of the radio interface.

A potential user may off course buy a mobile equipment, but he may also lease or borrow the equipment or purchase it through other channels. Fixed Mobile Stations are permanently installed in a car and may have a maximum allowed RF output of up to 20W. Portable ( bag phones) can emit up to8 W and handheld portable units up to 2 W. With Second Generation mobiles ( on the market since 1993), the GSM system is becoming more and more attractive. Hand-portable units are becoming much smaller and are coming with numerous features on it. This is giving the system a boost popularity, especially in those markets with a particular demand for small mobiles such as in Asian and Pacific areas.

Base Station Subsystem groups the infrastructure machines, which are specific to the radio cellular aspect of GSM. The BSS is in direct contact with the mobile station through the radio interface. As such, it includes the machines in charge of transmission and reception on the radio path, and the management thereof. On the other side, the BSS is in contact with the switches of Network Subsystem (NSS). The BSS includes two types of machines :

The counterpart to a mobile station within a cellular network is the base transceiver station ( BTS), which is the mobile's interface to the network. Each cell site is equipped with a BTS. A BTS is usually located in the center of a cell. A cell site is used to refer to the physical location of radio equipment that provides coverage within a cell. The transmitting power of the BTS determines the absolute cell size. The BTS houses the radio transceivers that define a cell and handles the radio-link protocols with the Mobile station. BTSs are placed in the field to transfer a call to a customer's handsets, and there are between one and sixteen transceiver, each of which represents a separate RF channel. A BTS may cover an area of 30 - 40 sq kms. However, in a congested, urban location, the BTS coverage area is much smaller. BTS can be considered as complex radio modems and have little other function. A list of hardware located at a cell site includes power sources, interface equipment, radio frequency transmitters and receivers, and antenna systems.

Base station controller is in contact with the switches of NSS. It monitors and controls several base stations, the number of which depends on the manufacturer and can be between several tens and several hundred of stations. A typical BSC can manage from one BTS to the entire BTS in service area, depending on their traffic capacity. The chief tasks of the BSC are frequency administration, the control of a BTS, and exchange functions, it handles radio - channels setup, frequency hopping, and handovers. The BSC is the connection between the mobile station and the Mobile Service Switching Centre and is in charge of all radio interface management through the remote command of the BTS and the mobile station, mainly the allocation and release of radio channels and the handover management. The BSC is connected, on one side, to several BTSs and on the other side, to the Network and Switching Sub System ( more appropriately to a Mobile Switching Centre). A BSC is in fact a small switch with substantial computational capability. The hardware of the BSC may be located at the same site as the BTS, at its own standalone site, or at the site of the Mobile Switching Centre ( MSC). BSC and BTS together form a functional entity some times referred to as the Base Station Subsystem.

The NSS includes the main switching functions, as well as the data basis needed for subscriber data and mobility management. The main role of NSS is to manage the communications between the GSM users and the other telecommunications network users. The NSS is responsible for performing call processing and subscriber-related functions.

The MSC is the interface of the cellular network to the PSTN. MSC performs the telephony switching functions of the system, it acts like a normal switching node of the PSTN, and additionally provides all the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber . MSC is the primary switching interface between the mobile telephone systems, and the PSTN. It is capable of routing calls from the fixed network - via the BSC and the BTS- to an individual mobile station. The MSC also provides the network with specific data about individual mobile stations. The MSC interfaces with BSS on one other side ( through which it is in contact with GSM users) and with the external networks on the other. The NSSs also need to interface with the external networks to make use of their capability to transport user data or signaling between GSM entities. In particular, the NSS make use of a signalling support network, at least partly external to GSM, usually referred to as the SS7 network.

The HLR is a database about subscribers, it stores the identity and user data of all the subscribers belonging to the area of related MSC. These are permanent data, such as the International Mobile Subscriber Number ( IMSI)of an individual user, authentication key, including a subscriber's service profile, location information, activity status and some temporary data. Temporary data on the SIM include such entries as (1) the address of the current visitor location register (VLR), which currently administers the mobile stations (2) the number to which the calls must be forwarded (if the subscriber select call forwarding), and (3) some transient parameters for authentication and ciphering .

The IMSI is permanently stored on the SIM card. The IMSI is one of the pieces of important information used to identify a subscriber within GSM system. The first three digits of the IMSI identify the Mobile Country Code ( MCC) and the next two digits are the mobile network code ( MNC). Up to ten additional digits of the mobile subscriber identification number ( MSIC) complete the IMSI.

The VLR contains the relevant data of all mobiles currently located in a serving (G)MSC. It is the database that contains temporary storing subscription data for those subscribers currently situated in the service area of the corresponding MSC as well as holding data on their location at a more precise level than the HLR. The VLR is always integrated with MSC. The permanent data are the same as data found in the HLR; the temporary data differ slightly. For example, the VLR contains the temporary mobile subscriber identity ( TMSI), which is used for limited periods of time to prevent the transmission of the IMSI via the air - interface. The substitution of the TMSI for the IMSI serves to protect the subscriber from high-technology intruders and helps point to the location of the mobile station through the cell identity.

The VLR has to support the (G)MSC during a call establishment and an authentication procedure as it furnishes data specific to the subscriber. Locating subscriber data in the VLR, as well as in the HLR, reduces the data traffic to the HLR, because it is not necessary to ask for these data every time they are needed. Another reason for storing the identical data at two different locations ( in the HLR & VLR) is that each serves a different purpose. The HLR has to provide the GMSC with the necessary subscriber data when a call is coming from the public network. The VLR, on the other hand serves the opposite function, providing the host (G)MSC with the necessary subscriber data when a call is coming from mobile station.

The Authentication center ( AC) is related to the HLR. It provides the HLR with different set of parameters to complete the authentication of a mobile station. The AC knows exactly which algorithms it has to use for a specific subscriber in order to calculate input values and issue the required results. Since all the algorithms for the authentication procedures are stored within AC, they are protected against abuse. The SIM card issued in area assigned to AC contains the same algorithms for authentication as the AC does. If the AC provides input and output parameters for these algorithms to either the HLR or the VLR, either location register can verify ( authenticate ) the mobile station.

The equipment identity register ( EIR) is a database that contains a list of all valid mobile equipment on the network, where each mobile station is identified by its International Mobile Equipment Identity ( IMEI). An IMEI is marked as invalid if it has been reported stolen or is not type approved . Within the EIR we find all the serial numbers of the mobile equipment that is either stolen or, due to some defect in their hardware, may not be used in a network. The idea is to check the identity at each registration or call setup of any mobile station, and then depending on its IMEI, admit or bar access of the mobile station to the system. The implementation of EIR is relatively a new security feature of the GSM system.

The Operation & Maintenance Centre ( OMC) has access to both the (G)MSC and the BSC, handles error messages coming from the network, and control the traffic load of the BSC and the BTS. The OMC configures the BTS via the BSC and allows the operator to check the attached components of the system. As the cells become smaller and the number of base stations increases, it will not be possible in the future.