Architecture of the GSM network

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Architecture of the GSM network
MobileStation
BaseStationSubsystem
Network Subsystem

Following is the simple architecture diagram of GSM Network. [pic]
A GSM network is composed of several functional entities, whose functions and interfaces are defined. Figure 1 shows the layout of a generic GSM network.  The GSM network can be divided into three broad parts. The Mobile Station is carried by the subscriber, the Base Station Subsystem controls the radio link with the Mobile Station.  The Network Subsystem, the main part of which is the Mobile services Switching Center, performs the switching of calls between the mobile and other fixed or mobile network users, as well as management of mobile services, such as authentication.  Not shown is the Operations and Maintenance center, which oversees the proper operation and setup of the network.  The Mobile Station and the Base Station Subsystem communicate across the Um interface, also known as the air interface or radio link.  The Base Station Subsystem communicates with the Mobile service Switching Center across the A interface.

,---------------------------------------------------------------------------, | Um A | | Interface A-bis Interface | | | Interface | ,--------------------, | | | ,-----, ,-----, | | | | ,----------|----------, | | | VLR | | HLR | | ,-------, | | ,-----, | ,-----, | | `-----' `-----' | | Other | | | | SIM | | | | BTS | | ,-----, | | | ,-----, | | MSCs | | | `--,--' | `-----'---| | | | | |--------------'-------' | | | , | : | | BSC |-----------| MSC | | | | ,-----, /| | ,-----,---| | | | | |--------------,------, | | | MS |' | ,---| BTS | | `-----' | | | `-----' | / PSTN / \ | | `-----' |/ | `-----' | | ,-----, ,-----, | \ ISDN / | | ' `----------|----------' | | | EIR | | AC | | `------' | | | `-----' `-----' | | | | Base Station Subsystem `--------------------' | | Network Subsystem | `----------------------------------------------------------------------------' SIM Subscriber Identity Module HLR Home Location Register MS Mobile Station VLR Vistor Location Register BTS Base Transceiver Station EIR Equipment Identity Register BSC Base Station Controller AC Authentication Center MSC Mobile services Switching Center PSTN Public Switched Telecomm Network VLR Visitor Location Register ISDN Integrated Services Digital Network FIGURE 1

Mobile Station

The mobile station (MS) consists of the physical equipment, such as the radio transceiver, display and digital signal processors, and a smart card called the Subscriber Identity Module (SIM).  The SIM provides personal mobility, so that the user can have access to all subscribed services irrespective of both the location of the terminal and the use of a specific terminal.  By inserting the SIM card into another GSM cellular phone, the user is able to receive calls at that phone, make calls from that phone, or receive other subscribed services.

The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI).  The SIM card contains the International Mobile Subscriber Identity (IMSI), identifying the subscriber, a secret key for authentication, and other user information.  The IMEI and the IMSI are independent, thereby providing personal mobility.  The SIM card may be protected against unauthorized use by a password...
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