GSM

Following is the simple architecture diagram of GSM Network.
The added components of the GSM architecture include the functions of the databases and messaging systems:

• Home Location Register (HLR)
• Visitor Location Register (VLR)
• Equipment Identity Register (EIR)
• Authentication Center (AuC)
• SMS Serving Center (SMS SC)
• Gateway MSC (GMSC)
• Chargeback Center (CBC)
• Transcoder and Adaptation Unit (TRAU)

Following is the diagram of GSM Netwrok alongwith added elements.
The MS and the BSS communicate across the Um interface, also known as the air interface or radio link. The BSS communicates with the Network Service Switching center across the A interface.

GSM network areas:

In a GSM network, the following areas are defined:

• Cell: Cell is the basic service area: one BTS covers one cell. Each cell is given a Cell Global Identity (CGI), a number that uniquely identifies the cell.
• Location Area: A group of cells form a Location Area. This is the area that is paged when a subscriber gets an incoming call. Each Location Area is assigned a Location Area Identity (LAI). Each Location Area is served by one or more BSCs.
• MSC/VLR Service Area: The area covered by one MSC is called the MSC/VLR service area.
• PLMN: The area covered by one network operator is called PLMN. A PLMN can contain one or more MSCs.

GSM System Architecture

In GSM system the mobile handset is called Mobile Station (MS). A cell is formed by the coverage area of a Base Transceiver Station (BTS) which serves the MS in its coverage area. Several BTS together are controlled by one Base Station Controller (BSC). The BTS and BSC together form Base Station Subsystem (BSS). The combined traffic of the mobile stations in their respective cells is routed through a switch called Mobile Switching Center (MSC). Connection originating or terminating from external telephone (PSTN) are handled by a dedicated gateway Gateway Mobile Switching Center (GMSC). The architecture of a GSM system is shown in the figure 2.1 below.

Figure 2.1: GSM Architecture (Source: Bettstetter et. all)

In addition to the above entities several databases are used for the purpose of call control and network management. These databases are Home Location Register (HLR), Visitor Location Register (VLR), the Authentication Center (AUC), and Equipment Identity Register (EIR).
Home Location Register (HLR) stores the permanent (such as user profile) as well as temporary (such as current location) information about all the users registered with the network. A VLR stores the data about the users who are being serviced currently. It includes the data stored in HLR for faster access as well as the temporary data like location of the user. The AUC stores the authentication information of the user such as the keys for encryption. The EIR stores stores data about the equipments and can be used to prevent calls from a stolen equipments.
All the mobile equipments in GSM system are assigned unique id called IMSI (International Mobile Equipment Identity) and is allocated by equipment manufacturer and registered by the service provider. This number is stored in the EIR. The users are identified by the IMSI (International Module Subscriber Identity) which is stored in the Subscriber Identity Module (SIM) of the user. A mobile station can be used only if a valid SIM is inserted into an equipment with valid IMSI. The ``real'' telephone number is different from the above ids and is stored in SIM.

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 or personal identity number.

Base Station Subsystem

The Base Station Subsystem is composed of two parts, the Base Transceiver Station (BTS) and the Base Station Controller (BSC).  These communicate across the specified A�bis interface, allowing (as in the rest of the system) operation between components made by different suppliers. The Base Transceiver Station houses the radio tranceivers that define a cell and handles the radio�link protocols with the Mobile Station.  In a large urban area, there will potentially be a large number of BTSs deployed.  The requirements for a BTS are ruggedness, reliability, portability, and minimum cost.
The Base Station Controller manages the radio resources for one or more BTSs.  It handles radio�channel setup, frequency hopping, and handovers, as described below.  The BSC is the connection between the mobile and the Mobile service Switching Center (MSC).  The BSC also translates the 13 kbps voice channel used over the radio link to the standard 64 kbps channel used by the Public Switched Telephone Network or ISDN.

Network Subsystem

The central component of the Network Subsystem is the Mobile services Switching Center (MSC).  It acts like a normal switching node of the PSTN or ISDN, and in addition provides all the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber.  These services are provided in conjuction with several functional entities, which together form the Network Subsystem.  The MSC provides the connection to the public fixed network (PSTN or ISDN), and signalling between functional entities uses the ITU�T Signalling System Number 7 (SS7), used in ISDN and widely used in current public networks. The Home Location Register (HLR) and Visitor Location Register (VLR), together with the MSC, provide the call�routing and (possibly international) roaming capabilities of GSM.  The HLR contains all the administrative information of each subscriber registered in the corresponding GSM network, along with the current location of the mobile.  The current location of the mobile is in the form of a Mobile Station Roaming Number (MSRN) which is a regular ISDN number used to route a call to the MSC where the mobile is currently located.  There is logically one HLR per GSM network, although it may be implemented as a distributed database.
The Visitor Location Register contains selected administrative information from the HLR, necessary for call control and provision of the subscribed services, for each mobile currently located in the geographical area controlled by the VLR.  Although each functional entity can be implemented as an independent unit, most manufacturers of switching equipment implement one VLR together with one MSC, so that the geographical area controlled by the MSC corresponds to that controlled by the VLR, simplifying the signalling required.  Note that the MSC contains no information about particular mobile stations - this information is stored in the location registers.
The other two registers are used for authentication and security purposes.  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.  The Authentication Center is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and ciphering of the radio channel.