The GSM technical specifications define the different entities that form the GSM network by defining their functions and interface requirements.

The GSM network can be divided into four main parts:

• The Mobile Station (MS).
• The Base Station Subsystem (BSS).
• The Network and Switching Subsystem (NSS).
• The Operation and Support Subsystem (OSS).

The architecture of the GSM network is presented in the figure below:

Architecture of the GSM network

Mobile Station

A Mobile Station consists of two main elements:

• The Subscriber Identity Module (SIM): It is protected by a four-digit Personal Identification Number (PIN). In order to identify the subscriber to the system, the SIM card contains amongst others a unique International Mobile Subscriber Identity (IMSI). User mobility is provided through maping the subscriber to the SIM card rather than the terminal as we done in past cellular systems.

• Mobile equipment/terminal (ME): There are different types of terminals (MN) distinguished principally by their power and application:

  • `fixed' terminals mainly installed in cars. Their maximum allowed output power is 20W
  • portable terminals can also be installed in vehicles. Their maximum allowed output power is 8W.
  • handheld terminals; their popularity is owed to their weight and volume, which is continuously decreasing. According to some specification these terminals may emit up to 0.8W. However, as technology has evolved their maximum allowed power ouput is limited to 0.1W.

Base Station Subsystem

The BSS provides the interface between the ME and the NSS. It is in charge of the transmission and reception. It may be divided into two parts:

• Base Station Controller (BSC): It controls a group of BTSs and manages their radio ressources. A BSC is principally in charge of handoffs, frequency hopping, exchange functions and power control over each managed BTSs.
• Base Transceiver Station (BTS) or Base Station: it maps to transceivers and antennas used in each cell of the network. It is usually placed in the center of a cell. Its transmitting power defines the size of a cell. Each BTS has between 1-16 transceivers depending on the density of users in the cell.

NSS

Its main role is to manage the communications between the mobile users and other users, such as mobile users, ISDN users, fixed telephony users, etc. It also includes data bases needed in order to store information about the subscribers and to manage their mobility. The different components of the NSS are described below.

• MSC: the central component of the NSS. The MSC performs the switching functions of the network. It also provides connection to other networks.
• GMSC: A gateway that interconnects two networks: the cellular network and the PSTN. It is in charge of routing calls from the fixed network towards a GSM user. The GMSC is often implemented in the same machines as the MSC.
• HLR: The HLR stores information of the suscribers belonging to the coverage area of a MSC; it also stores the current location of these subscribers and the services to which they have access. The location of the subscriber maps to the SS7 address of the Visitor Location Register (VLR) associated to the MN.
• VLR: contains information from a subscriber's HLR necessary to provide the subscribed services to visiting users. When a subscriber enters the covering area of a new MSC, the VLR associated to this MSC will request information about the new subscriber to its corresponding HLR. The VLR will then have enough data to assure the subscribed services without needing to ask the HLR each time a communication is established. The VLR is always implemented together with a MSC; thus, the area under control of the MSC is also the area under control of the VLR.
• Authentication Center (AuC): It serves security purposes; it provides the parameters needed for authentication and encryption functions. These parameters allow verification of the subscriber's identity.
• Equipment Identity Register (EIR): EIR stores security-sensitive information about the mobile equipments. It maintains a list of all valid terminals as identified by their International Mobile Equipment Identity (IMEI). The EIR allows then to forbid calls from stolen or unauthorized terminals (e.g, a terminal which does not respect the specifications concerning the output RF power).
• GSM Interworking Unit (GIWU): The GIWU provides an interface to various networks for data communications. During these communications, the transmission of speech and data can be alternated.

Operation and Support Subsystem (OSS)

It is connected to components of the NSS and the BSC, in order to control and monitor the GSM system. It is also in charge of controlling the traffic load of the BSS. It must be noted that as the number of BS increaseswith the scaling of the subscriber population some of the maintenance tasks are transferred to the BTS, allowing savings in  the cost of ownership of the system.