The Global System for Mobile communications (GSM) is a digital cellular communications system. GSM was originally developed in Europe (from the beginning it was denoted as group special mobile) is now spread around the world.
The GSM radio link uses both frequency-division multiple access (FDMA) and time-division multiple access (TDMA) to share the bandwidth among the users. With this combination, more channels of communications are available, and all channels are digital.
The GSM service is available in four frequency bands:
450-MHz, 900-MHz, 1800-MHz, 1900-MHz
GSM Network Elements
A GSM network consists of the following network components:
- Mobile station (MS)
- Base transceiver station (BTS)
- Base station controller (BSC)
- Base station subsystem (BSS)
- Mobile switching center (MSC)
- Authentication center (AuC)
- Home location register (HLR)
- Visitor location register (VLR)
The mobile station (MS) is the starting point of a mobile wireless network. The MS can contain the following components:
- Mobile terminal (MT)—GSM cellular handset
- Terminal equipment (TE)—PC or personal digital assistant (PDA)
- The MS can be two interconnected physical devices (MT and TE) with a point-to-point interface or a single device with both functions integrated.
Base Transceiver Station
When a subscriber uses the MS to make a call in the network, the MS transmits the call request to the base transceiver station (BTS). The BTS includes all the radio equipment (i.e., antennas, signal processing devices, and amplifiers) necessary for radio transmission within a geographical area called a cell. The BTS is responsible for establishing the link to the MS and for modulating and demodulating radio signals between the MS and the BTS.
Base Station Controller
The base station controller (BSC) is the controlling component of the radio network, and it manages the BTSs. The BSC reserves radio frequencies for communications and handles the handoff between BTSs when an MS roams from one cell to another. The BSC is responsible for paging the MS for incoming calls.
Base Station Subsystem
A GSM network is comprised of many base station subsystems (BSSs), each controlled by a BSC. The BSS performs the necessary functions for monitoring radio connections to the MS, coding and decoding voice, and rate adaptation to and from the wireless network. A BSS can contain several BTSs.
Mobile Switching Center
The mobile switching center (MSC) is a digital ISDN switch that sets up connections to other MSCs and to the BSCs. The MSCs form the wired (fixed) backbone of a GSM network and can switch calls to the public switched telecommunications network (PSTN). An MSC can connect to a large number of BSCs.
Equipment Identity Register
The equipment identity register (EIR) is a database that stores the international mobile equipment identities (IMEIs) of all the mobile stations in the network. The IMEI is an equipment identifier assigned by the manufacturer of the mobile station. The EIR provides security features such as blocking calls from handsets that have been stolen.
Home Location Register
The home location register (HLR) is the central database for all users to register to the GSM network. It stores static information about the subscribers such as the international mobile subscriber identity (IMSI), subscribed services, and a key for authenticating the subscriber. The HLR also stores dynamic subscriber information (i.e., the current location of the mobile subscriber).
Associated with the HLR is the authentication center (AuC); this database contains the algorithms for authenticating subscribers and the necessary keys for encryption to safeguard the user input for authentication.
Visitor Location Register
The visitor location register (VLR) is a distributed database that temporarily stores information about the mobile stations that are active in the geographic area for which the VLR is responsible. A VLR is associated with each MSC in the network. When a new subscriber roams into a location area, the VLR is responsible for copying subscriber information from the HLR to its local database. This relationship between the VLR and HLR avoids frequent HLR database updates and long distance signaling of the user information, allowing faster access to subscriber information.
The HLR, VLR, and AuC comprise the management databases that support roaming (including international roaming) in the GSM network. These databases authenticate calls while GSM subscribers roam between the private network and the public land mobile network (PLMN). The types of information they store include subscriber identities, current location area, and subscription levels.
Network and Switching Subsystem
The network and switching subsystem (NSS) is the heart of the GSM system. It connects the wireless network to the standard wired network. It is responsible for the handoff of calls from one BSS to another and performs services such as charging, accounting, and roaming.Figure shows a GSM network and the network elements.
Advantage of GSM
- One of the advantages of GSM is that it offers a subscriber identity module (SIM), also known as a smart card. The smart card contains a computer chip and some non-volatile memory and is inserted into a slot in the base of the mobile handset.
- The memory on the smart card holds information about the subscriber that enables a wireless network to provide subscriber services. The information includes:
- The subscriber’s identity number
- The telephone number
- The original network to which the subscriber is subscribed
- A smart card can be moved from one handset to another. A handset reads the information off the smart card and transmits it to the network.
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