Global System for Mobile Communication (GSM) is a set of ETSI standards specifying the infrastructure for a digital cellular service. As a fully digital system, GSM allows both speech and data services and allows roaming across networks and countries. These features made GSM a very popular system, not only in european countries but also elsewhere in the world.
In the mobile network evolution path shown above, the GSM system is the starting point to be address in this series of articles.
1. GSM Network Architecture
2. GSM Network Entities
2.1 Mobile Station (MS)
The Mobile Station (MS) is the user equipment in GSM, i.e. the cellular phone itself. The MSs in GSM are independent from networks-providers. The identity of the subscriber is obtained from a SIM (Subscriber Identity Module) that has to be inserted into the MS to make it work. The SIM contains the IMSI (International Mobile Subscriber Identity) which uniquely identifies the subscriber to the network. It also contains information necessary to encrypt the connections on the radio interface. The MS itself is identified by an IMEI (International Mobile Equipment Identity), which can be obtained by the network upon request. Without the SIM, calls to and from the mobile station is not allowed, except that the alls to the international emergency number, 112, is allowed without the SIM.
2.2 Base Transciever Station (BTS)
The Base Transciever Station (BTS) is the entity corresponding to one cellular site communicating with the Mobile Stations. Usually, the BTS will have an antenna with several TRXs (radio transcievers) that each communicate on one radio frequency. Speech and data transmissions from the MS is encoded in the BTS from the special encoding used on the radio interface to the standard 64 kbit/s encoding used in telecommunication networks.
2.3 Base Station Controller (BSC)
A Base Station Controller (BSC) controls the magnitude of several hundred BTSs for subscriber registration, location update, call setup and handover control etc. It is the entrance point for an MS to gain access to the overall mobile network services.
2.4 Mobile Switching Centre (MSC)
The Mobile Switching Centre (MSC) is a typical telecom switch with extended functionalities to support mobile services. The basic function of the MSC is to switch speech and data connections between BSCs, with other MSCs or other GSM-networks and external non-mobile-networks. It also takes care of all mobility management tasks.
2.5 Visitors Location Register (VLR)
For each MSC, there is an associated VLR to store the information about all subscribers that are roaming within its service coverage. The subscriber infromation is obtained either through the MS initiated location update procedure when it enters the covered area or through the notification from the subscriber's Home Location Register (HLR). The VLR serves the routing of all service initiated or targeted to the MS.
2.6 Home Location Register (HLR)
The HLR is the home register of a subscriber. It is where the subscriber information, allowed services, authentication information, location information etc. of a subscriber are permanently stored.
2.7 Equipment Identity Register (EIR)
The Equipment Identity Register (EIR) is an optional register. Its purpose is to register IMEIs of mobile stations in use. By implementing the EIR the network provider can implement a widely range of control functions to the served MSs, such as blacklisting malfunctional or stolen mobile stations etc.
2.8 Gateway MSC (GMSC)
A Gateway Mobile Switching Centre (GMSC) provides the edge function within the mobile network. It terminates the PSTN (Public Switched Telephone Network) signalling and traffic formats and converts to protocols employed in mobile networks. For mobile terminated services, it interacts with the HLR (Home Location Register) to perform routing functions.
3. GSM Interfaces and Protocols
4. GSM Air Interface Channel Structure
