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GSM Architecture - General Understanding of 2G GSM communication

Alan Antony

Global System for Mobile communications (GSM) is a general standard architecture set up by the European Telecommunication Institute to demonstrate the various protocols involved in 2nd Generation (2G) telecommunication networks used in user equipments (mobile phones). Its further expansion to include data communication leads to the evolution of GPRS, EDGE etc.

The increasing demand of high volume data transfer along with the normal mobile communication leads to the emergence of 3G (UMTS) and 4G Long term evolution high standards. Even though 3G, 4G are the latest trends, GSM still exists as the backbone of the entire communication protocols. 

Here,  I am not going deep into any GSM interfaces , channel allocations or any frequency handovers  involved in cellular communication .But this article will give an outline about the general GSM architecture, which will help a novice enthusiast to get a good understanding about the telecommunication world.

There are various elements in the GSM - which can communicate together to form the entire GSM system.  The GSM architecture can be grouped into four main areas:

  • Mobile Station  
  • Base-station subsystem      
  • Network and Switching Subsystem    
  • Operation and Support Subsystem

Mobile Station

The mobile stations and mobile equipment's are the main entities in GSM architecture.  The mobile phone in the cellular network is the only one element that the user sees and operates. The mobile phone has two main parts – its hardware components and the SIM card. The hardware components consist of its display unit, battery, signal receiver/transmitter etc. It also consists of a unique device identity number called IMEI (International Mobile Equipment Identity). This is imposed by the manufacturer on the phone and is not supposed to change.  The SIM card contains user details which can communicate with the carrier network. It also contains a unique International mobile subscriber identity (IMSI).

Base-station subsystem

The whole cellular communication is associated with the Base-station subsystem. It consists of two main parts – BTS and BSC.

Base Transceiver station (BTS): The BTS consists of the RF transmitters, receivers and its associated antennas which can directly communicate to the user equipment's (mobile phones). The BTS communicates with the mobiles with it’s um interfaces along with its associated protocols.

Base Station Controller (BSC): The BSC controls a group of BTS’s.  It is an important component in GSM communication which can control Radio resources, channel allocations and the RF handovers from one BTS to another. This is very important when a user moves from one mobile tower range to another mobile tower range and this is basically the frequency transmission between one BTS to another BTS.

Network Switching Subsystem

The Network switching sub system is the core network in the GSM architecture. It actually controls the whole mobile communications and it has the following major elements:

Mobile switching service centre (MSC): The MSC works like a switching node in a normal PSTN/ISDN land lines. It provides extra services when it comes to cellular communication which includes registration, authentication, call location, inter MSC handover, call routing - the call can be routed from a mobile network to normal land lines. The inter- MSC communication helps to enable call to different mobile networks.

Home Location Register (HLR) & Visitor Location Register (VLR): The HLR consists of an administrative database for a user. When a user switches on the mobile phone, it then registers with the network and it gives an idea to the network about its nearest BTS .So that the calls can be routed appropriately. Even if the phone is not using, it registers with the Network HLR periodically to keep track of its current location. The VLR consists of selected details from HLR and that helps to track selected services from the service providers. Both HLR and VLR are the integral parts of MSC which makes the communication faster and easier.

Equipment Identity Register (EIR) and Authentication (AUC): The EIR collects the unique IMEI number from the user equipment. Every time when the mobile registers with the network, it checks the EIR to identify the status of the device which can be of mainly three states- allowed access, barred access, monitored access. The AUC is a protected database, which contains the secret key used for the authentication and ciphering of Radio Channels.

Gateway mobile switching (GMSC) and SMS Gateway (SMS-G): The GMSC is the gateway service which contacts the HLR and finds the exact mobile station roaming number (MSRN) and helps to route the calls to the correct visited MSC. The SMS-G is the gateway service for short messages. It provides a fixed access point to the short message service center.

Operation and Support Subsystem

The OSS in the GSM architecture is the one that connects to the components of NSS and BSC. It monitors the entire GSM network and controls the traffic loads to the BSS.

All the above components along with its associated protocols work together and make the cellular communication possible. 

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