GSM stands for Global System for Mobile Communication. This is the most famous system for mobile communication in the world. More than 200 countries in the world are using this system for mobile communication. It is fully digital and allows both voice and data transfer. It also allows roaming services across the networks and countries.
We can broadly divide the GSM into three parts.
1. The Mobile Phone itself
2. The Base Station Sub System which controls the radio link with the mobile phone
3. The Network Sub Station where the switching of calls between mobile users and between mobile and fixed lines takes place.
1. The Mobile Station (MS) or the Mobile Phone
This consists of two parts i.e. the mobile phone itself and the Subscriber Identity Module or popularly known as the SIM card.
The mobile phones in the GSM are independent from the service providers. The SIM contains IMSI (International Mobile Subscriber Identity), which is used to identify the subscriber. This IMSI is a unique number to a particular subscriber globally. It gives the details of the country and the service provider. Personal Identity Number (PIN) protects the SIM card.
The mobile phone is identified by a 15 digit IMEI Number (International Mobile Equipment Identity). In most of the mobile phones typing *#06# will show the IMEI Number. No two mobile phones in the world supposed to have the same IMEI Number.
The first six digits of the IMEI no. is the Type Approval Code (TAC), the next two digits are the Final Assembly Code (FAC), next six digits represents Serial Number and the last digit is the check digit.
2. The Base Station Subsystem
This also consists of two parts i.e. BTS and BSC. BTS stands for Base Transceiver Station. The BTS houses the radio transceivers that define a call and handles the radio link protocol with the mobile. Speech and data transmission from mobile phones are recorded here.
BSC stands for Base Station Controller. This controls the magnitude of several hundred BTSs. The BSC takes care of a number of different procedures regarding call setup, location update etc. for each mobile phone.
3. The Network Subsystem
The main component of the network sub system is MSC (Mobile Switching Centre). The basic function of MSC is to switch voice and data connection between BSCs, other MSCs and other networks. It performs the registration, authentication, location identity and call routing to a roaming subscriber etc. It contains Home Location Register (HLR), Visitor Location Register (VLR), Equipmet Identity Register (EIR) and Authentication Centre.
HLR is the home register of the subscriber which contains the subscription information, services provided, authentication details and the location of the subscriber.
VLR stores data about customers who are roaming within the area of the MSC.
EIR registers the IMEI number of the mobile in use.