GSM-Mobile Communication

What is GSM?

GSM is the most popular standard for mobile phones in the world. GSM (Global System for Mobile communication) is a digital mobile telephony system.

The first GSM network was launched in 1991 and it is the most successful digital mobile telecommunication system that is widely used in India and other parts of the world.

How GSM is working?

GSM uses a combination of FDMA and TDMA. The GSM system has an allocation of 50 MHz bandwidth in the 900 MHz frequency band. Using FDMA, this band is divided into 124 channels each with a carrier bandwidth of 200 KHz. Using TDMA, each of these channels is further divided into 8-time slots. Therefore with a combination of FDMA and TDMA, we can realize a maximum of 992 channels for transmitting and receive. Let’s look at the detailed working of GSM.

GSM Architecture

The GSM technical specifications define different entities that form the GSM network by defining their functions and interface requirements. Here is a list of GSM Components and elements.

Cells: Cells are formed by the radio areas covered by a BTS (Base Transceiver Station). Several BTSs are controlled by one BSC.

1. MS (Mobile Station):

An MS is used by a mobile subscriber to communicate with the mobile network. Several types of Mobile Stations are existed, each allowing the subscriber to make and receive calls.

The function of MS is the transmission of a signal from MS to BTS (using uplink) and reception of a signal from BTS to MS (using downlink). The mobile station consists of:

1. Mobile Equipment (ME): “Cellular phone without SIM card” The mobile equipment has a unique international mobile equipment identity (IMEI) which is used by EIR. The numbers of GSM terminal types are defined within the GSM specification. They are distinguished primarily by their power output rating.
2. Subscriber identity module (SIM): SIM cards used in phones are smart processor cards. It possesses a processor and a small memory. The SIM stores permanent and temporary data about the mobile, the subscriber, and the network. It contains a serial_no, PIN, PUK (Pin Unblocking Key), an authentication key (Ki), IMSI (International Mobile Subscriber Identity).

2. BSS (Base Station Subsystem) :

BSS contains two components:

1. BTS (Base Transceiver Station): It comprises all radio equipment (e.g.: antenna, signal processing & amplifier required for transmission). It is placed in the center of a cell. Its transmitting power defines the size of a cell. It is connected to MS via Um interface and connected to BSC via Abis Interface. It manages the radio resources for BTS. It handles & handover the radio frequency, radio channel set up from one BTS to another.
2. BSC (Base Station Controller): It connects the BTS and MSC of NSS. It manages radio resources for one or more BTS. It handles and Handover the radio frequency, radio channel setup from one BTS to another BTS.

3. NSS (Network Switching Subsystem):

The NSS combines the call rotating switches (MSC and GMSC) with the database registered required to keep track of subscribers’ movements and use of the system. Key elements of NSS are:

1. MSC (Mobile Switching Center): The mobile-services switching center is an exchange that performs all the switching and signaling functions for mobile stations located in a geographical area designated as the MSC area. These are high-performance digital ISDN switches. It is used for connection between mobile phone to mobile phone within the same network. It is used for connection between mobile phone to fixed phone within a network. It manages BSC within a geographical area.
2. GMSC (Gateway MSC): Connection for another network MSC handles all the signaling needed for connection set up and connection release.
3. HLR (Home Location Register): The HLR is a centralized network database that stores and manages all mobile services belonging to a specific operator. It provides a call routing and roaming facility by combining with MSC and VLR. It is considered as a Database that stores the information about the subscriber within the coverage area of MSC. Information includes the current location of the mobile & all the services providing information, when a phone is powered off this information is stored in HLR. It is also a database but contains a temporary copy of some of the important information stored in HLR. If a new MS user comes into the location area, then VLR will provide relevant information by bringing it from HLR.
4. VLR (Visitor Location Resister): It is a temporary storage device of the GSM network. It stores subscribers’ subscription information for MS which are within the particular MSC service Area. There is only one VLR for each MSC service area.
5. Equipment Identity Register (EIR): EIR is a database that keeps track of the equipment that exists at the mobile station, i.e. It can be an IMEI number used for the tracking of the device.
6. Authentication Centre (AuC): AuC is a database that is used to hold authentication and encryption keys for all the subscribers.

4. OSS (Operation and Support Subsystem)

It contains the necessary functions for network operation and maintenance. Key elements of OSS are:

1. OMC (Operation and maintenance center): It is connected to different components of NSS & to BSC. It controls the traffic load of BSS.

5. GSM Interfaces:

1. Um Interface (MS to BTS): The Um radio interface (between MS and base transceiver stations [BTS]) is the most important in any mobile radio system. It addresses the demanding characteristics of the radio environment. The physical layer interfaces to the data link layer and radio resource management sublayer in the MS and BTS.
2. Abis Interface (BTS to BSC): The interconnection between the BTS and the BSC is through a standard interface, Abis. The primary functions carried over this interface are traffic channel transmission, terrestrial channel management, and radio channel management.
3. A-Interface (BSC to MSC): The A-interface allows interconnection between the BSS radio base subsystem and the MSC. The physical layer of the A interface is a 2-Mbps standard Consultative Committee on Telephone and Telegraph (CCITT) digital connection.