GSM (2G)

Global System for Mobile Communication (GSM)

 GSM (Global System for Mobile Communications, originally Groupe SpécialMobile), is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe the protocols for second-generation (2G) digital cellular networks used by mobile phones, first deployed in Finland in July 1991. It is a digital cellular technology used for transmitting mobile voice and data services.

GSM is a circuit-switched system that divides each 200 kHz channel into eight 25 kHz time-slots. GSM operates on the mobile communication bands 900 MHz and 1800 MHz in most parts of the world. In the US, GSM operates in the bands 850 MHz and 1900 MHz.

GSM Architecture

The Base Station System (BSS)

All radio-related functions are performed in the BSS, which consists of base station controllers (BSCs) and the base transceiver stations (BTSs).

Base Transceiver Station : The BTS handles the radio interface to the mobile station. The BTS is the radio equipment (transceivers and antennas) needed to service each cell in the network. A group of BTSs are controlled by a BSC.

Base Station Controller : The BSC provides all the control functions and physical links between the MSC and BTS. It is a high-capacity switch that provides functions such as handover, cell configuration data, and control of radio frequency (RF) power levels in base transceiver stations. A number of BSCs are served by an MSC.

                The Base Station Controller (BSC) requires a new hardware called the packet control unit (PCU). The PCU directs the data traffic to the GPRS network.

Network switching subsystem (NSS)

Mobile Switching Center (MSC): The MSC performs the telephony switching functions of the system. It controls calls to and from other telephone and data systems. It also performs such functions as toll ticketing, network interfacing, common channel signaling, and others

.

Visitor Location Register (VLR): The VLR is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to interrogate the HLR each time.

Authentication center (AUC): A unit called the AUC provides authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call. The AUC protects network operators from different types of fraud found in today's cellular world.

Equipment Identity Register (EIR): The EIR is a database that contains information about the identity of mobile equipment that prevents calls from stolen, unauthorized, or defective mobile stations. The AUC and EIR are implemented as stand-alone nodes or as a combined AUC/EIR node.

Home Location Register (HLR): The HLR is a database used for storage and management of subscriptions. The HLR is considered the most important database, as it stores permanent data about subscribers, including a subscriber's service profile, location information, and activity status. When an individual buys a subscription from one of the operators, they are registered in the HLR of that operator.

Interfaces:

Um interface

          Um interface connects the MS and BTS (air interface)

Abis-interface

      Its the interface between the BSC (Base Station Controller) and the BTS. The interface comprises traffic and control channels.

Functions implemented at the Abis-interface are:

- Voice-data traffic exchange.

- Signaling exchange between the BSC and the BTS.

- Transporting synchronization information from the BSC to the 

A-interface
Its the interface between the BSS and the MSC. It manages the allocation of suitable radio resources to the MSs and mobility management.

B-interface

This interface is between between the MSC and the VLR

       VLR uses the MAP/B protocol. Most MSCs are associated with a VLR, making the B interface "internal". Whenever the MSC needs access to data regarding a MS located in its area, it interrogates the VLR using the MAP/B protocol over the B interface.

C-interface

It connects  HLR and GMSC. Also between MSC and HLR.

D-interface

It connects HLR and VLR.

E-interface

It connects MSC and another MSC or G-MSC.

F-interface

It connects EIR and MSC and between EIR and G-MSC.

G-interface

It connects VLR and another VLR.


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