GSM Core Network Overview

The GSM Core Network consists of several functional nodes for controlling, switching, routing of the audio, video calls, sms and mms services in efficient ways, considering the speed, security, services simultaneously. GSM network broadly divided into three sub parts

BSS: Base Station Subsystem

This system takes care of all the radio related functions between a mobile and the network. It consists of the Base transceiver station(BTS, basically the tower) and a base station controller(BSC).

The BSS is the section of the traditional cellular telephone network which is responsible for handling traffic and signalling between a mobile phone and a Network switching subsystem. The BSS carries out the transcoding of the speech channels, allocation of the radio channels to the mobile phones, paging, quality management of the transmission and reception over the air interface and many other tasks related to the radio network. The BSS is composed of the two parts the BTS and BSC. These communicate across the standardized Abis interface. The Abis interface is generally carried by a DS-1, ES-1, or E1 TDM circuit. Uses TDM subchannels for traffic (TCH), LAPD protocol for the BTS supervicion and telecom signaling and carries synchronization from the BSC to the BTS and MS. The BSC and the Main Switching center communicate across the A interface. It is used for carrying the traffic channels and the BSSAP user part of the SS& stack. Although  there are usually transcoding units between BSC and MSC, the the signaling communication takes place between these two ending points and the transcoder unit does'nt touch the SS& information, only The voice or CS data are trascoded or rate adapted.

NSS: Network Switching Subsystem

This system takes care of the connection, authentication, encryption and supervision of a call. It consists of many elements/entities. the main one being a Mobile Station Controller which handles many BSCs. Other entities being a Home Location Register(HLR), Visitor Location Register(VLR), Authentication center(AUC), Equipment identity register(EIR), Short message services cente(SMSC), Multimedia messaging service center(MMSC) etc.

The first subsytem of the GSM network is the NSS. It carries out switching functions and manages the communications between mobile phones and the Public Switched Telephone Network. it is also responsible for the subscriber data handling, charging and controll of the calls. It is owned and deployed by mobile phone operators and allows mobile phones to communicate with each other and telephones in the wider telecommunication network. The architecture closely resembles the telephone exchange, but there are additional functions which are needed because the phone are not fixed in one location. The NSS also referred to as the GSM core network, usually refers to the circuit switched core network, used for traditional GSM services such as voice calls, SMS and circuit switched data calls. There is also an overlay architecture on the GSM core network to provide packet-switched data services and is known as the GPRS core network. This allows mobile phones to access to services such as WAP, MMS, and Internet access. All mobile phones manufactured today have both circuits and packet based services, so most operators have the GPRS network in addition to the standard GSM core network.

OSS: Operation Support Subsystem

This system responsibility includes operations and management of the whole system.

The Operation and Maintenance Center(OMC) is connected to all the equipment in the switching system and to the BSC. The implementation of the OMC is called the Operation and support system(OSS).

The OSS is the functional entity from which the network operator monitors and controls the system. The purpose of the OSS is to offer the customer cost-effective support for centralized, regional and local operation and maintenance activities that are required for the GSM network. An important function of the OSS is to provide a network overview and support the maintenance activities of the different operation and maintenance organizations. Here are some of the OMC functions:

• Administration and commercial operation(subscription, end terminals, charging and statics).
• Security Management
• Network configuration, Operation and performance management.
• Maintenance tasks.