GSM Core Network Architecture

GSM network is composed of several functional entities, whose functions and interfaces are specified. GSM core network consists of the several functional modules such as MSC, HLR, VLR, AUC, GMSC etc
These components are used for efficient switching and control of the calls. Previously the core network was supporting the voice calls and slow speed data calls but with the evolution of the technology now core network support high end voice calls and high speed data calls also.

GSM Core Network Components

MSC (Mobile Switching Center)

The central component of the network subsystem in the Mobile Switching Center(MSC). It acts like a normal switching node of the Public Switched Telephone Network (PSTN) or International Switched Data Network(ISDN), and additionally Provides all the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, handovers and call routing to the roaming subscriber. The MSC provides the connection to the fixed network (such as the PSTN or ISDN). Signaling between functional entities in the network subsystem uses Signaling System Number 7(SS7), used for trunk signaling in ISDN and widely used in current public networks.

The MSC performs the telephony switching functions of the system. It controls the calls to and from other telephone and data systems. It also performs such functions as toll ticketing, network interfacing, common channeling and others. The MSC co-ordinates the setting up of the calls to and from the GSM users. It is the telephone Switching office for MS originated or terminated traffic and provides the appropriate bearer services, teleservices and supplementary services.

It controls a number of BSS within a specified geographical coverage area and gives the radio subsystem access to the subscriber and equipment databases. Each MSC manages dozens of cell sites and their base stations. Large systems may have two or more MSCs. The MSC carries out the several different functions depending on its position in the network. When the MSC provides the interface between the PSTN and the BSS in the GSM network it is called the Gateway MSC.

Some important functions carried out by the MSC are Call Processing including control of data/voice call setup, inter BSS & inter MSC handovers, control of mobility management, Operation & maintenance support including databse management, traffic metering and man machine interface & managing the interface between GSM & PSTN network.

This is the main component of the NSS as the BSC co-ordinates with it. This component control the entire network. It interacts with the other databases and PSTN. The MSC set up and release the end to end connections, handles mobility and handover  requirements during the call and takes care of the charging and the real time pre-paid account monitoring. The responsibilities of this entity included checking if a customer has a valid account or not, what profile he is in, what kind of services he had been activated for etc. In short, it provides the needed user registration and authentication information.

HLR (Home Location Register)

The HLR is a central database that contains details of each mobile phone subscriber that is authorized to use the GSM network. The HLRs store details of every SIM card issued by an operator. Each SIM has a unique identity no Called an IMSI which is the most important key or the primary key to each HLR record.

The HLR is a database used for torage and management of subscriptions. The HLR is considered the most important database, asit 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, he or she is registered in the HLR of that operator.The HLR contains the master database of the all customers in the PLMN. This data is remotely accessed by the MSCs and VLRs in the network. A PLMN may contains more than one HLR, in which each HLR contains a portion of the total subscriber database. There is only one database record per subscriber. The subscriber data may be accessed by the IMSI or the MSISDN, which is basically nothing but the phone number of the subscriber which he/she uses. The MSISDN is the primary key to the HLR record. The HLR data is stord for as long as a subscriber remains with the operator. Other keys of this database apart from MSISDN and IMSI are: TMSI(temporary IMSI), IMEI( International Mobile Equipment Identity), Name and address of the subscriber, Current service subscription profile, Current location(MSC/VLR address), Authentication and encryption keys, Mobile country code(MCC) and MNC (Mobile Network code).

VLR (Visitor Location Register)

The VLR is a database that contains temporary information about subscriber that is needed by the MSC in order to service visiting subscribers. The data stored in the VLR has either been received from the HLR, or collected from the MS. 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.The data includes most of the information stored at the HLR, as well as more precise location and status information. The additional data in VLR are:

Mobile status (busy/free/no answer etc)

Location Area Identity (LAI)

Temporary Mobile Subscription Identity (TMSI)

Mobile Station Roaming Number (MSRN)

With Regards

Technocrats E Services

Jalandhar