Localization is a process by which a mobile station is identified, authenticated and provided service by a mobile switching centre (MSC) through the Base Station Controller (BSC) and the Base Transceiver Station (BTS) either at the home location of the Mobile station or at a visiting location. The user desires instantaneous connection setup for a call and service-on-demand even after they move. 

The mobile service providers can provide service to the user only after identifying the mobile station (MS) of the user and verifying the services subscribed to by the user or services presently allowed to that MS.

The localization mechanism of the GSM system fulfills the requirements. 

The Network and Switching Subsystem (NSS) periodically updates the location of those Mobile Stations (MSS) that are not switched off and are not struck off from the list of subscribers to the given mobile service. 

The SIM in a mobile station MS, stores location-area identification (LAI), LAI is location information that is updated by the MSC. It covers the MS's current location area.

The SIM even a temporary mobile subscriber identity (TMSI) assigned by the VLR associated with the current MSC. 

The location update is recorded at the VLR (Visitor Location Register) and the LAI is updated at the SIM card in MS, through the Mobile Switching Centre (MSC), BSC, and BTS covering its current location.

The updating process of the VLR is as follows: Each MSC is related to a VLR(Visitor Location Register) and HLR (Home Location Register). An MSCA has an HLRA at which a mobile station is originally registered on subscription to a mobile service.

The HLRA copies the HLR information of an MS to VLRB when the MS moves out to the area covered by MSCB. 

The copying saves time as MSCB gets instantaneous information about MS, from VLRB instead of fetching it from HLRA. Otherwise, the time could be spent in searching and fetching information for relocated MS, from HLRA is saved. The storing of LAI into the SIM from the VLR through MSCB, BSC, and BTS saves time. These processes, thus help the MS in getting service from MSCB instantaneously.

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