what is SIM?

SIM : Subscriber Identity Module

From the user's point of view, the first and most important database is inside the mobile phone the  Subscriber Identity Module (SIM). The SIM is small memory device mounted on card and contain user specific identification.

In the GSM network the SIM card identifies the user just like traveler uses a passport to identify himself.

The SIM card contains 

•  identification number of user.
• list of the services.
• contains tools needed for authentication and ciphering.
• storage space for messages and phone number.

when new subscriber switches on his phone in an area where a local operator provides network service know( VLR) Visitor Location Register the area is connected though an air interface to database.

The HLR stores the basic data of the subscriber on a permanent basis.The only variable data in the HLR is current location(VLR address) of the subscriber.