GSM architecture:

The GSM network can be divided into five main groups:

1. Mobile station(MS): It include mobile equipment(ME) and the subscriber identity module(SIM)
2. Base station subsystem(BSS): It include Base   transceiver station(BTS) and the base station controller(BSC)
3. The network and switching subsystem(NSS): it consists of VLR and HLR.
4. The operation and support subsystem(OSS)

The functionality of HLR(home location register): it is a central database. It contains the following information:

1. each mobile phone subscriber that is authorized to use the GSM core network
2. every SIM card issued by the mobile phone operator.
3. GSM services that the subscriber has requested or given.
4. Current location of subscriber.
5. Call divert settings applicable for each associated MSISDN.
6. Manage the mobility of subscriber by means of updating their position is administrative areas called location areas, which are identified with a LAC.

The functionality of visitor location register(VLR):

1. To inform the HLR that a particular subscriber has arrived in the particular area covered by the VLR.
2. To track where the subscriber is within the VLR area when no call is ongoing.
3. To allow or disallow which services the subscriber may use.
4. To allocate roaming numbers during the processing of incoming calls.
5. To delete the subscriber record when a subscriber explicitly moves to another, as instructed by the HLR.