BLOCK DIAGRAM OF GSM

 Mobile Station (MS). The GSM mobile stations are portable radiotelephony
 units that can be used on any GSM system as vehicular and/or handheld
 terminals.

Base Station System (BSS). The base station system comprises
a base station controller (BSC) and one or more subtending base
transceiver stations (BTS)

Mobile Switching Center (MSC). The mobile switching center
for GSM can be viewed as a local ISDN switch with additional capabilities to
support mobility management functions like terminal registration, location
updating, and handoff.

Home Location Register (HLR). The HLR represents a centralized
database that has the permanent datafill about the mobile subscribers
in a large service area (generally one per GSM network operator).





 Visiting Location Register (VLR). The VLR represents a
temporary data store, and generally there is one VLR per MSC. This register
contains information about the mobile subscribers who are currently in the
service area covered by the MSC/VLR.