Once again apologies for using somebody else’s image.

POTS – Plain Old Telephone System

MSC – Mobile Switching Center

BSS – Base Station SubSystem

HLR – Home Location Register

VLR – Visitor Location Register

From our land line we dial a mobile number. Our telephone exchange identifies that its a mobile number and its of a particular operator. The call is routed to that operators Gateway MSC. The gateway switching center has to determine where to route the call. So it queries the HLR to find out the current MSC serving the target subscriber. The HLR returns the destination MSC of the subscriber and the call is routed there. The destination MSC upon receiving the call request finds the exact location (Cell ID) by querying the VLR. The VLR upon receiving a the request checks whether the destination subscriber is with allowed to recieve calls, and is available with in its BSS. If yes the cell ID is returned. And the call is established.

This is just a small description and not very accurate. Wait for more posts as I learn more about this technology.