When a mobile phone makes an outgoing call, it is easy: the network just has to allocate a channel and send the call to the destination number. Incoming calls are not so easy. The network has to find the mobile before the caller rings off!
Finding the Mobile
The incoming call is routed to the Gateway Mobile Switching Centre (GMSC), which asks the NPDB if it has a record for that number (and for details if it has) and then asks the HLR where to send it.
The HLR knows which sim card is associated with that phone number, and remembers which VLR/MSC is currently looking after it, whether on the home network, or the VLR of a roaming partner.
The VLR/MSC returns the current location and status of the mobile and this information passes back to the GMSC, The GMSC passes the call to the appropriate MSC, which in turn passes it to the BSC, which tells the BTS to page the mobile on the Paging Channel (PCH) to say there is a call for it.
Trying to Connect You…
Once the BTS has paged the mobile, it offers a channel and waits for the mobile’s response. If it finds the mobile, it authenticates the mobile’s identity and then the call is set up.
See the Air Interface pages for details.
If it doesn’t reach the mobile, usually the call is passed back to the voicemail system, where the caller can leave a message. If this is not in use, the caller gets a recorded announcement saying that the mobile is not available.