CALL DELIVERY
Call Delivery occurs when someone calls to a
cellular phone. This example assumes that the person is roaming in a system
away from their home system. The general steps for this process are:
1. Someone dials the phone number of the
cellular phone.
2. The call is routed to the subscriber's
home switch. The home switch sends a query to the Home Location Register (HLR)
to determine the location of the phone. The HLR recorded the location of the
phone when the phone registered in the visited system.
3. The HLR sends a data message across the
signaling network to the visited system requesting a temporary local directory
number (TLDN) for routing the call.
4. The visited switch returns a data
message with a TLDN to the HLR. The HLR sends the TLDN to the home switch for
routing across the public telephony network.
5. When the visited switch receives a call
to that TLDN, it associates it to the phone, sends out a regular cellular page
and delivers the call to the phone.
The TLDN is a regular telephone number that is
routable through the public network. The visited switch usually assigns TLDNs
on a per-call basis, when it receives the routing request from the home system.
The visited switch uses a timer to determine how long it keeps the TLDN
associated with that subscriber. The timer lasts long enough for the home
switch to route a call to that number across the public network.