System and method for delivering incoming calls to mobile units in border cells using packet-based links

A method for setting up an incoming call directed to a mobile unit, wherein the mobile unit has roamed from a first base station coverage area, associated with a base station controlled by a serving mobile switching center (SMSC), to a border base station coverage area, associated with a base station controlled by a border mobile switching center (BMSC), the SMSC defining mobile switching center where the mobile unit is registered and believed to be located and the BMSC defining a mobile switching center where the mobile unit is located but not registered, the method comprising:

• sending an intersystem setup request message from the SMSC to the BMSC, the intersystem setup request message including a data link connection identifier (DLCI) indicating a packet address of voice coder resources assigned by the SMSC for the incoming call;
• sending the DLCI to the base station controlled by the BMSC, the base station having established a traffic channel for communicating with the mobile unit, the base station thereby establishing a packet link between the packet address of the voice coder resources and the traffic channel;
• sending an intersystem setup response message from the BMSC to the SMSC, the intersystem setup response message indicating the packet link is established;
• and
• connecting, by the SMSC, the incoming call to the packet link.

A communication system comprising:

• a serving mobile switching center (SMSC) defining a mobile switching center where a mobile unit is registered and believed to be located, the SMSC operable to determine, upon receiving an incoming call for a mobile unit, that the mobile unit has roamed outside an area served by the SMSC, the SMSC including a voice coder adapted for allocating voice processing resources for the incoming call;
• a border mobile switching center (BMSC) defining a mobile switching center where the mobile unit is located but not registered, the BMSC operable to determine, responsive to a query of the SMSC, that the mobile unit is within an area served by the BMSC, and to inform the SMSC that the mobile unit is within an area served by the BMSC;
• a base station connected to the BMSC for providing a wireless link to the mobile unit;
• and
• a packet network for providing a packet link between the SMSC and BMSC, the packet link being established responsive to the SMSC sending a packet address of the voice coder to the BMSC, and the BMSC sending the packet address to the base station.