er of the new MSC area and consider all the combinations of one base station in the new MSC area and one base station in an old MSC area so that exchanging the two still leaves contiguous MSC regions. We consider exchanging these two base stations and ask if the exchange reduces the backhaul cost. If it does, then we repeat the process until we find no im- provement in the calculation. This generally will find good assignments, but it can make a good change in one step that prevents a better change later on. We can paint ourselves into a corner. A more sophisticated heuristic looks further ahead in the assignment-exchange process. We can look at all the pairs of border cells where we can exchange their MSCs and keep the MSC regions contiguous. We calculate the backhaul cost difference of each exchange of base stations and pick the one that reduces cost the most. We repeat this process until no further improvement is found. Even this heuristic might not reach the true optimal minimum-cost solution that a full-blown mathematical program would find, but the difference in cost should be very small. As the system grows and adds base stations, it is even more unlikely that the result over time will be much different. 43.3 Dealing with New MSC Borders The job of assigning base stations to MSCs should take into account inter-MSC border signaling for call processing and transport for handoffs. The MSC border area is a busy area. There are inter-MSC handoffs, the transport required to support these handoffs, and all their associated signaling messages for measurements and actual handoffs. In addition to handoffs, we have call registrations and setup activity in the border zones creating inter-MSC communication. The other component of inter-MSC communication is roaming traffic that has little to do with the border areas. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given