C to convert speech between the base station packets and the PSTN full-rate pulse code modulation (PCM) links. Packet switching is going to be harder to engineer because there are SO many func- tions using packet data. All the base station links, all the PPDN traffic, and all the sig- naling are going to be using packet links. The MSC must have enough capacity to han- dle all these data flows. We must form an estimate of the total packet data requirement at the MSC based on forecasts of data services. MSC equipment should include provision for electrical power and backup equipment for reliability. We need the same sort of reliability studies we did for base stations in Sec. 4.8. MSC reliability is very important because hundreds of cells can go out of ser- vice with an MSC failure. We should be prepared for the mix of services to change over time as the market grows for 3G wireless telephone service. As people get used to the idea, they may de- cide to buy more and more wireless data service over time. As wireless data service be- comes part of the technical environment we all live in, the demand may grow much faster than the demand for voice telephone calls. This growth in data service will change the requirements for MSC equipment. The result of MSC equipment planning is a clear picture of what equipment will be required for each MSC in the wireless telephone system. This picture should be based on forecasts of traffic and the mix of services. There also should be provision in the equipment plan for growing wireless traffic and shifts in the mix of services subscrib- ers use. 34.4 Assigning Base Stations to MSCs For systems large enough to have multiple MSCs, the assignment of base sta- tions to MSCs can be a complicated process. The basic idea is to distribute the 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 at the website. Mobile Switching Center (MS