base station relieves the computational load at the MSC. Today the base station con- trollers are sophisticated enough to handle handoffs within their domain without any participation from the MSC. This means that sector-to-sector handoffs (or handoffs within a cell between server groups described in Sec. 5.2.3) are done completely in the base station. 12 Aside from custom calling, the landline support of local number portability (LNP) may require similar kinds of switch-to-switch coordination as wireless roaming. LNP allows people to keep their phone numbers even when they change address and therefore is part of the general trend away from location-based and toward person-to-person telephony. 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. The PSTN and Telephone Switching Key Telephony Concepts Even with the base stations measuring call quality and determining handoffs, both deciding when to look for handoff and where to go, the MSC is still busy with handoff work. When a base station considers a call for handoff, the MSC is used as a base- station-to-base-station message switch to request and receive radio measurements. As a handoff is being considered from one MSC to another, there is communication between the two base stations and between the serving and target MSCs. When a handoff goes from one MSC to another, we have two telephone switches both involved in the same local-loop end of the same call, an event unheard of in landline telephony. The call goes from the anchor MSC to the serving MSC, as described in Chap. 15. If the call is an incoming roaming call, then three MSCs can be involved (home, anchor, and serving), all terminating the same call, and even more MSCs can be involved if this call is handed off to yet another MSC. In code division multiple access (CDMA), a soft handoff, with a transition process from one serving site to 