igital 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. Telephony Engineering Concepts Key Telephony Concepts The connection not only can go from sector to sector and from cell to cell, it also can go from one MSC to another. When this happens, we have one MSC setting up the call, connected to the PSTN, and a connection to another MSC that actually serves the call. We call the setup switch the anchor MSC to distinguish it from the serving MSC. The call can even go from one wireless system to another SO that the anchor and serv- ing MSCs are in different systems. This requires close signaling coordination and a connection between the two MSCs. When a handoff to a new MSC is being considered, we call the not-yet-handed-off switch a target MSC. The details of inter-MSC handoff are specified by the ANSI-41 standard, as described in Chap. 15. 12.5.5 Roaming Roaming is unique to cellular systems, although the recent service of local number portability (LNP) is bringing similar issues to the landline network. The roaming sub- scriber expects services similar to home and expects to receive calls dialed to the home telephone number. This is effected by close coordination in the signaling network and maintaining sub- scriber records in the home system. We call the home system repository of subscriber information the home location register (HLR). Anytime and anywhere a user terminal changes status, its HLR record is updated. The HLR keeps track, minute by minute, of the current location of the user terminal. It also knows whether the user terminal is available for call delivery. This allows the home MSC to make the appropriate choice in response to an incoming call: sending the call to the user terminal if it is registered on the home network, sending it to an alternate MSC if it is available through roam- ing, or directing the callING party to voice mail (or sending a busy si