occurs when the user terminal receives and acts on an instruction to change its frequency or timing. As soon as the user terminal switches over, the new receiving base station picks up the signal. The new base-station-to-MSC link is already established, and there is no interruption in the call. Semisoft handoff makes use of the MSC's ability to maintain two links for the same mobile telephone call to eliminate the dead space of a hard handoff. However, semisoft handoff is a type of hard handoff in that the user terminal must make a change (of transmission type, of frequency, or of timing) to establish the new radio link and accomplish the handoff. The switchover oc- curs at the moment the user terminal makes that change and the semisoft handoff function has already put the new link across the landline portion of the wireless net- work in place. 14 Another way to make the distinction between soft handoff and softer handoff would be to say that soft handoff is a telephony switching function at the MSC, whereas softer handoff is a radio filtering function at the base station. 15 The authors also consider gelato, a semisoft ice cream, to be a type of hard ice cream. 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 11.4.8 Call statistics On the PSTN, as well as on wireless networks, call statistics are kept for the purposes of traffic engineering and system troubleshooting, as well as to support customer billing. The call-tracking issues are, of course, much more complicated in the wireless world. Engineers need data to track call activity by time of day, by base station, and by sector. Ineffective attempts and lost calls need to be tracked. And handoff activity is critical capacity issue that needs to be measured. In addition, wireless billing is much more complicated than landline billing. 