ve up just because a piece of equipment stops working. When a call needs a piece of equipment that is not available, the switch has to know what to try next, a shift to al- ternate equipment, a fast busy signal, or a recording. The software in the telephone switch has this knowledge programmed into it. Most of the software in a telephone switch is for situations that never or seldom occur. 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 Call processing also includes the internal activity of trunk selection for each call. The local loops at each end are determined by the calling party or called party, respectively, but the intermediate links are selected and managed by telephone switches. 11.3.3.2 Accounting. The local office switch also has the billing function, not al- ways as popular among customers as call processing. Calls have to be tracked and mon- itored not only to be paid for but also for planning the growth and maintenance of the telephone network. The switch generates the data for these business functions and stores them as AMA records on magnetic tape or in a computer data file. The other accounting function is account verification, which ensures that subscribers who do not pay their bills are not allowed to make calls. This function requires that the system read and evaluate the subscriber's identification data from the system at the beginning of the call. This is considerably more complicated than storage of data about a call, which requires only writing data, not reading them. 11.4 Mobile Switch Functions The two big differences between landline switching and mobile switching are the need to switch calls while they are in progress and the need to support a radio link across the air interface rather than a landline local loop. These are not cosmetic or adminis- trative dif