ng and one for receiv- ing. This means that during the call, there is no way for the mobile telephone to main- 3The average is the root mean square (RMS) described in Sec. 4.3. 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. Analog Wireless Telephony (AMPS) Analog Wireless Telephony (AMPS) tain a link on the control channel. Rather than add a third radio, the AMPS designers made this two-way link do the full job of maintaining a cellular telephone call. In ad- dition to maintaining the audio voice, the channel handles link supervision, messages for cellular overhead, and subscriber call-processing messages. Call setup is done on separate paging and access channels, which are not used during a call. 6.4.1 Supervisory audio tone (SAT) How does a telephone system make sure that it is still communicating? As telephone users, we have the same issue: Silent periods longer than a few seconds make us ner- vous. Both ends of the telephone call have to know when the call has ended. Also, the FM channel has brief outages due to the Rayleigh fading described in Sec. 2.1.4. We not want to hear somebody else's conversation during those brief outages. Maintaining and confirming continuity on a link is called supervision. In AMPS, we do this with a supervisory audio tone (SAT) that modulates the FM carrier along with the voice. The SAT is set out of the telephone audio band, at 5970, 6000, or 6030 Hz. This tone is sent by the base station and returned by the mobile telephone, as described later in Sec. 25.1, and it is filtered out so that the customer never hears it. Should the SAT tone be absent or the wrong frequency of SAT be detected, the re- ceiver mutes the audio so that the listener does not hear radio static or, worse, another telephone call. 6.4.2 Blank and burst Once an AMPS call is on a voice link, the base station and mobile telephone communi- c