ate through a series of blank and burst messages. These messages are 100 ms (one- tenth of a second) of direct-binary frequency shift keying (FSK) data; a 1 is an increase of 8 kHz in the carrier, and 0 is a decrease of 8 kHz. The initial sequence of a blank and burst message is recognized by the audio receiver, and it mutes the audio so that the subscriber does not hear a "bzzzt" sound when these messages are sent. Blank and burst messages are used for cellular overhead signaling such as handoff and power-control messages. The alert message to ring the mobile telephone is also a blank and burst message as AMPS establishes a voice link before the telephone rings. Subscriber call processing during a call typically starts with a flash for call waiting or three-way calling. Three-way calling also sends its dialed digits via blank and burst. The blank and burst messages are designed to be robust because typically they are used in poor radio conditions, especially for handoffs. Some cellular systems use sub- band digital signaling, frequencies below 300 Hz, for their own digital messages, but this is not part of the AMPS standard. 6.4.3 Narrowband AMPS Introduced by Motorola in 1991, narrowband AMPS (N-AMPS) replaces the 30-kHz FM channel with three 10-kHz FM channels. Compared with AMPS, the sound quality is diminished by the narrower channel. However, N-AMPS has several advantages. Greater capacity. A three-for-one channel split actually increases capacity more than three times because of traffic engineering issues discussed in Chap. 21. 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) Key Radio Concepts Compatibility. N-AMPS can use the same control channels, and dual-mode mobile tele- phones can be used to operate under both standards. Improved supervision. The SAT tone is replaced by a 200-bit-per-second signa