ions. ARQ is also sometimes used to guarantee data integrity with GSM communications. Fax data are more error-tolerant than computer data. In computer data, flipping a single bit can corrupt a program or render a data file unreadable. In fax data, flipping a single bit will turn one dot from black to white, or vice versa, and the fax will still be readable. A low error rate im- proves the appearance of the fax and also reduces the likelihood that crucial control characters will be transmitted incorrectly. 4Codecs are used for music-quality audio and for video as well. The term codec refers to the com- ponent that encodes and decodes the audio or video source into a digital signal. In cellular tele- phones, the component is a hardware chip or chipset. On a computer, a codec may be implemented in software, perhaps as a device driver. Codecs code and decode a signal according to one or more standards. The standards often, but not always, perform compression. The term codec is also sometimes used to refer to the standard rather than to the device. 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. Worldwide CDMA Standards Standards for Cellular Systems RS2 have a rate one-quarter mode for even simpler speech patterns. And they all have a rate one-eighth mode that is used when the subscriber is not speaking. 10.1.3 cdmaOne radio bandwidth cdmaOne operates using pairs of carriers for two-way communication, employing fre- quency division duplex (FDD) with a 1.25-MHz carrier in the forward direction and an- other 1.25-MHz carrier in the reverse direction. Each cdmaOne carrier uses a total of 2.5 MHz for bidirectional communication. Each pair of carriers theoretically could sup- port 64 channels, each with its own Walsh code, but installed equipment usually sets a maximum of about 30 channels, and practical limits created by interference typically limit