 pairing of channels permits true, full-duplex 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. Access Technologies Chapter 6 telephony. Most analog systems have since gone by the wayside in favor of the far more popular, efficient, and battery-friendly digital systems. During the developmental battles, two techniques emerged. The first, time-division multiple access, or TDMA, resembles FDMA in that it divides the available frequency spectrum into channels. That, however, is where the resemblance ends. In TDMA each of the analog channels carries telephone calls that are time-division multiplexed- that is, they share access to the channel. As in FDMA, a control channel is reserved for communication between the network and mobile users. The biggest advantage that TDMA systems have over FDMA systems is that they support significantly more users. If each channel is divided into four time slots, then the system capacity is quadrupled. And while TDMA electronics are significantly more complex, the fact that they are digital means that they can easily evolve as technology advances. The second digital access technique is called code-division multiple access, or CDMA. CDMA systems are dramatically different from FDMA and TDMA systems in that they do not channelize the available band- width-instead, they allow all users to access and use the available spec- trum simultaneously. This technique is called spread spectrum transmission. Spread-spectrum techniques are described in detail later in this chapter. Not only are digital systems more secure than narrowband technolo- gies, they also support significantly larger numbers of simultaneous users. In fact, whereas FDMA systems support a single user per fre- quency slot, CDMA systems can support hundreds. Radio-based telephony has enjoyed a wild, tumultuous ride along the way to its position today as a mai