 as well, including Trellis, Reed-Solomon, the Viterbi algorithm, Turbo coding, and the Walsh Code. A good source for more information on all these techniques is Wikipedia (www.wikipedia.org). Duplexing Techniques Another way in which the operations of wireless networks differ, and hence also the standards differ, is the duplexing technique-the procedure for separating the incoming and outgoing conversations. Duplex transmission refers to the ability to send information in both directions over a single channel. Two transmitters cannot use the same channel at the same time because they will interfere with one another, but there are two techniques for enabling full-duplex transmission: Frequency Division Duplex (FDD) and Time Division Duplex (TDD). With FDD, two separate channels are assigned for transmission: the transmit chan- nel and the receive channel. FDD requires more bandwidth than TDD. FDD is the legacy technique used for point-to-point links supporting voice, which is symmet- ric and defined by predictable traffic. Hence, FDD is the most widely used tech- nique in cellular networks. Because there is much equipment based on legacy FDD point-to-point radio links, FDD is a top priority for some manufacturers. Chapter 13 Wireless Communications Basics In TDD, which is found in various standards across all the technology domains, including WWANs, WMANs, WLANs, and WPANs, the two ends take turns sending on the same channel. TDD can be designed as a contention system, where an access protocol (such as CSMA/CA) minimizes the chance of collision. TDD can also be designed for no contention, where time slots are defined, and each direction uses assigned time slots; this is referred to as a "ping-pong" technique. TDD makes practi- cal the reallocation of spectral resources from uplink to downlink or vice versa. The ability to adapt the spectrum resources to the actual traffic pattern significantly improves frequency resource utilization. These characteristics make TDD an attrac- tive alternative in support of triple 