modulation. Multicarrier transmission is a technique that divides the available spectrum into many subcarriers, with the transmission rate reduced on each subcarrier. OFDM is similar to FDM in that multiple-user access is achieved by subdividing the available bandwidth into multiple channels that are then allocated to users. However, OFDM is a special case of FDM. An FDM channel can be likened to the water flow out of a faucet, where the water comes out as one big stream and can't be subdivided. The OFDM channel, on the other hand, can be compared to a shower, where the water flow is composed of a lot of little streams of water. This analogy also highlights one of the advantages of OFDM: If you put your thumb over the faucet, it stops all the water flow, but that is not the case with the shower, where some of the streams of water will still get through. In other words, FDM and OFDM respond differently to interference, which is minimized in the case of OFDM. (OFDM is discussed in more detail in Chapter 15, "WMANs, WLANs, and WPANs.") Discrete Multitone (DMT)-DMT is a multicarrier scheme that allows variable spectral efficiency among the subbands it creates. Therefore, it is used in wireline media, where noise characteristics of each wire might dif- fer, as in the wires used to carry xDSL facilities. Because spectral efficiency can be optimized for each individual wire with DMT, DMT has become the preferred choice for use with xDSL. Simplex, Half-Duplex, and Full-Duplex Data Transmission Information flow takes three forms: simplex, half-duplex, and full-duplex (see Figure 5.5). With simplex transmission, information can be transmitted in one direction only. Of course, simplex does not have great appeal to today's business communi- cations, which involve two-way exchanges. Nonetheless, there are many applica- tions of simplex circuits, such as a doorbell in homes. When someone presses a doorbell button, a signal goes to the chimes, and nothing returns over that pair of wires. Another example of a simplex application