nk, but they are divided by frequency-that is, each signal rides on a different portion of the frequency spectrum. Frequency, which is an analog parameter, implies that the type of link used with FDM is usually an analog facility. A disadvantage of frequency division muxes is that they can be difficult to reconfig- ure in an environment in which there's a great deal of dynamic change. For instance, to increase the capacity of channel 1 in Figure 1.14, you would also have to tweak channels 2, 3, and 4 to accommodate that change. In an enterprise that has a high degree of moves, additions, and changes, an FDM system would be expensive to maintain because it would require the addi- tional expertise of frequency engineering and reconfiguration. Today's environment doesn't make great use of FDM, but it is still used extensively in cable TV and radio. In cable TV, multiple channels of programming all coexist on the coax com- ing into a home, and they are separated based on the frequency band in which they travel. When you enter a channel number on your set-top box or cable-ready TV, you're essentially selecting the channel (set of frequencies) that you want your television to decode into a picture and sound stream to watch. The second muxing technique to be delivered to the marketplace was TDM. There are various levels of TDM. In the plain-vanilla TDM model, as shown in Figure 1.15, a dedicated time slot is provided for each port, or point of interface, on the system. Each device in a predetermined sequence is allotted a time slot during which it can Channel 1 Channel 2 Modem Channel 3 Modem Channel 4 Figure 1.14 FDM Multiplexing ABCDABCDABCDABC Figure 1.15 TDM transmit. That time slot enables one character of data, or 8 bits of digitized voice, to be placed on the communications link. The allocated time slots have to be framed in order for the individual channels to be separated out. A problem with a standard time division mux is that there is a one-to-one cor- relation between each port and time slot, SO if the devic