ion when considering the use of a fiber-optic multiplexer. One key feature to con- sider is the fact that a fiber-optic multiplexer includes a built-in fiber- optic modem. This means that when your organization must acquire the ability to transmit multiple logical channels of data simultaneously over one physical channel, you can consider two equipment configura- tions. First, you can acquire a pair of traditional TDMs that are restrict- ed to having electrical interfaces. Then you can connect the high-speed interface of each TDM to an optical modem, with each fiber-optic modem connected to a common optical fiber. The top portion of Figure 8.5 illustrates the use of traditional TDMs with fiber-optic modems. In contrast, the lower portion of Figure 8.5 illustrates the use of a pair of fiber-optic modems to accomplish a similar networking Chapter Eight a. Using traditional TDMs and fiber-optic modems. Figure 8.5 Two methods are available to transmit multiple channels optical fiber over a common fiber-optic cable. b. Using fiber-optic multiplexers. optical fiber FOMux FOMux Legend: time division multiplexer fiber-optic modem FOMux fiber-optic multiplexer capability. The word "similar" is used in this context to reflect the fact that the number of channels supported by traditional TDMs com- monly exceeds the number of channels supported by fiber-optic mul- tiplexers. Thus, in certain situations the configuration shown at the top of Figure 8.5 may represent a more flexible network solution to satisfy many organizational requirements. Actually, there is a third method that will permit multiple channels of data to flow over single optical fiber. That method is wavelength division multiplexing (WDM) or its more sophisticated cousin, dense wavelength division multiplexing (DWDM). Because both WDM and DWDM are employed primarily on wide area networks, we will not consider this technology for intrabuilding communications; however, this does not mean that it cannot be used. In addition to considering the number of channels s