ch as a channel service unit. Unfortunately, very rarely do the communications requirements of an organization remain static. Thus, requirements can be expected to vary over time to include the need for additional T1 or E1 lines that must be connected to equipment that will be installed at other locations within a building. Because it is relatively expensive for the communications carrier to change the routing of an underground or aboveground main cable into a building, they will more than likely continue to route new cir- cuits to a common DEMARC point, even if the customer would prefer another location. Thus, once again the use of a pair of fiber-optic modems and optical fiber may represent the solution to obtaining the connectivity required within a building. Employment Features Figure 8.1 illustrates the basic employment of a pair of fiber-optic modems to extend the transmission distance between two copper inter- faces. In examining Figure 8.1, note that the type of copper interface built into the fiber-optic modem is not specified as this is one of the features that separates one type of fiber-optic modem from another. Also note that the optical fiber is specified as either multimode or sin- gle-mode. While some optical modems are capable of supporting either type of fiber, other devices are restricted for use with a specific type of device. Thus, the type of optical fiber supported by the optical modem can be an important selection criterion. Chapter Eight Multimode or single-mode Figure 8.1 Fiber fiber optic cable pair Fiber Using a pair of fiber Modem Modem modems to extend the transmission distance between devices with Legend: electrical interfaces. E/I Electrical Interface Because LEDs require a larger core than do lasers, the type of transmitter-LED or laser-will normally govern the type of fiber supported. Other important optical modem selection criteria include the data transmission rate supported at the electric interface, the type of data, asynchronous or synchronous supported at the interface, and