 sequence of eight or four binary zeros. The reason for this substitution is the need for T1 and E1 copper circuits to have a minimum "ones" density to ensure that repeaters have a suffi- cient number of one bits on which to synchronize their operation. B8ZS coding is used on T1 lines, while HDB3 coding is used on E1 lines. UTILIZATION The TC8300p is well suited for many potential applica- tions. For example, you could use a pair of TC8300p fiber-optic multi- plexers to interconnect two or more routers and/or PBXs located in different buildings within a campus or industrial complex where it is possible to install your own fiber. Within a building, the local telephone company will often install a demarcation point where a T1 or E1 line terminates. At that location you could install a T1 or E1 electrical inter- face fiber-optic multiplexer while locating routers or digital PBXs beyond the relatively short distance normally allowed from the DEMARC point. Figure 8.7 illustrates an example of how a pair of TC8300p fiber- optic multiplexers could be used to enable routers and a PBX to be located at a considerable distance from the copper DEMARC point where the high-speed lines normally terminate within a building. In examining Figure 8.7, note that although only three T1 lines are shown as multiplexed, the TC8300p is capable of supporting four T1 or E1 circuits. Similar to the fiber-optic modem that we described previously, the fiber-optic multiplexer includes various LEDs and DIP switches. The LEDs indicate the loss of a signal, the occurrence of a bipolar violation, Figure 8.7 Router Using fiber-optic mul- tiplexers to extend Single-mode or multimode fiber the termination optic cable points of high-speed FOMux FOMux copper circuits. Legend: FOMux a TC8300p fiber-optic multiplexer Fiber in the Building the status of received data, and other conditions that can be helpful when troubleshooting a problem. In contrast, the DIP switches are used to select a line code for a specific channel or to initial a remote loopback f