ultiplexers combine lower rate data signals into a multiplexers higher rate aggregate signal. Add/drop multiplexers add or drop out digital tributary Cross-connect system signals to/from the aggragate data stream. Digital cross-connect equipment switch data streams from one time slot to another or from one port to another. Repeaters amplify and regenerate signals on the line. Optical line systems terminate optical fibers and convert a signal from electrical to optical and vice versa. Microwave radio systems convert digital data into high frequency radio signals. To network management system (NMS) Figure 4.32 Transmission equipment and system topology. 4.8.1 Modems A modem is a piece of equipment that includes a modulator and demodula- tor. Modems are used to transmit digital signals over an analog channel. Functionality of voice-band modems is described in Chapter 6 and they are used to transmit and receive data from a PC to/from an analog telephone channel. The microwave radio systems are sometimes also called modems because they send digital information over a microwave radio link, and in order to do this, they also carry out modulation and demodulation processes. 4.8.2 Terminal Multiplexers Terminal multiplexers (TMs) or multiplexers combine digital signals to make up a higher bit rate for high-capacity transmission (Figure 4.32). The digital multiplexing hierarchies in use are PDH and SDH, which are replacing older generation PDH systems. These multiplexing schemes were described in Section 4.6. 4.8.3 Add/Drop Multiplexers A transmission system in the network may be just a point-to-point system or it may be built as a chain or as a ring system as shown in Figure 4.32. These Introduction to Telecommunications Network Engineering configurations make efficient use of the high system capacity feasible when only a small fraction of the total transmission capacity is needed on each equipment site. The add/drop multiplexers are used in these configurations to take out (drop) some channels from the high-rate data str