ems converted for the 39.5.2.1 Half duplex vs full duplex U.K. market provide nationwide operation over dedicated and wire leased lines at speeds up to 19.2kbit/s. From the U.S. perspective, A train travelling on a single track illustrates half duplex operation coverage of the U.K. would constitute a limited distance! Some over two wire circuits, because while it is travelling in one direction, modems, described as limited distance devices, use the baseband travel in the other direction is not possible. The train would have to technique and are subsequently limited to the local exchange oper- be removed from the track at its destination before another could be ating restrictions. sent in the opposite direction. Half duplex operation on four wire The types of modem SO far have dealt exclusively with digital data circuits can also be illustrated using the same analogy if we now using baseband techniques that carry one channel per line. Broad- have a second track, SO there is one for each direction, but one train band signalling has its origins in the cable television industry and is not allowed to start before the other is finished. carries many channels per cable (Minoli, 1991). A 300MHz broad- Continuing with two parallel tracks, full duplex operation is band system can handle seventy five thousand 4kHz voice channels, equivalent to the trains being able to operate independently of each but can also carry data, text and image. Using a two cable im- other on their own track. In practice, full-duplex operation is pro- plementation to link devices to the network, the interface is pro- vided over four wire lines where the outward and inward *tracks' are vided by a radio frequency transmitter/receiver called an RF physically separate or over two wires where either the modem splits modem. Radio modems are now being developed for data trans- the physical circuit into two channels of different frequency or uses mission over cellular links and packet radio. Packet radio accom- echo cancellation. modates asynchronous and s