Data Communications Networking Devices - 4th Ed.pdf

632 _________________________ WIDE AREA NETWORK DATA CONCENTRATION EQUIPMENTFDM utilizationAs mentioned previously, one key advantage in utilizing FDM equipment is theability afforded the user in installing multipoint circuits for use in a communicationsnetwork. This can minimize line costs since a common line, optimized inrouting, can now be used to service multiple terminal locations. An example ofFDM equipment used on a multipoint circuit is shown in Figure 6.4, where a threechannelFDM is used to multiplex traf®c from terminals located in three differentcities. Although the entire frequency spectrum is transmitted on the circuit, thechannel set at each terminal location ®lters out the preassigned bandwidth for thatlocation, in effect producing a unique individual channel that is dedicated byfrequency for utilization by the terminal at each location. This operation is analogousto a group of radio stations transmitting at different frequencies and setting aradio to one frequency so as always to be able to receive the transmission from aparticular station.Figure 6.4 Frequency division multiplexing permits multipoint circuit operations.Eachterminal on an FDM multipoint circuit is interfaced through the multiplexer to an individualcomputer portIn contrast to poll and select multipoint line operations where one computer portis used to transmit and receive data from many buffered terminals connected to acommon line, FDM used for multipoint operations as shown requires one computerport for each terminal. Such terminals do not, however, require a buffer areato recognize their addresses, nor is poll and select software required to operate inthe computer. When buffered terminals and poll and select software are available,polling by channel can take place, as illustrated in Figure 6.5. In this example,