Data Communications Networking Devices - 4th Ed.pdf

708 _________________________ WIDE AREA NETWORK DATA CONCENTRATION EQUIPMENTcharacter of each message B, F, and J, respectively) to make up its second frame.And the sampling continues in this way until traf®c on the line is reversed to allowtransmission from the computer to the terminals. The demultiplexing side of theTDM operating on the receiving side of the network) assembles incomingmessages and distributes them to their proper terminals or computer ports.An FDM divides up the transmission link's total bandwidth into a number ofdistinct frequency bands, each of which is able to carry a low-speed channel. TheFDM accepts and moves transmissions from all of its terminals and portssimultaneously and continuously.A line-sharing networkis connected to the host computer by a local link, throughwhich the host polls the terminals one by one. The central site transmits the addressof the terminal to be polled throughout the networkby way of the sharing unit. Thisis illustrated in Figure 6.55b). The terminal assigned this address 01 in thediagram) responds by transmitting a request to send RTS) signal to the computer,which returns a clear to send CTS), to prompt the terminal to begin transmittingits message ABCD in diagram). When the message is completed, the terminaldrops its RTS signal, and the computer polls the next terminal.Throughout this sequence, the sharing device continuously routes the signals toand from the polled terminal and handles supporting tasks, such as making sure thecarrier signal is on the line when the terminal is polled and inhibiting transmissionfrom all terminals not connected to the computer.Device differencesThere are two types of devices that can be used to share a polled line: modemsharingunits and line-sharing units. They function in much the same way toperform much the same taskÐthe only signi®cant difference being that the linesharingunit has an internal timing source, while a modem-sharing unit gets itstiming signals from the modem it is servicing.A line-sharing unit is mainly used at the central site to connect a cluster ofterminals to a single computer port, as shown in Figure 6.56. It does, however,playa part in remote operation, when a data stream from a remote terminal cluster formsone of the inputs to a line-sharing unit at the central site to make it possible to runwith a less expensive single-port computer.In a modem-sharing unit, one set of inputs is connected to multiple terminals orprocessors, as shown in Figure 6.56. These lines are routed through the modemsharingunit to a single modem. Besides needing only one remote modem, amodem-sharing networkneeds only a single two-wire for half-duplex) or four-wirefor full-duplex) communications link. A single link between terminals and hostcomputer allows all of them to connect with a single port on the host, a situation thatresults in still greater savings.If multiplexing were used in this type of application the outlay would likely begreater, because of the cost of the hardware and the need for a dedicated hostcomputer port for each remote device. A single modem-sharing unit, at the remotesite, is all that is needed for a sharing system, but multiplexers come in pairs, one foreach end of the link.