Data Communications Networking Devices - 4th Ed.pdf

6.8 MODEM- AND LINE-SHARING UNITS _____________________________________________ 707A similar deviceAnother device which functions similarly to modem- and line-sharing units,although its operation and usage is different, is a control unit. This device is built toworkwith a speci®c type of terminal device, usually manufactured by the vendorthat produced the control unit. In addition, while modem- and line-sharing unitsoperate basically transparent to the data ¯ow, control units scan and interpret thedata, looking for and operating upon orders received in the information ¯ow. Foradditional information on control units the reader is referred to Section 6.10.OperationA TDM operates continuously to sample in turn each channel feeding it, either bitby bit or character by character; this produces an aggregate transmission at a speedequal to the sum of the speeds of all its terminals.A conventional TDM operation is illustrated in Figure 6.55a). For example, amultiplexer operating character by character assembles its ®rst frame by taking theletter A from the ®rst terminal, the letter E from the second, and the letter I fromthe third terminal. During the next cycle, the multiplexer takes the secondFigure 6.55 Multiplexing versus line-sharing.a) Time division multiplex network.b)Modem-sharing network.Multiplexer needs: a time or frequency division multiplex systema) requires one computer port for each terminal and a multiplexer at each end.A sharingsystem b) needs only one computer port.Because it requires terminals to be polled, asharing system can be cost-effective for interactive operation, but may not be so for longmessages such as are likely to move in remote job entry or remote batch types ofapplications