Data Communications Networking Devices - 4th Ed.pdf

468 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTFigure 4.61 Multiport modem application example. A pair of four-channel multiportmodems services two CRTs, a 1500 cps printer, and three low speed terminal devices overa single transmission linemultiplexed 1200 bps channels with the three synchronous unmultiplexed channelsinto a single multiplexed synchronous data stream. At the central site where thecomputer is located, the multiport modem at that end splits the 28 800 bps streaminto four data streams; one data stream is then channeled through another 3-channel, traditional TDM, whose output data streams in turn are connected to thecomputer. The 3-channel TDM takes the 7200 bps synchronous data stream fromthe multiport modem and demultiplexes it into three 1200 bps asynchronous datastreams, which are passed to the appropriate computer ports. The remaining datastreams produced by the demultiplexer in the multiport modem are connected tothree additional computer ports. As this example demonstrates, the high-speedmultiport modem's utilization in conjunction with other communications componentspermits a wide degree of ¯exibility in the design of a data communicationsnetwork.A few examples of multiport modem and channel combinations available arelisted in Table 4.28. It should be noted that not all multiport modem channelcombinations listed in Table 4.28 may be available for a particular vendor's modemthat operates at the indicated aggregate throughput, since some vendors offer only a4-port multiplexer with their modem, while other vendors may offer an additionalnumber of ports.Although most manufacturers of multiport modems produce equipment thatappears to be functionally equivalent, you should exercise care in selectingequipment because of the differences that exist between modems but are hard toascertain from vendor literature. Although the modes of operation available forutilization are quite similar regardless of manufacturer, the number of ports or datachannels supported by a multiplexer can differ signi®cantly. Another varianceconcerns the use of an independently controlled carrier signal in some multiportmodems. By using multiport modems that permit an independently controlledcarrier signal for each channel, you can combine several polled circuits and furtherreduce leased line charges.