Data Communications Networking Devices - 4th Ed.pdf

6.1 MULTIPLEXERS _______________________________________________________________ 639Figure 6.10 Hub-bypass multiplexing.This method should be considered when a numberof terminals have the requirement to communicate with more than one locationAlthough hub-bypass multiplexing can be effectively used to connect collocatedterminal devices to different destinations, if more than two destinations exist a moreef®cient switching arrangement can be obtained by the employment of a portselector or a multiplexer that has port selection capability. The reader is referred toSection 6.9 for information concerning port selection.Front-end substitutionAlthough rarely utilized, a TDM may be installed as an inexpensive front end for acomputer, as shown in Figure 6.11. When used as a front end, only one computerport is then required to service terminal devices which are connected to thecomputer through the TDM. The TDM can be connected at the computer center,or it can be located at a remote site and connected over a leased line and a pair ofmodems or DSUs. Since demultiplexing is conducted by the computer's software,only one multiplexer is necessary.Figure 6.11 TDM system used as a front end.When a TDM is used as a front-endprocessor, the computer must be programmed to perform demultiplexingDue to the wide variations in multiplexing techniques of each manufacturer, nostandard software has been written for demultiplexing; and, unless multiplelocations can use this technique, the software development costs may exceed thehardware savings associated with this technique. In addition, the software overheadassociated with the computer performing the demultiplexing may degrade itsperformance to an appreciable degree and must be considered.