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Unformatted text preview: ltiplexing is cost effective. One serious disadvantage with multiplexing relates to a transmission line failure. If the line goes out, everything is dead. With individual lines only one terminal is likely to be lost. Concentrators A concentrator performs the same function as a multiplexer, but concentrators have the ability to actually reduce the number of signals. For example, 100 signals from different devices coming into the concentrator could leave as only 70 or 80 signals. The signals are concentrated to form a fewer number of signals. This requires intelligence. This intelligence normally takes the form of microprocessors or even minicomputers. Thus, a concentrator is basically an intelligent multiplexer. The advantage of using a concentrator is that devices of varying speeds and types are connected to the concentrator, which in turn is connected to the host computer by high-speed lines. Concentrators are especially useful where data communication cost is high, such as long-distance international communications. In general, concentrators are more expensive than multiplexers. For some longdistance and international data communication applications, however, the use of concentrators is cost justified. Front-End Processors (FEP) If information is to be moved from point-to-point in a computer network, then some type of control is necessary. Network processing (also known as communications processing) is the control of information movement between the various locations of the network. This is a substantial task. In the early network days, the communications processing job was handled by the main computer. This began to place heavy additional loads on the CPU. Thus, the ability of the computer to do information processing, i.e., the job for which it was intended, was seriously degraded. As a result, manufactures introduced the frontend processor. This is a separate processor designed specifically to handle the network processing task (see Figure 17.14). A front-end processor is usually installed...
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