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Solution
 
¡ôConcentrators and Multiplexers
 

Conceptually the simplest way of linking a number of remote locations to single host processor is to provide a separate communications link to each remote location. The cost of the communication links in this type of network can quickly become prohibitive and various approaches have been adopted to overcome the problem-two of the approaches being concentrators and multiplexers.

Concentrators

The concentrator is effectively a computer in its own right which-as its name implies-concentrates the data travelling to or form a number of remote locations into ¡®bulk load¡¯. Thus a concentrator may ¡®collect¡¯ data form a number of relatively slow-speed terminals using appropriate low speed and probably asynchronous links and interleave them for transmission over a higher performance link to the host processor.

Being intelligent, concentrators may be programmed to perform a variety of tasks, which frequently include data validation, and for this purpose storage devices containing a variety of files may be attached, similarly for data flowing outwards from the host processor, that is, towards the terminal, output formatting may be performed, for example to provide appropriate screen layouts for visual display units. Finally the possibility of using concentrators to provide at least some of the essential back-up and/or fail safe options in a network should be mentioned. Thus a concentrator may perform sufficient processing to enable ¡®its¡¯ part of the network to continue essential processing if one or some of some of the communications link(s)to it be come inoperable. Furthermore, it may store essential data if a link or device on either the inboard(host processor)or outboard (terminal)side of it goes down..

Multiplexers

A multiplexer is basically an unintelligent unit which performs only the basic role of reducing total communications link costs but has no other function in network. In this sense the multiplexer plays solely economic role with regard to data communications links whereas, as we have seen, a concentrator may have a functional (back-up, data validation, etc.) as well as an economic role. This distinction will probably become less clear as an increasing number of multiplexers are based on programmable minicomputers and development of hybrid devices seems likely.

There are two main techniques used in multiplexing frequency division and time division.

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