Data Communications Networking Devices - 4th Ed.pdf

698 _________________________ WIDE AREA NETWORK DATA CONCENTRATION EQUIPMENTFigure 6.49 Fron-end processors.A front-end processor can be considered the heart of ahost or main frame based computer network, relieving the host computer system of manysoftware burdens, performing such functions as code conversion, character blocking, andcharacter deblockingoperate with that computer and its operating system. A typical front-end processorcon®guration is illustrated in Figure 6.49. In examining the hardware con®gurationof a typical front-end processor shown in Figure 6.49, note that the multi-linecontroller MLC) represents a grouping of single line controllers, and a single linecontroller represents a serial port, buffer area, and logic which enables received bitsto be grouped into characters for parallel operations and parallel formed charactersto be placed bit by bit onto a serial communications line. By performing serial toparallel and parallel to serial conversion, as well as adding appropriate communicationsprotocol characters to a data stream, performing CRC computations andsimilar communications functions, the front-end processor of¯oads a considerableamount of communications processing from a host computer. This frees the hostcomputer of a large portion of the burden of communications processing, enablingmore of its CPU cycles to be devoted to processing application programs.MLCs enable a mixture of asynchronous and synchronous lines to be serviced atdata rates ranging from 50 bps to 2.048 Mbps. By con®guring each port for aspeci®c protocol the MLC also enables multiple protocols to be supported atdifferent data rates over a grouping of line terminations.Another device encountered on some front-end processors is a local communicationsmultiplexer which provides for time division multiplexing by character,to and from the front-end processor, for a variety of low-speed terminals attransmission rates up to 9600 bps. These local multiplexers can handle terminalswith differing communications speeds and code settings, with the characterdemultiplexing performed by software in the front-end processor. In addition toperforming networkand communications processing activities that are normallyassociated with front-end processors, owing to the larger memory and word size,quite often they can be used to perform message switching functions by the additionof modular software.The operating system which supervises the overall control and operation of allsystem functions is the key element of a front-end processor. Although numeroussoftware elements must be evaluated, major consideration should be given todetermining supported line protocols as well as supported processor communications.In the ®rst area, most vendors divide their supported line protocols intoseveral categories or classes of support. Normally, category one refers to vendor-