Data Communications Networking Devices - 4th Ed.pdf

730 _________________________ WIDE AREA NETWORK DATA CONCENTRATION EQUIPMENTFigure 6.76 Constructing a circuit-switched network.A port selector with an automaticgroup select feature enables the utilization of a common network segment to accessgeographically separated computer systemsin Macon while the entry of a `2' would cause the user to be routed out of the portselector and into a multiplexer that would connect to the computer in Miami. Thus,in this example, the port selector enables the utilization of a common networksegment for obtaining access to geographically distant computer systems.Additional features available on selectors include diagnostic modules and busyoutswitches. Selector diagnostic modules are used to determine which line isconnected to which port and to display key diagnostic information for that particularcross-connect while busy-out switches permit user ¯exibility in changingexisting contention ratios.6.12 PROTOCOL CONVERTERSA wide range of protocol converters are marketed, ranging in scope from devicesthat convert the physical and electrical interface from one standard to another, todevices that may perform ®ve levels of conversion to include the physical andelectrical interface. Table 6.16 lists the different levels of operational conversionthat may be performed by a protocol converter.Table 6.16 Operational conversion performed byprotocol convertersOperational conversionLevelDevice functionality 1Device operation 2Protocol 3Data code/speed 4Physical and electrical interface 5