Data Communications Networking Devices - 4th Ed.pdf

6.11 PORT SELECTORS ___________________________________________________________ 723Some port selectors are designed only to contend for asynchronous, teletypeterminal traf®c; other devices can contend for both asynchronous and synchronoustraf®c within the same unit, with each type of traf®c being contended for by one ormore computer ports servicing that mode of transmission.OperationThe utilization of port selectors permits terminals to be added to a networkwithouta corresponding increase in the number of computer ports. In addition, the utilizationof this device may permit a system contraction whereby a number of computerports become unnecessary and can be returned to the manufacturer.The basic function performed by a port selector can be viewed as a dynamic dataswitch similar to telephone rotaries stepping switches that sequentially search foravailable telephone lines), except that the selector provides appropriate interfacesbetween computers and terminals to route a large calling terminal population to alesser number of called computer ports. Some selectors have additional featuresspeci®cally applicable to data networks. Although users tend to confuse portselectors with port-sharing units, their applications are for speci®c line environmentsthat result from the utilization of different types of terminals for speci®capplications. Port-sharing units are used in polled networks where the computercontrols the traf®c ¯ow, and terminals must have a buffer area to recognize polls totheir address. Port selection units are used in contention networks, in whichterminals transmit to the host on a random basis; and the access to any port isnormally on a ®rst-come, ®rst-served basis.Computer site operationsFrom a networkviewpoint, a port selector is similar to a blackbox with N line sideinput connections and n port side output connections, with N n as shown inFigure 6.68. The port selector continuously scans all line side connections forincoming data from terminals connected to that side. At the same time, the selectormaintains a status checkof available ports so that when a terminal becomesoperational and requests access, the selector connects the terminal to an availableFigure 6.68 Port selector at computer site.At line side, port selectors can interface avariety of channelsÐfrom multiplexers, dial-in lines, leased lines, and direct connect lines.As terminal traf®c becomes active, the port selector attaches the circuit to a prede®ned orrandomly selected computer port for the duration of the transmission