Data Communications Networking Devices - 4th Ed.pdf

6.9 PORT-SHARING UNITS _________________________________________________________ 715beyond the failure out of action. In the con®guration in Figure 6.61 however, failureof modem or outage on the line will only cut out a terminal on that segment. Failureof a computer port or of the port-sharing unit would of course bring down the entirenetwork, but these devices are stable and such failures are fairly unusual.Port-sharing as a supplementPort-sharing units may also be used alongside modem-sharing units. If modemsharingunits alone are used, a situation can arise where there are not enough portsto serve the network, as in Figure 6.62a). If each modem-sharing unit serves its fullcomplement of terminals and all the computer ports are in use, expansion of thenetwork, even by just one port, may require a second mainframe computer.This problem can also be dealt with by the use of a port-sharing unit at the centralsite which by cutting down the number of ports currently needed allows a networkto expand without additional computer ports. This is illustrated in Figure 6.62b)which shows how one port-sharing unit with a two-modem interface can free acomputer port from the con®guration shown at the top of that illustration.One versatile feature of port-sharing units is an option that allows the unit toaccept a local interface instead of the normal RS-232 DCE interface, so that up totwo local terminals may be operated without modems at the central site, as shown inFigure 6.63.Figure 6.63 Connecting local peripherals.A local interface option to port-sharing lets localand remote sites be served by the same port.Peripherals are polled as if they were atremote sitesWhile both modem-sharing units and port-sharing units are similar in the waythey are used, there is an important difference in the normal placing of their interfaces.Table 6.15 provides, a comparison of the characteristics of a port-sharing unitwith those of modem- and line-sharing units.A similar deviceAnother device which has a function similar to port-sharing units, although itsoperation and usage differ, is a port selector or port contention) unit. This unit