Data Communications Networking Devices - 4th Ed.pdf

742 ___________________________________________________________ SPECIALIZED DEVICESFallback switchesThe fallback switch is a rapid and reliable means of switching other networkcomponents from on-line to standby equipment. The EIA version selects either apair of 24-pin-connected components, which, as shown in Figure 7.1, may beterminals, modems, DSUs or channels on a front-end processor.Figure7.1 Fallback switches. Top: EIA 24-pin) fallback switch. Bottom: telco fallbackswitch. The EIA fallback switch transfers 24 leads at a time,while the telco switch transfersthe two or four leads associated with telephone linesIn the ®rst example, two terminals share a single modem. This con®gurationmight be required, for example, when terminals have the same transmission speedbut use different protocols, so that each communicates with a different group ofremote terminals or computers.In the second example, one terminal is provided with access to two modems orDSUs, one of which is redundant but needed for uptime reliability. Alternatively,the ®rst modem or DSU might enable a terminal to transmit to another device atone data rate during one portion of the day, while the second lets it talk to a differentdevice at a different data rate during other periods of the day. Then, depending onoperational requirements, one terminal with a fallback switch for modem or DSUselection could be more practical than installing two terminals.In a third application, an EIA fallback switch Figure 7.1, top right) permits amodem or DSU to be transferred between front-end processors. Although called aline-transfer device by some manufacturers, in effect what you obtain is a devicethat selects which front-end processor will service the modem or DSU.A telco fallback switch similarly allows the user to select one of two sets oftelephone lines. As shown in Figure 7.1 bottom), it can select one line from amongvarious combinations of dedicated and dial-up lines that may have been installed to®t the needs of a particular application. Thus, for a large data transfer applicationwhich is of a critical nature, the telco fallback switch could be connected to a pair ofleased lines, one of which is used as an alternative circuit in the event of an outage onthe primary circuit.