Data Communications Networking Devices - 4th Ed.pdf

7.1 DATA COMMUNICATIONS SWITCHES ____________________________________________ 747Figure7.6 Multiple fallback switches. Multiple fallback switches,either EIA or telco,allowmany terminals to share one modem or DSU) or many modems or DSUs) to share onetelephone lineterminal to be connected to any one of four modems or DSUs, while at the right,any one of four terminals may be connected to a single modem or DSU. Similarly,Figure 7.6 bottom) shows how the 1 of 4 telco fallback switch allows either fourmodems or DSUs to share a single line or four lines to share a single modem orDSU.Chaining switchesNo manufacturer produces a complete line of readymade switches, but it is oftenmore convenient to deal with and install switches from a single maker. You can doso by developing the switching functions required from combinations of onevendor's switches. In Figure 7.7 left), for instance, four fallback switches arechained together to perform the function of a bypass switch. A single backupterminal can be used to replace any one of four primary terminals. In Figure 7.7right), four fallback switches are chained so that a single backup modem or DSUmay be used by any terminal if its primary modem or DSU fails.