Data Communications Networking Devices - 4th Ed.pdf

746 ___________________________________________________________ SPECIALIZED DEVICESAdditional derivationsA number of additional switching functions have been developed from the fourcategories of switches previously discussed. For instance, a spare componentbackup switch is basically a pair of fallback switches contained in one housing. Asshown in Figure 7.5 top left), this switch permits a normal and a backup mode ofoperation. The normal mode permits data to be transferred through the primarycomponent, whereas the backup mode switches the data ¯ow through the sparecomponents.Figure7.5 Backup switch variations. Top left: backup switch. Top right: backup switch,3of 4 EIA interface. Bottom: backup switch,4 of 4 EIA interface. Paired EIA and telco fallbackswitches,which often come in one package,provide both a backup and a normal mode ofoperationIn another con®guration Figure 7.5, top right), a pair of modems or DSUs arethe primary and spare components connected to one terminal, and the switch selectsthe modem or DSU to be used in transferring data between the terminal and thetelco line. Because three EIA interfaces are involved, this con®guration is called a 3of 4 EIA interface bypass switch. In a 4 of 4 EIA interface Figure 7.5, bottom),four interface devices are connected to the switch. In this con®guration, the switchselects one of two encoders to encode terminal data for transmission through anattached modem or DSU.A second common switch derivation is a multiple fallback switch Figure 7.6).Besides the EIA and telco versions, this switch is manufactured in a 1 of n version,with n being the number of possible selections. Figure 7.6 top) shows two possiblecon®gurations for a 1 of 4 EIA fallback switch. At the left, the switch allows the