Data Communications Networking Devices - 4th Ed.pdf

750 ___________________________________________________________ SPECIALIZED DEVICESoverall network availability than the installation of typical redundant equipment.Furthermore, the cost of switches themselves can be kept low if the applicationallows a reasonable time interval for an operator to perform manual switchingcompared with the higher cost of master-controlled remotely operated switches.Switching applicationsWhen network equipment fails, a variety of communications switches can quicklyget the network back into operation. They rapidly bring redundant equipment intoplace to meet established requirements for overall network availability. The cost ofproviding the switches can range from less than a few hundred dollars to well over$50 000. What makes the price vary so much rests on the answers to such questionsas: Which devices are most likely to fail? What tangible and intangible effects will a failed network device, such as aconcentrator, have on the organization's operation? Would the operational loss be so great that it warrants the cost of includingbackup equipment and transmission lines? When a network component goes down, how much downtime, if any, is allowableto activate backup devices and get the network back into full-scale operation? To obtain speedy network recovery, what are the best types of switches for theapplication, and where should they be placed in the network?The signi®cance of these and similar questions, and their answers, will becomeapparent during the discussions of typical redundancy/switching con®gurationsthat follow.The four basic types of communications switchesÐfallback, bypass, crossover,and matrixÐcome in two versions: EIA for switching digital signals and telco forswitching two-wire and four-wire telephone lines. Chaining these switchesprovides a variety of extended switching functions. Furthermore, the switchescan be activated or controlled in local or remote manual modes, in an unattendedremote mode in which the switch is activated by a speci®ed ASCII-character code,and in a computer-controlled remote mode. Switches becomes more expensive ingoing from local manual mode up to computer-controlled mode. Changing anetwork from primary to backup mode manually may, however, take 10 or 15minutes, while a computer-controlled switch can activate all switch connectionsessentially instantaneously and automatically from a remote location.In the ®rst part of this section, the use of switches to substitute spares for suchdevices as modems or DSUs) and terminals was discussed. In the second part, thediscussion will center around the rami®cations of switching between dualcollocatedrouters. Here, one router may be assigned completely to back up theother unit, or each router may be servicing its own networks during normaloperation. In either case, on failure of one router, the other takes over all duties if ithas enough capacity to do so. Although this discussion involves the use of routers,its points can be directly applied to most networking devices the user may wish toobtain a level of redundancy for, including statistical and intelligent multiplexers.