Data Communications Networking Devices - 4th Ed.pdf

728 _________________________ WIDE AREA NETWORK DATA CONCENTRATION EQUIPMENTThe installation of a port selector to front-end a line transfer device is shown inFigure 6.74. Note that not only are the number of front-end processor portsreduced by 32 16 per processor) but also that the size of the line transfer device canbe reduced, since a 32 by 64 switch is now needed instead of a 48 by 96 switch if aselector was not used.Figure 6.74 Port selector increases savings for redundant con®gurations.Using a portselector to front-end the line transfer device of a dual processor not only doubles thenumber of ports that can be eliminated but also reduces the size of the switch requiredLoad balancingA key advantage obtained from the utilization of port selectors is the ability tobalance the communications load when an installation has two or more front-endprocessors. This load balancing can be accomplished simply by wiring the cablesfrom the port side of the selector in an alternate manner to each front-endprocessor. An installation with dual front-end processors would cable the leadsfrom ports 1, 3, 5, 7, 9, ..., N-1 from the port selector to the ®rst front-end processorand the leads from ports 2, 4, 6, 8, ..., N to the second front-end processor.Selector featuresA wide range of standard and optional port selector features should be considered.First, the basic size or capacity of the port selector and its expansion capability, ifFigure 6.75 Grouping selector contention.Users from New York City always requireimmediate access so no contention is performed.Although 32 local terminals are connectedto the selector, management has decided that at most 24 will be active at one time and hasdirected a 4:3 contention ratio be established for this group