Data Communications Networking Devices - 4th Ed.pdf

710 _________________________ WIDE AREA NETWORK DATA CONCENTRATION EQUIPMENTFigure 6.57 Extending the connection.Line- or modem-sharing units form a single linkbetween a host computer and terminals.This system contains a modem-sharing unit withinputs from the terminals at its own site as well as from remote terminals.A line-sharing unitat the central site can handle either remote site devices or local devices more than 50 feetaway from the host computer, which is the maximum cable length advisable under the RS-232/ITU V.24 standardsmodems when terminals are clustered, only one or at most a few DCE optionsshould be used with a modem-sharing unit, as it could defeat the economics ofclustering the terminals to utilize a common modem.It is advisable to checkcarefully into what types of modem can be supported bymodem-sharing units, since some modems permit a great deal more ¯exibility ofnetworkdesign than others. For instance, if the sharing unit can workwith amultiport modem, the extra modem ports can service remote batch terminals ordedicated terminals that frequently handle long messages. An example of this¯exibility of design is shown in Figure 6.58. Some terminals that cannot acceptexternal timing can be ®tted with special circuitry through which the timingoriginates at the terminal itself instead of at the modem.Other sharing devicesSections 6.9 through 6.11 cover other sharing devices. Section 6.9 explores the useof port-sharing units, which are used in polled networks for the programmedselection of the computer port. Section 6.10 examines control units which althoughfunctioning similarly to sharing units are different in operation and usage. Section6.11 discusses port selection or port contention) units, where the unit providesrandom access to the computer ports under its control. The most signi®cantdifferences in the various types of sharing units lies in their placement and functionin the options available to them. Unlike line-, modem-, and port-sharing units, theport selection devices operate by time-sharing or contention. Access to any one portis provided on a ®rst-come, ®rst-served basis whenever a port is available. With