Data Communications Networking Devices - 4th Ed.pdf

626 _________________________ WIDE AREA NETWORK DATA CONCENTRATION EQUIPMENTin a loss of productivity, if the users are within your organization, or the possibleloss of customers, if users are from outside the organization. If too many ports areinstalled, many ports will be underutilized, probably resulting in the expenditure offunds for unnecessary hardware.Owing to the importance of equipment sizing, a review of the mathematicsassociated with such problems is presented in Appendix A. The material presentedin that appendix can be used for sizing such devices as multiplexers, concentrators,and front-end processors as well as for determining an appropriate con®guration fora remote access server.6.1 MULTIPLEXERSWith the establishment of distributed computing, the cost of providing requiredcommunications facilities became a major focus of concern to users. Numerousnetworkstructures were examined to determine the possibilities of using specializedequipment to reduce communications costs. For many networks where geographicallydistributed users accessed a common computational facility, a centrallocation could be found which would serve as a hub to linkthose users to thecomputer. Even when transmission traf®c was low and the cost of leased lines couldnot be justi®ed on an individual basis, quite often the cumulative cost of providingcommunications to a group of users could be reduced if a mechanism was availableto enable many communications devices to share common transmission facilities.This mechanism was provided by the utilization of multiplexers whose primaryfunction is to provide the user with a reduction of communications costs. Thisdevice enables one high-speed line to be used to carry the formerly separate transmissionsof a group of lower speed lines. The use of multiplexers should beconsidered when a number of devices communicate from within a similar geographicalarea or when a number of leased lines run in parallel for any distance.EvolutionFrom the historical perspective, multiplexing technology can trace its origination tothe early development of telephone networks. Then, as today, multiplexing was theemployment of appropriate technology to permit a communications circuit to carrymore than one signal at a time.In 1902, 26 years after the world's ®rst successful telephone conversation, anattempt to overcome the existing ratio of one channel to one circuit occurred. Usingspeci®cally developed electrical networkterminations, three channels were derivedfrom two circuits by telephone companies.The third channel was denoted as the phantom channel, hence the name `phantom'was applied to this early version of multiplexing. Although this technologypermitted two pairs of wires to effectively carry the load of three, the requirementto keep the electric network ®nely balanced to prevent crosstalk limited itspracticality.