Data Communications Networking Devices - 4th Ed.pdf

158 ______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSNetwork characteristicsFour of the major characteristics of an ISDN network are listed in Table 1.36.These characteristics can also be considered as driving forces for the implementationof the network by communications carriers.Table 1.36ISDN characteristicsIntegrates voice, data and video servicesDigital end-to-end connection resulting in high transmission qualityImproved and expanded services due to B and D channel data ratesGreater ef®ciency and productivity resulting from the ability to have several simultaneouscalls occur on one lineDue to the digital nature of ISDN,voice,data,and video services can beintegrated,alleviating the necessity of end-users obtaining separate facilities foreach service. Since the network is designed to provide end-to-end digital transmission,pulsescan be easily regenerated throughout the network,resulting in thegeneration of new pulses to replace distorted pulses. In comparison,analog transmissionfacilities employ ampli®ers to boost the strength of transmission signals,which also increases any impairments in the signal. As a result of regenerationbeing superior to ampli®cation,digital transmission has a lower error rate andprovides a higher transmission signal quality than an equivalent analog transmissionfacility.Due to basic access in effect providing three signal paths on a common line,ISDN offers the possibilities of both improvements to existing services and anexpansion of services to the end-user. Concerning existing services,current analogtelephone line bandwidth limitations normally preclude bidirectional data transmissionrates over 33.6 kbps occurring on the switched telephone network. Incomparison,under ISDN each B channel can support a 64 kbps transmission ratewhile the D channel will operate at 16 kbps. In fact,if both B channels and the Dchannel were in simultaneous operation a data rate of 144 kbps would be obtainableon a basic access ISDN circuit,which would extend current analog circuit datarates by a factor of 4.Since each basic access channel in effect consists of three multiplexed channels,different operations can occur simultaneously without requiring an end-user toacquire separate multiplexing equipment. Thus,an end-user could receive a callfrom one person,transmit data to a computer and have a utility company read theirelectric meter at a particular point in time. Here,the ability to conduct simultaneousoperations on one ISDN line should result in both greater ef®ciency andproductivity. Ef®ciency should increase since one line can now support severalsimultaneous operations,while the productivity of the end-user can increase due tothe ability to receive telephone calls and then conduct a conversation while transmittingdata.