Data Communications Networking Devices - 4th Ed.pdf

164 ______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSFigure 1.94 2BIQ coding exampleDibit Quats00 301 110 ‡ 311 ‡ 1comparison,from the U interface to the S/T interface the data rate is 192 kbps,with 48 kbps used for synchronization and line balancing.The future of ISDNAlthough the use of ISDN has many distinct advantages,especially in the areas ofdial Internet access and the use of multiple calls grouped together to obtainsuf®cient bandwidth required for videoconferencing,the use of this digital servicehas signi®cantly fallen below expectations. Among the reasons provided for theslower than expected growth in the use of ISDN are its cost and pending competitionin the form of digital subscriber lines DSLs). Concerning its cost,installation of an ISDN line can easily exceed $300 and usage is typically billed at 5or 10 cents/minute. When compared to the cost of a conventional analog line thatcan support bidirectional modem transfer at up to 33.6 kbps without incurring a $3to $6 per hour usage surcharge,the price/performance ratio associated with the useof ISDN leaves much to be desired. Concerning DSL competition,although in itsinfancy,several types of digital subscriber lines have been developed that enabletransmission rates up to 8 Mbps on the local analog loop between a subscriber andthe serving telephone company of®ce. Since DSL represents a modulation technology,itwill be covered in detail in Chapter 5.At the time this book revision was prepared,several ®eld trials of DSLtechnology were being conducted and a few communications carriers and InternetService Providers were offering commercial services using the technology. Althoughit is probably premature to predict the ultimate effect of DSL upon ISDN,it should be noted that ISDN represents a circuit switched and packet switchedtechnology that enables calls to be routed. In comparison,DSL technology islimited to providing a high speed point-to-point access from a subscriber to atelephone company central of®ce. Thus,the primary competition between the twowill probably occur in the market for obtaining fast Internet access. This is because