t achievable, the modem will fall back to whatever maximum speed it can fulfill. Furthermore, no premises wiring changes are required, and since this device is really nothing more than a faster modem, the average customer is comfortable with migration to the new technology. This is certainly demonstrated by sales volume: As was mentioned before, most PCs today are automatically shipped with a 56K modem. The Integrated Services Digital Network (ISDN) has been the prover- bial technological roller coaster since its arrival as a concept in the late 1960s. Often described as "the technology that took 15 years to become an overnight success," ISDN's level of success has been all over the map. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Access Technologies Access Technologies Internationally, it has enjoyed significant uptake as a true, digital local loop technology. In the United States, however, because of competing and often incompatible hardware implementations, high cost, and spotty availability, its deployment has been erratic at best. In market areas where providers have made it available at reasonable prices, it has been quite successful. In fact, one application that has made its mark with ISDN is videoconferencing, which we'll discuss shortly. ISDN Technology The typical non-ISDN local loop is analog. Voice traffic is carried from an analog telephone to the CO using a frequency-modulated carrier; once at the CO the signal is typically digitized for transport within the digital network cloud. On the one hand, this is good because it means that there is a digital component to the overall transmission path. On the other hand, the loop is still analog and, as a result, the true promise of an end-to-end digital circuit cannot be realized. The circuit is only as good as the weakest link in the chain, and the weakest link is clearly the ana