"As digital technology began to penetrate into long-distance transmission and switching systems in the 1960's and 70's, telephone companies conceived of a network that would offer end-to-end digital transmission. Digital bits would enter, traverse, and exit the network; in contrast to POTS service, the network would not have to convert calls from analog to digital internally. Such a service offering is "integrated" because, by handling signals in digital form, it could transparently handle different applications: for example, data bits, generated by computers, fax machines, etc. could be sent directly; voice bits would be generated by special telephones able to digitize voice signals at the boundary of the network."Implementation of ISDN is hampered by the irregularity of the local loop plant. ……loops are of different lengths, depending on the subscriber's distance from the Central Office. Different gauges of wire may be used within a single loop, and the number of splices may also vary. ………
"Within such an environment, achieving reliable transmission of bits is a challenge. For the same digital transmission equipment to work across the variety of loops found in the plant, either a plethora of configuration settings must be provided and manually adjusted, or the equipment must be able to adapt dynamically to the conditions found on the particular line. Since the former approach is economically infeasible, the advent of ISDN had to wait for the development of self-adaptive filtering techniques implementable with VLSI technology in the late 1980's. The result, standardized by the American National Standards Institute (ANSI) in 1988, is the Basic Rate Access Digital Subscriber Line.
"Basic Rate Access provides a total of 144 Kbps of bidirectional digital transmission over the baseband channel from 0-50 KHz. Because it overlaps with POTS' baseband spectrum (0-4 KHz), ISDN cannot co-exist with POTS on the same wire pair. Once a subscriber loop has been upgraded to ISDN service, existing POTS equipment is no longer usable as is. ISDN cannot be provided over loops with loading coils or loops longer than18,000 feet (5.5 km), with bridged taps included in the loop length. Based on loop surveys conducted by Bellcore, Waring estimates that loading coils still exist on 25% of subscriber loops, and that the loops with loading coils are mainly the same as the loops longer than 18,000 feet. By this estimate, Basic Rate Access can be provided over no more than 75% of North American
subscriber loops."The 144 Kbps provided by Basic Rate Access is subdivided into two bearer, or B channels, and one delta, or D channel. Each B channel transmits bits at 64Kbps and is expected to carry voice or data. The D channel, which transmits at 16 Kbps, was envisaged for out-of-band signaling (for example, setting up a call by exchanging digital 'packets' of instructions, instead of detecting on-and off-hook signal transitions. The result is referred to as 2B+D, or an ISDN 'U' interface.
"……Since ISDN is designed for both voice and data, it incorporates both circuit and packet switching. A Central Office switch that offers ISDN service can be configured to switch either circuits or packets on either B channel independently. The D channel can only be packet-switched. To provide this functionality as well as digital transmission to the subscriber, an ISDN line card must be used. A matching card, called an NT1, is also needed at the subscriber end of the line. This card not only handles digital transmission but also formats and interprets the signaling packets sent
over the 'D' channel."
