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Islands of ISDN were the result of these various implementations. A given carrier, using the hardware and software of a given manufacturer, could not easily achieve full connectivity with another carrier deploying another version of ISDN. Weary of delays in the standards process, some carriers (e.g., Southwestern Bell) developed and implemented proprietary versions of ISDN, further contributing to the problem. During the past several years, this problem has been mitigated through cooperation of the manufacturers and carriers, with active involvement of Bellcore [8-23] and [8-24].

Availability of ISDN was slow to develop, as the carriers were reluctant to invest in the technology unless they were convinced that a market existed for the services and/or that the technology offered internal cost savings. ISDN is not inexpensive to deploy, a fact which unfortunately has been reflected in high end-user circuit and equipment costs. Availability recently has increased to a significant extent, with the RBOC’s and GTE having made considerable commitments to its deployment. The independent telephone companies are increasingly making ISDN a priority, although they understandably lag far behind the RBOCs due to cost-of-service factors [8-25].

Regulators generally have required that the LECs pass on the cost of ISDN infrastructure to ISDN users, rather than allowing them to average those costs across the entire rate base. In other words, they have viewed ISDN as an optional service that must pay its own way, or that the carriers should absorb any associated losses. As the carriers have been unwilling to do so, ISDN rates have remained high.

Rates for ISDN access were not tariffed at attractive levels compared with the cost of basic services. Again, the regulators were, and still are, largely responsible. The carriers also bear responsibility as well, as they have not been willing to absorb initial losses in order to stimulate the growth of the service offering. Ameritech, for instance, charges $135 for installation and $34.64 per month for a low-speed ISDN BRI circuit; an additional $20 per month applies if the premise is more than three miles from an ISDN central office. Southwestern Bell is the prize winner, at $578 for installation and $57.70 per month [8-26]. In New York, NYNEX charges $14 per month in addition to the normal line charge, plus installation or upgrade charges which vary from about $50 to $312 [8-27]. Rates for PRI vary considerably as well. In some cases based on a per-channel charge and in other cases on a flat rate per PRI [8-28]. In some states, additional per-minute surcharges apply to ISDN; starting with call setup, rather than call connection. The charges typically are higher for a data call than for a voice call.

Equipment costs were high, as the manufacturers were constantly investing in R&D to maintain ISDN compliance with developing standards. Additionally, the limited demand for ISDN caused the manufacturing runs to be small—lower costs are achieved through increased volume. As an example, Terminal Adapters, which provide the interface to an ISDN circuit for non-ISDN equipment, have come down in price from about $1,000 in 1993 to an average of less than $500 today. That is an improvement, but still a substantial investment for a small business or residential user [8-29].

Marketing by the LECs was not effective. Not only were costs maintained at unattractive levels, but advertising and promotion were limited, and availability clearly was not high. Further, meaningful and cost-effective applications were not identified and stressed. With typical lack of foresight, the LECs placed heavy emphasis on the low-speed BRI version, which is suitable only for residence, small business, and SOHO (Small Office/Home Office) application. High-speed PRI was not emphasized heavily as a replacement of T1 trunking. ISDN Centrex was touted heavily, but with limited success. Centrex ISDN marketing was heavily slanted toward CO-based Local Area Networking, which was a dismal failure.

While ISDN has frustrated the industry, in general, there currently are over 200,000 ISDN access lines worldwide. Some industry pundits forecast that more than 750 million will be installed by the year 2000—the author does not share that level of enthusiasm. ISDN is much more mature in Europe and certain parts of the Pacific Rim than it is in the United States. In those regions, deployment was encouraged by the regulators and even subsidized by the governments. Additionally, marketing was much more effective, focusing on PRI, rather than BRI [8-24].

Standard Interfaces and Channel Types

The current version of ISDN is Narrowband ISDN (N-ISDN). (Broadband ISDN (B-ISDN), which is still on the drawing boards, is discussed in Chater 11.) ISDN currently is available in essentially two interface varieties BRI (2B+D) and PRI (23B+D in North America, and 30B+D in Europe and many other countries) (see Figure 8.9). In each case, the ITU specifies the electrical characteristics, signaling, coding and frame formatting. There is one variation on the theme; H-channel, designed for high-bandwidth applications, bonds multiple B channels.


Figure 8.9  ISDN BRI and PRI interfaces.

Basic Rate Interface (BRI)

Basic Rate Interface (BRI) also is known as Basic Rate Access (BRA) and 2B+D. BRI provides 2 B channels (Bearer, or information-bearing channels), each operating at the clear-channel rate of 64 Kbps by virtue of SS7 nonintrusive signaling. Each B channel can carry digital data, digitized voice (PCM-encoded at 64 Kbps or a lower rate), or a mixture of low-speed (subrate) data as long as it all is intended for the same destination. BRI also provides a D (Data) channel at 16 Kbps, which is used for control, messaging, and network management. The D channel also generally is made available for packet data transmission and low-speed telemetry when not in use for signaling purposes. Cost-effective applications include credit card authorization, which involves very small bursts of data [8-30]. BRI is primarily used for residential, small business, Centrex, and telecommuting applications that are not particularly bandwidth-intensive. The B channels can be aggregated or bonded, to provide up to 128 Kbps to a given conversation, such as a videoconference; additionally, multiple BRIs can be bonded for even greater capacity. Whether bonded or not, ISDN BRI provides multiple channels over a single physical loop, which is a great advantage.


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