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Inexpensive alternatives are now available to provide ISDN access over copper-based local loops.
Throughout the past decade, and especially within the past few years, significant resources have been invested in a number of countries in order to implement nationwide ISDN service. In Germany, for example, all newly-installed telephone lines are ISDN lines, and it is expected that by the year 2000 the same policy would have been adopted in many other countries. The growth in ISDN is obviously driven by business-grade digital telephone services, but also by modern applications such as Internet access, videoconferencing, switched backup for leased lines, internetworking and telecommuting (with connection-on-demand and bandwidth-on-demand features). Regardless of how attractive these services may be, however, the bottom line is that extensive, country-wide ISDN coverage is necessary in order to justify the massive investments in ISDN infrastructure.
The launch of ISDN service typically begins with only a few ISDN-capable switches. Usually, these switches are ordinary public telephone exchanges that have been upgraded to include ISDN features (or already have been built with them) which are located in larger central offices. At the same time, smaller outlying central offices continue to depend on regular PBXs. At a cost of approximately US$ 200 to US$ 300 per BRI port to upgrade to a regular digital switch, it is clear that ubiquitous ISDN coverage is prohibitively expensive for most carriers, especially in developing countries. An excellent alternative solution to this serious problem is to offer ISDN services over non-ISDN links. But how could such a solution even be implemented?
Customers can access ISDN lines once BRI is provided at city-wide, regional and national levels. A digital subscriber line (DSL) or ISDN BRI U interface is linked to existing twowire lines to connect business or residential users to the nearest central office. Problems arise, however, when the customer premises are located more than 5.5 kilometres away from the nearest ISDN access node. In suburban environments, the nearest switch is often a town or two...