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PR-ISDN is a highly standardized telephony service that can be delivered by a variety of technical means. The 2 Mb/s means of delivery is often referred to as an 'E1' line (or microwave link), or in North America as a 'T1' for the 1.544 Mb/s approach. These 2 or 1.544 Mb/s links may be used for purposes other than its main application of providing 30 exchange lines to an ISDN PABX. They may be split into 64 kb/s channels and used for telephony using signaling protocols other than ISDN. Alternatively a 2 or 1.544 Mb/s link they may be used to connect routers for carrying Internet and other packet-switched traffic. The latter two examples are not using the link with ISDN protocols, but the term PR-ISDN may nonetheless erroneously be applied to such installations.
In some countries it is possible for customers to lease copper pairs between their sites - and so to run HDSL links over them with their own HDSL modems. This was a cost-effective means of linking offices for WAN and telephony and in particular for linking Internet Service Providers. Some carriers, for instance Telstra in Australia, have responded by withdrawing the promise of direct copper connectivity for these leased lines and replacing it with a guarantee only of voice frequency performance. This precludes its use for HDSL and is justified in terms of the carrier finding it more cost-effective to implement such leased 'lines' via digital switching and transmission technology designed for voice telephony rather than maintaining an increasingly burdensome network of copper wires. This argument has some validity, especially when the potential cross-talk and line-quality issues of HDSL are considered. However, in Telstra's case, the carrier is deploying ADSL - which typically involves higher frequencies and energies than HDSL - and customers and regulators, have resisted the change.
HDSL is the only 'xDSL' that was widely deployed in the late 1990s, with 300,000 services in the USA in 1998, primarily for T1 1.5 Mb/s links for data and PABX connections.
ISDN Historical Overview
ISDN was conceived as a global any-to-any network. The initial phase was for communications taking place in 64 kb/s permanent or telephone call 'pipes', but the longer-term vision of Broadband ISDN (B-ISDN, not to be confused with BR-ISDN) is for much greater and more flexible data rates, with global packet switching (or 'cell' switching, meaning short fixed length packets) using ATM (Asynchronous Transfer Mode) rather than the slower and less flexible 64 kb/s circuit-switched telephone exchanges.
ISDN was designed by the telecommunications carriers, with a view of all users being linked by permanent or temporary bi-directional pipes. This makes more sense than it would for tone or pulse dialed telephone calls, since with ISDN signaling a call can be set up - at least within the one national telephone network - in half a second or so. The exchange software to achieve this is impressive, since it may involve half a dozen exchanges collaborating in that fraction of a second to reserve and connect 64 kb/s channels in multiple inter-exchange fiber links across the country.
Theoretically ISDN call and service rental costs could be only marginally higher than analogue services, but since it is a relatively new technology which required substantial investment by monopoly carriers, these low costs have not always eventuated. Regulatory pressure may force these prices down, for instance by declaring such services to be made available to other service providers at a price based on cost and a given profit margin, rather than the higher price the carrier prefers to charge.
In the late 1990s the future of telecommunications was generally regarded - or perhaps hoped - to be converging towards a single global approach to switching and addressing with a variety of data link technologies for the long distance links and the 'last mile' links of the Customer Access Network. From the mid 1970s, the telecommunications carriers saw ISDN as this future global network. However this was a view based on the centrality of the circuit-switched communications, as exemplified by telephony. It did not adequately foresee the need for packet-switched network communications.
The ability of an Internet connected computer to send a packet to any other such computer in the world, and receive packets from any of them, in less than half a second for the round trip, is not feasible or cost-effective by making an ISDN call to that remote computer.
Consequently ISDN has become a model for making a seamless digital telephone network - which is now recognized as just a part of the larger telecommunications picture.
ISDN, especially PR-ISDN, is important for large organizations for connecting their PABXs. It is also vital for any ISP or other organization which wants to communicate with 56 kb/s modems.
This 56 kb/s capability (typically in the downstream direction alone, with 33.6 kb/s upstream) is hardly any different from BR-ISDN's 64 kb/s. Since such analogue calls typically incur no time-charges (except in many European countries), these modems diminish the appeal of BR-ISDN for many customers.
ISDN in any form is inadequate to deal with the 6 to 8 Megabits per second required for fast-action sports via MPEG-2 compressed video, or to cope with bandwidth-intensive Internet applications such as purchasing a CD's worth of music (500 Megabytes or more - 10 Megabytes per minute) and video-conferencing.
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