|
Modern telephone networks - including the exchanges, their inter-exchange links and the Remote Access Units which drive the twisted pairs of customers distant from the exchange - all work with data-streams of 64 kb/s (thousand bits per second). Sound (actually the voltage on the analogue line) is sampled 8,000 times a second and measured (analogue to digital conversion) with 8 bit (1 in 256) accuracy. A telephone call within one exchange or between two exchanges is carried by two 64 kb/s data streams, one in each direction. ISDN requires that these 64 kb/s datastreams be brought to the customer, rather than be converted to and from analogue audio signals at the exchange or Remote Access Unit.
Basic Rate ISDN
One approach to achieving this is to use a relatively ordinary twisted pair telephone line in a completely different way. This is called Basic Rate ISDN (BR-ISDN). It carries three bi-directional datastreams to the customer's site: two 64 kb/s 'B' or Bearer channels and one 16 kb/s 'D' signaling channel. Each B channel can support a telephone call, including a direct 64 kb/s link, via the telephone network, to a B channel of an ISDN service of another customer - including customers in other countries. (For historical reasons, the ISDN services of some North American carriers is limited to 56 kb/s for each B channel, due to design problems in their inter-exchange links.)
The 16 kb/s D signaling channel handles all call set-up and termination communications. For instance commands from the exchange to the customer equipment to ring the bell, or from that equipment to the exchange to set up or terminate a call. The D channel can also be used for other purposes, such as packet switched data.
The two B channels are independent. They could be used for two phone calls, including calls to analogue or mobile phones. One could be used for linking a computer at 64 kb/s to an Internet Service Provider (ISP) or to another computer, via one phone call, while the other might be carrying a voice, fax or modem call. Special customer equipment is required for an ISDN service, or adapters for analogue telephones and modems. For instance a computer interface card or router could, for instance, set up two calls with the two B channels to provide 128 kb/s access to an ISP or to a router at a corporate network. One of the main applications of BR-ISDN is to provide teleworkers with access to their office LAN at speeds significantly faster than the 33.6 kb/s limit of modems.
The pricing of ISDN services and calls varies widely - but it is generally more expensive than an analogue phone service, and even local calls incur time charges.
The direct circuit switched nature of an ISDN connection is ideal for video-conferencing - in contrast to the packet-switched approach of the Internet or a LAN, in which there is no guarantee of packet delivery. 128 kb/s enables reasonable quality video conferencing - but the cost of the two long-distance or international calls is significant. Larger, high quality, video-conferencing systems which are suitable for linking two or more meetings, typically use 348 kb/s - and so involve three BR-ISDN lines and six ISDN phone calls.
BR-ISDN conveys the data to and from a small, wall-mounted interface box called an NT-1 at the customer's site using frequencies up to about 80 kHz. This is twenty times higher than the maximum frequency that the twisted pair cables were intended to carry. BR-ISDN can only work reliably on cables in good condition up to five or so kilometers long - or more with the thicker cables in rural areas. Certain techniques that were appropriate for analogue telephony must be eliminated - such as loading coils and branching a pair into several streets.
Note that some North American carriers provide B channels and ISDN data calls at only 56 kb/s because their inter-exchange network can guarantee the integrity of only seven of the eight bits in each byte.
Primary Rate ISDN and HDSL
A second approach to bringing multiple 64 kb/s B channels to customer sites is Primary Rate ISDN (PR-ISDN). This uses the same D channel protocols as BR-ISDN for controlling calls - protocols which provide a much faster and more sophisticated interface between customer equipment and the telephone network than is possible with an analogue service. ISDN technical standards are complex and have taken many years to stabilize. Most countries outside North America have adopted the ETSI standards, and so a nearly global market has developed for ETSI-compatible customer equipment.
PR-ISDN is a 2,048 kb/s (2 Mb/s) service which provides thirty 64 kb/s B channels and one 64 kb/s D channel for signaling. (North American PR-ISDN is a 1,544 kb/s service comprising 23 B channels and one D channel.)
When PR-ISDN was first deployed in the late 1980s, it was delivered with two carefully chosen copper twisted pairs, over a distance limited to one kilometer unless expensive repeaters were installed. In the mid 1990s, several other methods became more attractive. Firstly, one or more PR-ISDN services could be delivered on a pair of optical fibers. Secondly, it could be delivered with HDSL (High Rate Digital Subscriber Line) - which uses two or three twisted pairs over four kilometers or so. Finally, the 2 Mb/s data rates of PR-ISDN can be delivered by relatively low cost, privately owned and in some cases unlicensed point-to-point spread-spectrum microwave links for up to ten kilometers or so.
Back to Main Menu Previous Page Next Page
|