The integrated services digital network (ISDN) concept is designed to allow voice and data to be sent in the same way along the same lines. Currently, most subscribers are connected to the switched telephone network by local loops and interface cards designed for analog signals. This is reasonably well-suited to voice communication, but data can be accommodated only by the use of modems.

As the telephone network gradually goes digital, it seems logical to send data directly over telephone lines without modems. If the local loop could be made digital, with the codec installed in the telephone instrument, there is no reason why the 64 kb/s data rate required for PCM voice could not also be used for data, at the user’s discretion.

The integrated services digital network concept provides a way to standardize the above idea. The standard encompasses two types of connections to the network. Large users connect at a primary-access point with a data rate of 1.544 Mb/s. This, you will recall, is the same rate as for the DS-1 signal described earlier. It includes 24 channels with a data rate of 64 kb/s each.

One of these channels is the D (data) channel and is used for commonchannel signaling, that is, for setting up and monitoring calls. The other 23 channels are called B (bearer) channels and can be used for voice or data, or combined, to handle high-speed data or digitized video signals, for example.

Individual terminals connect to the network through a basic interface at the basic access rate of 192 kb/s. Individual terminals in a large organization use the basic access rate to communicate with a private branch exchange (PBX), a small switch dedicated to that organization.

Residences and small businesses connect directly to the central office by way of a digital local loop. Two twisted pairs can be used for this, though more use of fiber optics is expected in the future.

Basic-interface users have two 64 kb/s B channels for voice or data, one 16 kb/s D channel, and 48 kb/s for network overhead. The D channel is used to set up and monitor calls and can also be employed for low-data-rate applications such as remote meter-reading.

All channels are carried on one physical line, using time-division multiplexing. Two pairs are used, one for signals in each direction.

Implementation of the ISDN has been slow, leading some telecommunications people to claim, tongue-in-cheek, that the abbreviation means “it still does nothing.” Several reasons can be advanced for this.

First, converting local loops to digital technology is expensive, and it is questionable whether the results justify the cost for most small users. Residential telephone users would not notice the difference (except that they would have to replace all their telephones or buy terminal adapters), and analog lines with low-cost modems or fax machines are quite satisfactory for occasional data users.

Newer techniques like Asymmetrical Digital Subscriber Line (ADSL), which is described in the next section, and modems using cable-television cable have higher data rates and are more attractive for residential and small-office data communication.

Very large data users often need a data rate well in excess of the primary interface rate for ISDN. They are already using other types of networks. It appears possible that the ISDN standard is becoming obsolete before it can be fully implemented.

With this in mind, work has already begun on a revised and improved version of ISDN called broadband ISDN (B-ISDN). The idea is to use much larger bandwidths and higher data rates, so that high-speed data and video can be transmitted. B-ISDN uses data rates of 100 to 600 Mb/s.

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