Integrated Services Digital Network (ISDN)

The need for data communications services grew throughout the 1970s. These services were provided (mostly to the companies rather than individuals) by the X.25-based packet switched data networks (PSDNs). By the early 1980s it was clear to the industry that there was a market and technological feasibility for integrating data communications and voice in a single digital pipe and opening such pipes for businesses (as the means of PBX access) and households. The envisioned applications included video telephony, online directories, synchronization of a customer’s call with bringing the customer’s data to the computer screen of the answering agent, telemetrics (that is, monitoring devices—such as plant controls or smoke alarms—and automatic reporting of associated events via telephone calls), and a number of purely voice services. In addition, since the access was supposed to be digital, the voice channels could be used for data connections that would provide a much higher rate than had ever been possible with the analog line and modems.

The ISDN telephone (often called the ISDN terminal) is effectively a computer that runs a specialized application. The ISDN telephone always has a display; in some cases it even looks like a computer terminal, with a screen and keyboard in addition to the receiver and speaker. Several such terminals could be connected to the network terminator (NT) device, which can be placed in the home or office and which has a direct connection to the ISDN switch. Non-ISDN terminals (telephones) can also be connected to the ISDN via a terminal adapter. As far as the enterprise goes, a digital PBX connects to the NT1, and all other enterprise devices (including ISDN and non-ISDN terminals and enterprise data network gateways) terminate in the PBX.

These arrangements are depicted on the left side of Figure 1. The right side of the figure shows the partial structure of the PSTN, which does not seem different at this level from the pre-ISDN PSTN structure. This similarity is no surprise, since the PSTN had already gone digital prior to the introduction of the ISDN. In addition, bringing the ISDN to either the residential or enterprise market did not require much rewiring because the original twisted pair of copper wires could be used in about 70 percent of subscriber lines (Werbach, 1997). What has changed is that codecs moved at the ultimate point of the end-to-end architecture—to the ISDN terminals—and the local offices did need to change somewhat to support the ISDN access signaling standardized by ITU-T. Again, common channel signaling predated the ISDN, and its SS No. 7 version could easily perform all the functions needed for the intra-ISDN network signaling.

Figure 1: The ISDN architecture.

As for the digital pipe between the network and the user, it consists of channels of different capacities. Some of these channels are defined for carrying voice or data; others (actually, there is only one in this category) are used for out-of-band signaling. (There is no in-band signaling even between the user and the network with the ISDN.) The following channels have been standardized for user access:

• A. 4-kHz analog telephone channel.

• B. 64-kbps digital channel (for voice or data).

• C. 8- or 16-kbps digital channel (for data, to be used in combination with channel A).

• D. 16- or 64-kbps digital channel (for out-of-band signaling).

• H. 384-, 1536-, or 1920-kbps digital channel (which could be used for anything, except that it is not part of any standard combination of channels).

The major regional agreements support two combinations:

• Basic rate interface. Includes two B channels and one D channel of 16 kbps. (This combination is usually expressed as 2B+D.)

• Primary rate interface. Includes 23 B channels and 1 D channel of 64 kbps. (This combination is accordingly expressed as 23B+D, and it actually represents the primary rate in the United States and Japan. In Europe, it is 30B+D.)

The ISDN has been deployed throughout mostly for enterprise use. The residential market has never really picked up, although there has been a turnaround because of the demand for fast Internet access (it is possible to use the 2B+D combination as a single 144-kbps digital pipe) and because ISDN connections are becoming less expensive.

Even before the ISDN standardization was finished, the ISDN was renamed narrowband ISDN (N-ISDN), and work began on broadband ISDN (B-ISDN). B-ISDN will offer an end-to-end data rate of 155 Mbps, and it is based on the asynchronous transfer mode (ATM) technology. B-ISDN is to support services like video on demand—predicted to be a killer application; however, full deployment of B-ISDN means complete rewiring of houses and considerable change in the PSTN infrastructure.

Although the ISDN has recently enjoyed considerable growth owing to Internet access demand, its introduction has been slow. The United States until recently trailed Europe and Japan as far as deployment of the ISDN is concerned, particularly for consumers. This lag can in part be explained by the ever complex system of telephone tariffs, which seemed to benefit the development of the business use in the United States. Another explanation often brought up by industry analysts is leapfrogging: by the time Europe and Japan developed the infrastructure for total residential telephone service provision, the ISDN technology was available, while in the United States almost every household already had at least one telephone line long before the ISDN concept (not to mention ISDN equipment) existed