What Is ISDN? (Integrated Services Digital Network)

Integrated Services Digital Network (ISDN) is a network technology that supports the digital transfer of simultaneous voice and data traffic along with support for video and fax over the public switched telephone network. ISDN gained popularity around the world during the 1990s but has largely been supplanted by modern long-distance networking technologies.

The History of ISDN

As telecommunications companies gradually converted their phone infrastructures from analog to digital, the connections to individual residences and businesses— referred to as the "last mile" network — remained on old signaling standards and copper wire. ISDN was designed as a way to migrate the transition to digital signals. Businesses especially found value in ISDN due to the larger number of desk phones and fax machines their networks needed to support.

Using ISDN for Internet Access

Many people first came to know of ISDN as an alternative to traditional dial-up internet access. Although the cost of residential ISDN internet service was relatively high, some consumers were willing to pay more for a service that advertised up to 128 Kbps connection speeds versus the 56 Kbps (or slower) speed of dial-up connections.

Hooking up to ISDN internet required a digital modem instead of a traditional dial-up modem, plus a service contract with an ISDN service provider. Eventually, the much higher network speeds supported by newer broadband internet technologies like DSL attracted most customers away from ISDN.

Although a few people continue to use it in lesser populated areas where better options aren't available, most internet providers have phased out their support for ISDN.

The Technology Behind ISDN

ISDN runs over ordinary telephone lines or T1 lines (E1 lines in some countries), and it does not support wireless connections. The standard signaling methods used on ISDN networks come from the field of telecommunications, including Q.931 for connection setup and Q.921 for link access.

Two Main Forms

The two main variations of ISDN are:

• Basic Rate Interface (BRI-ISDN): The form of ISDN that consumers recognize as an internet access option, BRI works over regular copper telephone lines and supports data rates of 128 Kbps for both uploads and downloads. Two 64 Kbps data channels called bearer channels (also called DS-0 links in telecommunications) carry the data while a 16 Kbps channel handles control information. Telecom providers sometimes call this service ISDN2 referring to the two-data channel setup.
• Primary Rate Interface (PRI-ISDN): This high-speed form of ISDN supports full T1 speeds of 1.544 Mbps and up to 2.048 Mbps on E1. On T1, PRI uses 23 parallel bearer channels, each carrying 64 Kbps of traffic, compared to two such channels for BRI. In Europe and Asia, providers often call this service ​ISDN30 as the E1 lines used in those countries support 30 bearer channels.

A Third Form

The third form of ISDN called Broadband (B-ISDN) was also defined. This most advanced form of ISDN was designed to scale up to hundreds of Mbps, run over ​fiber optic cables, and use ATM as its switching technology. Broadband ISDN never achieved mainstream usage.