This knowledge base written by MiniTool defines the meaning of ISDN. ISDN stands for Integrated Services Digital Network and is a network standard for communicating. Want to learn more information about ISDN? Just read this library!

What Does ISDN Stand For?

Work on ISDN began in 1980 and was formally standardized in 1988. By the time ISDN was released, newer faster networking systems were already available. ISDN took fewer market share then and only 25 million subscribers at a time in its peak period worldwide when 1.3 billion analog lines were in use. ISDN has largely been replaced with a DSL (Digital Subscriber Line) system with much better performance.

Advantages of ISDN

Part of the full name of ISDN, “integrated services”, indicates that it can deliver as least 2 connections simultaneously, in any combination of data, video, voice, and fax over a single line. Thus, multiple devices are allowed to be connected to the line.

Therefore, the ISDN network can satisfy most users’ needs of complete communication, except for broadband Internet access and entertainment television, at a much faster transferring speed without having to buy multiple analog phone lines.

Also, ISDN integrates transmission and switching in a carrier wave and telephone switching.

Tip: ISDN connection is popular in providing a backup line for business’s inter-office and internet connectivity.

ISDN Rate Interfaces

There are two types of rate interfaces for ISDN protocol, basic and primary.

Basic Rate Interface (ISDN BRI)

Basic rate interface (BRI) is the entry-level interface to the ISDN line, a 128 kbit/s service transferred through a pair of standard telephone copper wires. The 144 kbit/s overall payload rate is divided into one 16 kbit/s signaling channel (data channel or “d” channel) and two 64 kbit/s bearer channels (“b” channels). Sometimes, that is referred to as 2b+d.

BRI-ISDN specifies below network interfaces:

  • T interface: It is a serial interface between a computing device and a terminal adapter (TA), which is the digital equivalent of a modem.
  • U interface: It is a 2-wire interface between the exchange and a network terminating unit, which is usually the demarcation point in non-North American networks.
  • S interface: It is a 4-wire bus that ISDN consumer devices are connected to. The S & T reference points are usually implemented as a single interface labeled “S/T” on a network termination 1 (NT1).
  • R interface: It is the point between a non-ISDN device and a TA that provides translation to and from such a device.
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Primary Rate Interface (ISDN PRI)

The primary rate interface is the other ISDN interface access that is carried over T-carrier (T1) with 24-time channels or slots in North American and over E-carrier (E1) with 32 channels in most other countries, each channel offers a 64 kbit/s data transferring speed.

The available channels of the E1 carrier are divided into 30 bearer (b) channels, 1 data (d) channel, and 1 timing and alarm channel. This plan is usually referred to as 30B+2D.

Broadband Integrated Services Digital Network (B-ISDN)

B-ISDN is the most advanced form of ISDN Internet designed to scale up to hundreds of Mbps. Running over fiber optic cables and using asynchronous transfer mode (ATM) as its switching technology, yet, it has never become the mainstream network.

ISDN Reference Points

Reference points defined by ISDN standard refer to certain points between the end-user ISDN equipment and the telephone company.

  • R: The point between a TA and a non-ISDN terminal equipment 2 (TE2).
  • S: The point between a network termination type 2 (NT2) device and the ISDN terminal equipment 1 (TE1) or TA.
  • T: The point between network termination 1 (NT1) and the NT2
Tip: Most NT1 devices can perform the functions of the NT2.


In an ISDN connection, what is the size throughput did a single B channel provide?
64 kbit/s
How many channels are in an ISDN PRI connection?
24 or 32 in different areas.
Which type of ISDN circuit provides 23 B channels and 1 D channel?
ISDN primary (PRI-ISDN).
Which type of network medium is used by an Integrated Services Digital Network adapter?
Copper telephone wire.

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