Frequent Questions      

ISDN Protocols Overview



Integrated Services Digital Network (ISDN), as the name implies, provides integrated services that consist of telephony, data, and video transmission over ISDN. 

ISDN is of two types:

  • Basic Rate ISDN (BRI), and

  • Primary Rate ISDN (PRI)

Basic Rate ISDN consists of two 64kbps B-channels (B for Bearer) and one D-channel (2B+1D). B-Channels are used for transmitting user information (voice, data, or video), and D-Channel is used for transmitting control information. B-Channel offers a bandwidth of 64kbps, and D-Channel has a bandwidth of 16kbps. With 2B channels, BRI provides up to 128kbps uncompressed bandwidth. Note that the total bandwidth used by ISDN BRI is 192kbps. The remaining bandwidth [192 - (2B+D)] or 48kbps is used for framing. 

Primary Rate ISDN consists of 23 B-channels and one D-channel (23B+1D) for US or 30 B-channels and one D-channel (30B+1D) for Europe, Australia, India, and some other countries. The ISDN standard followed by Europe is also known as Euro ISDN, and standardized by  ETSI (European Telecommunications Standard Institute). The PRI D-Channel offers 64kbps bandwidth. 

There are several constituent standards that define ISDN.

I.430 Standard: It describes the Physical layer and part of the Data Link layer for BRI. 

Q.920 and Q.921 Standards: Together, they provide the Data Link protocol used over the D channel. 

Q 930, and Q.931 Standards: Documents the Network layer user-to-user and user-to-network interface. The functionalities offered include call setup and breakdown, channel allocation, and other optional services. 

G.711 Standard: It describes the standard 64 kbps audio encoding used by telcos.

ISDN Reference Points: 

ISDN standards specify several reference points that functionally separate the ISDN network. The ISDN devices need to comply with applicable reference point specifications. For example, a TE1 device such as an ISDN phone or a computer need to comply with reference point 'S' specifications. Various reference points specified in ISDN are given in the figure below:

R: This is the reference point between non-ISDN equipment and a Terminal Adapter (TA).

S: This is the reference point between user terminals and Network Termination Type2 (NT2).

T: This is the reference point between NT1 and NT2 devices.

U: This is the reference point between NT1 devices and line termination equipment of the Telco

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