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ISDN basics Tutorial

This page on ISDN tutorial covers ISDN types, ISDN architecture and more.

Telephone and mobile growth has increased through out the world. To provide better quality and to combine digital telephony with data transport services ISDN was introduced in 1979 along with ITU-T. ISDN is the set of protocols which helps digitize existing telephone network so that video,voice and text can be transmitted over these telephone lines. It is referred as Integrated Digital Services Network.

ISDN services are categorized into bearer services,teleservices and supplementary services.

•  Bearer services in which network does not manipulate user information. Voice,data and video utilize this service. It operates on OSI layer 1 to layer 3. These services are provided with the help of circuit switched,packet switched , cell switched and frame switched networks.

•  Teleservices in which network change the information contents. It operates on layer 4 to layer 7 of the OSI model. Telex,telephone and teleconferencing utilize this service.

•  Supplementary services utilize features of both bearer and teleservices. The applications are call waiting,message handling and reverse charging.

ISDN architecture

ISDN Architecture

There are three main types of channels used in the ISDN network viz. bearer(B), data(D) and hybrid(H) channels. Different data rates can be obtained by the user with combinations of these channels. One bearer channel supports 64 kbps, one data channel supports between 16 to 64 kbps. One hybrid channel supports 384 or 1536 or 1920 kbps data rates. There are two main types of digital subscriber loops supported in ISDN to fulfill user requirements.

Basic Rate Interface(BRI): Supports two B channels and 1 D channel. Hence supports about 192 kbps with 64 kbps B channel, 16 kbps D channel and 48 overhead.

Primary Rate Interface(BRI):Supports 23 B channels and 1 D channel. Hence supports about 1.544 Mbps with 64 kbps B channel, 64 kbps D channel and 8 overhead.
Refer ISDN BRI vs ISDN PRI➤ interface types.

As shown in the figure,TE1,TE2,TA,NT1 and NT2 are components used in a typical ISDN network with functions as described below.

•  Terminal Equipment-1 or TE1 is used to interface ISDN terminal with the network.

•   Terminal Equipment-2 or TE2 is used to interface Non-ISDN terminal with the network such as Plain Old Telephony.

•   Terminal Adapter or TA Allows non ISDN devices to be interfaced with ISDN network.

•   Network Termination-1 or NT1 is physical layer device which separates user premises from phone company.

•   Network Termination-2 or NT2 functions as per OSI layers 2 to 3. PBX and LAN are considered as NT-2 devices.

•   Reference points are used to identify interfaces between two ISDN elements.

15 digits are used in ISDN addressing as defined in E.164.


Signalling Types
EPABX system basics
Circuit Switching vs Packet Switching


ISDN 2B1Q signal format

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