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ISDN Basics: Integrated Services Digital Network Tutorial

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The growth of telephone and mobile networks has exploded worldwide. To improve quality and combine digital telephony with data transport services, ISDN was introduced in 1979 by the ITU-T. ISDN, or Integrated Services Digital Network, is a set of protocols that digitizes the existing telephone network, allowing the transmission of video, voice, and text over standard phone lines.

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

  • Bearer services: The network doesn’t manipulate the user’s information. Voice, data, and video utilize this service, which operates on OSI layers 1-3. These services are delivered using circuit-switched, packet-switched, cell-switched, and frame-switched networks.

  • Teleservices: The network changes the information contents. It operates on layers 4-7 of the OSI model. Telex, telephone, and teleconferencing make use of this service.

  • Supplementary services: These services utilize features of both bearer and teleservices, and include features such as call waiting, message handling, and reverse charging.

ISDN architecture ISDN architecture

ISDN Architecture

There are three main types of channels used in the ISDN network: bearer (B), data (D), and hybrid (H) channels. Users can achieve different data rates using combinations of these channels.

  • One bearer channel supports 64 kbps.
  • One data channel supports between 16 to 64 kbps.
  • One hybrid channel supports 384, 1536, or 1920 kbps data rates.

ISDN supports two main types of digital subscriber loops to meet user needs:

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

  • Primary Rate Interface (PRI): Supports 23 B channels and one D channel, delivering about 1.544 Mbps with 64 kbps B channels, 64 kbps D channel, and 8 kbps overhead.

As shown in the figure above, TE1, TE2, TA, NT1, and NT2 are the components in a typical ISDN network, and their functions are described below:

  • Terminal Equipment-1 (TE1): Used to interface ISDN terminals with the network.

  • Terminal Equipment-2 (TE2): Used to interface non-ISDN terminals, such as Plain Old Telephony (POTS) devices.

  • Terminal Adapter (TA): Allows non-ISDN devices to interface with the ISDN network.

  • Network Termination-1 (NT1): A physical layer device that separates user premises from the phone company.

  • Network Termination-2 (NT2): Functions as per OSI layers 2-3. PBX and LAN are considered NT-2 devices.

  • Reference points: Used to identify interfaces between two ISDN elements.

ISDN addressing uses 15 digits as defined in E.164.