ISDN and its architecture

ISDN

ISDN (Integrated Services Digital Network) is a set of ITU-T standards for end-to-end digital transmission of voice, data, video and fax over the ordinary telephone network, using a single digital access line.

Services/channels

• B channel: 64 kbps, carries user data or voice.
• D channel: 16 kbps (BRI) or 64 kbps (PRI), carries signalling and control.
• BRI (Basic Rate Interface): 2B + D = 144 kbps (for homes and small offices).
• PRI (Primary Rate Interface): 30B + D (E1, 2.048 Mbps) in Europe/Nepal, or 23B + D (T1) in North America.

Architecture

 TE1 ---------- S --+
                     |   +-----+  T  +-----+  U  +----------+
                     +---| NT2 |-----| NT1 |-----| Local    |--> ISDN
 TE2 --R-- [TA] - S -+   |(PBX)|     |     |     | exchange |    network
                         +-----+     +-----+     +----------+

Components:

• TE1 (Terminal Equipment 1): ISDN-compatible device such as a digital phone.
• TE2: non-ISDN device (an analog phone, PC with serial port).
• TA (Terminal Adapter): converts TE2 signals to ISDN format.
• NT1 (Network Termination 1): physical and electrical termination at the customer premises; connects the 4-wire user side to the 2-wire local loop.
• NT2: intelligent device (PBX, router) that performs switching and concentration at layers 2 and 3. Often combined with NT1 as NT12.
• LT/ET (Line Termination, Exchange Termination): the exchange end of the local loop.

Reference points: R (between TE2 and TA), S (TE1/TA to NT2), T (NT2 to NT1), U (NT1 to exchange line).

Example

A small office has an ISDN BRI line. A digital phone (TE1) and a PC with an ISDN adapter connect to the NT1 using the S/T bus. One B channel carries a phone call while the other carries Internet data at 64 kbps, and the D channel sets up the calls.