What is ISDN? Discuss different types of channels used in ISDN.

ISDN: ISDN stand for Integrated Service Digital Network. It is based on the concept of providing multiple channels in a single interface.

The transmission structure of any access link is constructed from the following types of channels:

(i) B Channel: 64 kbps

(ii) D channel: 16 or 64 kbps

(iii) H Channel: 384 (H0), 1536 (H 11) and 1920 ( H12) kbps.

(i) B channel: The B channel is the basic user channel. It can be used to carry digital data, PCM_ encoded digital voice or a mixture of lower- rate traffic, including digital data and digitized voice at a fraction of 64 kbps be destined for the same endpoint. Four kinds of connections can be set up over a B channel:

(i) Circuit switch: This is equivalent to switched digital service available today. The used places a call and a circuit-switched connection is established with another network user.

ii. Packet switched: The User is connected to a packet- switching node, and data are exchanged with other users via X.25

· France Mode: The user is connected to a frame relay node, and data are exchanged with other users via LAPF.

· Semi-permanent: This is a connection to another used set up by prior arrangement and not requiring a call establishment protocol. It is equivalent to a leased line.

(ii) D Channel: The D channel serves two purposes. First it carries signaling information to control circuit-switched calls on associated B channels at the user interface. In addition, the D channel may be used for packet switching or low-speed telemetry at times when no signaling information is waiting. The types of data traffic to be supported on B and D channels.

(iii) H Channel: H channels are provided for user information at higher bit rates. The user may used such a channel as a high speed trunk or subdivide the channel according to the user’s own TDM scheme.
Example of applications include fast facsimile, video, high- speed data, high-quality audio and multiple information streams at lower data rates.