Isdn Channels

ISDN consists of three types of communications channels. They are:

1. Bearer channel (B channel)

2. Delta channel (D channel), and

3. Hybrid channels (H channel).

These three ISDN channels are described below.

B channel. B channels are logical digital "pipes" which exist on a single ISDN line. B channel carry data and services at 64 kbps. It carries data in full duplex mode. Each B channels provide a 64 kbps clear channel, clear meaning that the entire bandwidth is available for data, B channels typically form circuit switched connections. B channel connection is an end-to-end physical circuit that is temporarily dedicated to transferring data between two devices.

The circuit switched nature of B channel connections; combined with their reliability and relatively high bandwith makes ISDN suitable for a range of applications including voice, video, fax and data. B channels are normally used for on-demand connection. As B channel operation based on circuit switching, it can be configured as semi permanent or "nailed up'' connections.

D channel. D channel can be either 16 or 64 kbps, depending on the needs of the user. The primary function of the D channel is to carry control signalling and administrative information for B channels to set up and tear down the calls. The D channel uses packet switched connection. The packet switched connection are best adapted to the intermittent but latecy sensitive nature of signalling traffic, accounting for the highly reduced call setup time of 1 to 2 seconds on ISDN calls.

Unlike the B-channel, which can function as a simple 'pipe', the D channel is associated with higher level protocols at layers 2 and 3 of OSI model which form the packet switched connections. The D channel provides the signalling information that is required for caller identification. It also includes low-rate data transfer and applications such as telemetry and alarm transmission.

H channels. H channels are suitable for high data rate applications such as video, teleconferencing and so on. Table 12.2 gives ISDN channel and its specifications.

Table 12.2. ISDN channels specifications

Channel

Bit rate (kbps)

Interface

Purpose

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