Worksheet 4 - ISDN Networking - B-Channels and D-Channels

B-channels are logical "pipes" in a single ISDN line. Two B-channels can be combined to download data at very high speeds to a PC. One of the B-channels can also be freed up for another device, such as a telephone or fax machine, with the remaining B-channel used for data transfer. Each provides a 64 Kbps clear channel whose entire bandwidth is available for data, since call setup and other signaling is done through a separate D-channel.
Although B-channels are normally used for on-demand connections, taking full-advantage of the circuit-switched networks upon which they are based, they can also be configured as semi-permanent or "nailed-up" connections which are always "up", much like a leased-line. This can cause a capacity problem for carriers and Internet Service Providers that is solved by an emerging standard called Always On / Dynamic ISDN (AO/DI).

The ISDN D-channel is used mostly for administrative signaling, to instruct the carrier to set up or terminate a B-channel call, to ensure that a B-channel is available to receive a call, or to provide signaling information for such features as caller identification. The D-channel uses packet-switched connections, which are best adapted to the intermittent but latency -sensitive nature of signaling traffic, thus accounting for the vastly reduced call setup time of 1 to 2 seconds on ISDN calls (vs. 10 to 40 seconds using an analog modem). The D-channel transmits at either 16 Kbps (for BRI service) or 64 Kbps (for PRI service).

Circuit-switched B-Channel Connections

Note: This worksheet is a logical continuation of the Worksheet 4.

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