Worksheet 5 - 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.

If you want to practice your listening and pronunciation, listen to the text using the audio player below. Notice the British accent of the speaker!