What is ISDN?
ISDN Communication Channels
In the ISDN interface, a number of communication channels are carried over a digital line. These communication channels can vary by the type of information they transmit and by the transmission speed they support. The number and type of channels used may vary.
The communication channels can be of the following types:
•B-Channel - 64 Kbps
•D-Channel - 16 or 64 Kbps
•H-Channel - 384, 1536, or 1920 Kbps (not yet commercially available)
Different combinations of these channels are used to support different types of usage (e.g., basic, primary and broadband).
B-Channel
The B-channel is a user (bearer) channel that carries a subscriber's digital traffic (e.g. digitized video, voice, audio, or digital data, or perhaps a mixture). The bandwidth of this channel is 64 Kbps. The control signals used to set up the connection, however, are not sent over the B-channel. The call establishment is done on the D-channel using common-channel signaling. This concept will be discussed shortly.
Two kinds of connections can be established over a B-channel:
•Circuit-Switched
•Packet-Switched
Circuit-switched connections are ideal for voice traffic for several reasons. Voice data is “delay-sensitive,” meaning that it must not be delayed, and that it must be received in the order in which it was transmitted. There is a call set-up time that is characteristic of circuit-switched connections. This time lag is quite acceptable when measured against the length of a typical telephone call.
In a packet-switched data network or PSDN, data to be transmitted is broken up into individual units called packets that are then routed from sender to receiver. The sender/receiver can be any type of terminal, printer, computer or other device that supports an interface to the network.
D-Channel
The primary function of the D-channel is to carry common-channel signaling information to manage and control circuit-switched calls on B-channels.
The D-channel can also be used for packet-switching or low-speed telemetry when it is not involved in control signaling. Message signaling takes priority over data packet-switching.
The D-channel functions at 16 Kbps for Basic Rate Interface or 64 Kbps for Primary Rate Interface. The speed is dependent on the end-user's interface.
