Hayes Microcomputer Products PC Card - Manual (Page 6)

DIVA T/A PC Card Reference Guide 6
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 which 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.