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Configuring Dial Services
3-2 114062 Rev. B
ISDN is primarily a dial service that uses switched connections as circuits. This
means that you can use channels and services on an as-needed basis instead of
having a connection up all the time, regardless of need. This choice ensures a
cost-effective use of phone lines as well as bandwidth efficiency.
ISDN Standards
The ITU-T (formerly CCITT) has defined a set of standards to ensure universal
implementation of ISDN technology. The following sections describe some of
these standards.
Basic Rate Interface
BRI transmits data at a rate of 64 Kb/s over each of two bearer channels, also
called B channels, which are dedicated to data transmission. Along with the two B
channels, BRI uses one data channel called the D channel. The D channel handles
all signaling information such as call setup requests. It transmits this information
at a rate of 16 Kb/s.
The networking industry also refers to BRI service as 2B + D to denote the
combination of the two B channels and one D channel. These three channels are
combined into one physical, twisted-pair line that terminates at your premises and
provides a total of 144 Kb/s bandwidth (Figure
3-2).
Figure 3-2. The BRI Interface
2B + D
B channel
B channel
D channel
64 Kb/s
64 Kb/s
16 Kb/s
144 Kb/s
DS0008A
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