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Configuring Dial Services
4-2
117353-B Rev. 00
In addition to offering integrated services, ISDN provides a number of standard,
universal interfaces to access ISDN services. With a defined set of interfaces, you
can purchase ISDN equipment and services from different providers and all the
parts of your network will have the same interfaces to connect to one another as
well as to the ISDN network.
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 4-2)
.
Figure 4-2. 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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