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Connectivity to ISDN is
provided by physical PRI and BRI interfaces. A single PRI or BRI
interface provides a multiplexed bundle of B and D channels
.
The ISDN bearer service channel—B channel—carries voice or data, usually in frame format. The D channel is the ISDN out-of-band signaling channel, which carries the user-network messages, such as call setup and teardown.
ISDN-BRI specifies the following:
-
There are two
64-kilobit-per-second (Kbps) bearer channels and one 16-Kbps data
channel service. BRI connects to an NT1 (Network Termination 1) for four-wire connection.
-
Framing and
synchronization are at 48 Kbps.
-
The total speed is two B
channels at 64 Kbps each (128), one D channel at 16 Kbps, plus
framing and synchronization at 48 Kbps. The total comes to 192 Kbps
(128 + 16 + 48 = 192).
-
It is intended to be used at small concentration points.
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It is referred to as a digital signal level zero (DS0) interface.
ISDN-PRI specifies the following:
-
There are 23 or 30 B
channels, at 64 Kbps each.
-
There is one D channel,
at 64 Kbps.
-
Framing and
synchronization use 8 Kbps (North America T1) or 64 Kbps (European
E1).
-
Total speed is 1.544
Mbps (T1) or 2.048 Mbps (E1)
Note: In an E1 PRI, there are actually 32 channels: 30 B channels, one D channel, and one synchronization channel. Also, in Europe, the D channel is carried in timeslot 16; in the United States, it is in timeslot 24. Table
displays the relationship between the digital signal level, speed, T designation, and number of channels.
In some cases, a DS0 can
carry only 56 Kbps, usually because of legacy telco equipment or a
signaling method called robbed-bit signaling (RBS). RBS and other
signaling methods are discussed later in this chapter in the "ISDN
Primary Rate Interface" section.
In Europe, the equivalent of
a T1 facility is an E1. E1 has 32 64-Kbps channels for a total of 2.048
Mbps.
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