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To monitor ISDN PRI messages on the D-channel, the protocol analyzer
must have software that can monitor or decode LAPD and Q.931 messages.
PRI D-channel backup
PRI D-channel backup provides an optional backup D-channel in addition to
the primary D-channel. This backup increases reliability and guarantees
continuous PRI service between any switching nodes or networks using
ISDN PRI.
Terminology
The terms primary D-channel and D1 are used interchangeably to describe
the primary D-channel for the PRI interface with a backup D-channel. The
terms secondary D-channel and D2 are used interchangeably to describe the
secondary D-channel for the PRI interface with a backup D-channel.
The term backup D-channel can refer to either the D1 or D2, depending on
the state of D1 and D2. For example, if D1 is in the active state, D2 is called
the backup D-channel. On the other hand, if D2 is in the active state, D1 is
the backup D-channel. D1 and D2 cannot be in the active state at the same
time.
Backup D-channel is a logical term, but primary D-channel and secondary
D-channel are terms assigned to circuits as datafilled in the trunk subgroup
table.
Background
A typical PRI interface supports a configuration of 23 B-channels and one
D-channel in a common DS-1 facility. This is called associated signaling of
the PRI interface.
When one D-channel controls a PRI interface with more than one DS-1
facility, it is called nonassociated signaling of the PRI interface. When the
nonassociated PRI interface is used, D-channel backup ensures the integrity
of the interface by providing a second D-channel and a warm switch of
activity (SWACT) capability.
In the DMS-250 switch product series, DTCI type XPMs are supported by
D-channel backup. The ISDN access controller type XPM is not supported
with D-channel backup.
Configuration for D-channel backup
When two or more DS-1 facilities are used with the PRI interface, a primary
D-channel is always present on one DS-1 and is labeled D1. On a different
DS-1 facility, a secondary D-channel, identified as D2, is also present. Both
primary and secondary D-channels can send signaling information in LAPD
Digital Switching Systems DMS-250 ISDN Reference Manual MWC14 (CSP14)
D-channel handler 3-5
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