Nortel DMS-100 Series Maintenance Manual page 105

Note: To perform full disable protection switching, both the DMS switch
and the RCS ends must be disabled for protection switching.
Scenarios of enabling and disabling protection switching
Scenario 1
All primary DS-1 links and the protection link are INSV. Faults are not
present on any links.
The B-link is disabled for protection switching.
The SMS and RCS detect a fault on the B-link.
A protection switch does not occur. The B-link is system busied.
Scenario 2
Protection switching is disabled for the A-link. A fault is present on the
A-link. The A-shelf is not protection switched. The A-link is system
busied. Faults are not present on any other DS-1 link. The protection link is
INSV.
A fault is detected on the B-link.
A protection switch does not occur. The DDL messaging is lost because
the A-link is defective and a protection switch to the B-, C- or D-shelf
cannot occur. The B-link is system busied.
The A-link is enabled for protection switching.
The protection link switches to the A-shelf.
The A-link returns to service.
Scenario 3
The protection link is switched to the B-shelf because of a defective B-link.
Other DS-1 links are not defective.
Protection switching is disabled for the B-shelf.
The protection link is released from the B-shelf. The B-link is system
busied.
Manually busy, switched primary links
If the B-, C- or D-shelf is automatically protection switched, the primary
link is not manually protection switched. The primary link that corresponds
is manually busied. The protection link is released. Refer to scenario 1.
The B-, C- or D-shelf is manually protection switched and the corresponding
primary link is manually busied. When this condition occurs, the protection
link is not released. Refer to scenario 2.
DMS-100 Family SMS Maintenance Manual XPM10 and up
Maintenance overview 1-89