Nortel DMS-100 Series Maintenance Manual page 91

Causes of automatic protection switching
Either an SMS or an RCS can initiate a protection switch after a primary link
failure is detected. The module that detects the failure initiates the
protection switch.
The SMS initiates a switch for the following faults:
bipolar violations (BpV) exceeding the threshold datafilled in table
CARRMTC
loss of signal that lasts for a time greater than the threshold datafilled in
table CARRMTC
removal of a DS-1 interface card
The RCS initiates a protection switch for the following two failures:
bipolar violations (BpV) that exceed the threshold setting on the line
interface unit (LIU) card
loss of DS-1 framing
The SMS initiates a protection switch back to the normal line under the
following conditions:
bipolar violations on the line fall below the threshold in table
CARRMTC
frame synchronization resumes without slips for a threshold time, as
datafilled in table CARRMTC when frame loss caused the protection
switch
the DS-1 card interfacing to the line is replaced when removal of the
DS-1 card caused the protection switch
The DS-1 A-link carries system messaging between the SMS and the RCS.
The DS-1 A-link has priority over the B-link, C-link and D-link. The
system always executes a switch request for link A. If the A-link fails and
cannot be restored, the RCS remains in-service if at least one other link
operates correctly.
Automatic protection switch requests for links other than the A-link are
treated on a first-come, first-serve basis. Two or more switch requests for
non-A-links can generate at the same time. A group of lines can fail
together. The protection link goes to the shelf that, in alphabetical order, is
first. The B shelf supersedes C, which in turn supersedes D.
If a fault is present and another fault occurs, protection switching works by
switching the lowest-lettered link. This condition applies to link B, link C
and link D. Protection switching only works for this condition if the end
DMS-100 Family SMS Maintenance Manual XPM10 and up
Maintenance overview 1-75