Automatic Protection
Switching (APS) is a protection mechanism for OTN networks that enables OTN
connections to switch to another circuit when a circuit failure occurs. A
protect circuit serves as the backup circuit for the working circuit. When the
working circuit fails, the protect circuit quickly assumes its traffic load.
In a linear protection
architecture, protection switching occurs at the two distinct endpoints of a
protected circuit. For a given direction of transmission, the head-end or the
tail-end of the protected signal performs a bridge function, and places a copy
of a normal traffic signal onto a protection entity when required. The tail-end
or the head-end performs a selector function, where it is capable of selecting
a normal traffic signal either from its usual working entity, or from a
protection entity.
The widely used
protection mechanism is the 1+1 architecture. Here, a single normal traffic
signal is protected by a single protection entity. The bridge at the head-end
is permanent. Switching occurs entirely at the tail-end.
In the case of
bidirectional transmission, it is possible to choose either unidirectional or
bidirectional switching. With unidirectional switching, the selectors at each
end are fully independent. With bidirectional switching, an attempt is made to
coordinate the two ends so that both have the same bridge and selector
settings, even for a unidirectional failure. Bidirectional switching always
requires an APS and/or protection communication channel (PCC) to coordinate the
two endpoints. Unidirectional switching can protect two unidirectional failures
in opposite directions on different entities.
Hierarchy in APS
There are different
levels of priority that can be set for the path to switch from a working
circuit to the protect circuit (or vice-versa). The hierarchy levels are
(listed priority-wise, with lockout having the highest priority):
-
Lockout - the path
continues to be in the working circuit, even if a failure is detected in the
working circuit, switch to the protect circuit is not permitted. If the path is
currently using the protect circuit, then it automatically switches back to the
working circuit.
-
Forced switch -
forces a switch from the protect circuit to the working circuit (even when the
protect circuit is down, this scenario can happen during a maintenance
activity).
-
Manual switch -
manually switches from the working circuit to the protect circuit or from the
protect circuit to the working circuit.
-
Exercise - enables
the APS protocol.
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