Transition Networks
S4224 Web User Guide
33558 Rev. C
Page 208 of 669
optimized (i.e., in terms of delay and resourcing) than the protection transport entity, it is better to revert
and glitch the traffic when the working transport entity is repaired.
In general, the choice of revertive / non-revertive will be the same at both ends of the protection group.
However, a mismatch of this parameter does not prevent interworking; it would be peculiar for one side to
go to WTR for clearing of switches initiated from that side, while the other goes to DNR for its switches.
Operationally, in non-revertive mode, in conditions where working traffic is being transmitted via the
protection entity, if local protection switching requests have been previously active and now become
inactive, a local DNR state is entered. Since this state now represents the highest priority local request, it
is indicated on the transmitted “Request/State” information and maintains the switch, thus preventing
reversion back to the released bridge/selector position in non revertive mode under NR conditions.
WTR Time
The Wait To Restore timing value to be used in revertive switching. The valid range is 1 - 720 seconds.
For example, set the WTR timer to 5 minutes so you are protected by primary line flapping.
In revertive mode, to prevent frequent operation of the protection switch due to an intermittent defect, a
failed working transport entity must become fault-free. After the failed working transport entity meets this
criterion, a fixed period of time elapses before a normal traffic signal uses it again. This period, called the
wait-to-restore (WTR) period, may be configured in 1 minute steps between 5 and 12 minutes; the default
value is 5 minutes.
An SF (or SD, if applicable) condition will override the WTR. To activate the WTR timer appropriately
even when both ends concurrently detect clearance of SF, when the local state transits from SF to NR
with the requested signal number 1, the previous local state, SF, should be memorized. If both the local
state and far-end state are NR with the requested signal number 1, the local state transits to WTR only
when the previous local state is SF. Otherwise, the local state transits to NR with the requested signal
number 0.
In revertive mode, when the protection is no longer requested (i.e., the failed working transport entity is no
longer in SF condition - or SD condition, if applicable, and assuming no other requesting transport
entities), a local wait-to-restore state is activated. Since this state becomes the highest in priority, it is
indicated on the APS signal (if applicable) and maintains the normal traffic signal from the previously
failed working transport entity on the protection transport entity. This state normally times out and
becomes a no-request state. The WTR timer deactivates earlier when any higher priority request pre-
empts this state.
Hold Off Time
The timing value to be used to make persistent check on Signal Fail before switching. This is in 100 ms.
and the max value is 100 (10 sec).
You can set the Hold-off timer to 0 so the switchover to backup happens immediately on connection
failure.
A hold-off timer is implemented to coordinate the timing of protection switches at multiple layers or across
cascaded protected domains. Its purpose is to allow either a server layer protection switch to have a
chance to fix the problem before switching at a client layer, or to allow an upstream protected domain to
switch before a downstream domain (e.g., to allow an upstream ring to switch before the downstream ring
in a dual node interconnect configuration so that the switch occurs in the same ring as the failure).
Each protection group has a configurable hold-off timer. When a new defect or more severe defect occurs
(e.g., a new SF), this event will not be reported immediately to protection switching if the provisioned
hold-off timer value is non-zero. Instead, the hold-off timer is started; when it expires, it is checked if a
defect still exists on the trail that started the timer. If it does, that defect is reported to protection switching.
The defect need not be the same one that started the timer. The hold-off timer applies to both the
Working and the Protection transport entities.
