Nortel Networks OPTera Metro 3500 - Manual (Page 291)

Hardware feature descriptions 3-63
Planning and Ordering Guide—Part 1 of 2 NTRN10AN Rel 12.1 Standard Iss 1 Apr 2004
Protection of path failures on a single OC-192 optical interface completes in
60 ms, but protection of simultaneous path failures on multiple optical
interfaces completes in less than 200 ms.
BLSR protection
BLSR protection switching is revertive. If a fiber cut occurs in either the
receive or transmit fibers of the active channel, or the transmitter or receiver
OC-192 optical interface circuit pack fails at either end of the fiber span of the
active channel, traffic is switched from the working channel to the protection
channel (usually from the short path to the long path on the other side of the
ring).
The Wait-to-Restore (WTR) bridge request is issued on both the long and short
paths when working channels meet the restoral threshold after a signal degrade
or signal fail condition. This request is used to maintain the current state during
the WTR period unless one or a combination of the following conditions
occurs:
• a bridge request of higher priority than WTR is received
• another failure is detected
• an externally initiated command becomes active
The WTR time is between 1 to 12 minutes (default is 5 minutes). The WTR
period is provisionable for each optical interface pair.
Note: You can provision an infinite WTR period, so that BLSRs
autonomously switch non-revertively.
Switching can also take place under user control. In BLSR user-initiated
switches, the user may initiate a lockout on either the working or protection
channels on a span. Both of these effectively ’lock’ traffic onto the working
channel. The lockout of the protection channel of the span also prevents any
protection switching from occurring anywhere in the ring.
Forced and manual switches on the working channels switch traffic to the
protection channel. A forced switch has a higher priority than a manual switch.
For the complete BLSR protection switching hierarchy, see
Table 2-27 on page
2-119 . Both forced and manual switches can be released.
In ring switches, the protection channels are shared among each span of the
ring. If a scenario arises where multiple points in a BLSR fail or nodes become
isolated, there is the potential for misconnected traffic. Services originally on
separate spans but sharing the same time slot may compete for the same
protection time slot. Squelching is a mechanism to prevent this.
For more information about squelching, see BLSR networks (2-fiber) on page
2-10 .