What is the Wait-to-Restore Period within a Protection-Switching System?
The purpose of this post is to describe and define the Wait-to-Restore period within a Revertive Protection-Switching system.
Introduction
All Protection Groups will perform Protection-Switching, to route the Normal Traffic Signal around a defective Working Transport entity anytime it is declaring a service-affecting or signal degrade (dDEG) defect with that Working Transport entity.
In other words, the Protection Group will route the Normal Traffic Signal through the Protection Transport entity for the duration it declares this defect condition.
Whenever a Revertive Protection Group clears that defect, it will switch the Normal Traffic Signal back to flowing through the Working Transport entity.
We call this second switching procedure (to return the Protection-Group to its NORMAL, pre-protection-switching state) revertive switching.
In contrast, a Non-Revertive Protection Group will NOT perform this revertive switch, and the Normal Traffic Signal will continue to flow through the Protection Transport entity indefinitely.
When the Tail-End Node clears the Service-Affecting Defect
Protection-Switching events are very disruptive to the Normal Traffic Signal. Each time we perform a protection-switching procedure, we induce a glitch (or a burst of bit errors) and signal discontinuity within the Normal Traffic Signal.
Therefore, Protection-Switching events should not be a common occurrence within any network.
To minimize the number of protection-switching events (occurring within a network), the Protection-Group will usually force the Tail-End Node to go through a Wait-to-Restore period after it clears the service-affecting or dDEG defect (which caused the Protection-Switching event in the first place) before it can proceed on to the next step and revert the protection-switching (and traffic).
In other words, the Tail-End Node (within a Protection-Group) will execute the following steps each time it clears a defect, which causes a protection-switching event.
- It clears the defect condition.
- The Tail-End circuit will then start a Wait-to-Restore Timer and will wait until this timer expires before it proceeds to the next step.
- If the Tail-End circuit declares another service-affecting defect while waiting for this Wait-to-Restore timer to expire, it will reset this timer back to zero and continue waiting.
- Once the Wait-to-Restore timer expires, the Tail-End circuit will revert the protection-switched configuration into the NORMAL configuration. In other words, the Normal Traffic Signal will (once again) travel along the Working Transport entity.
I show these same steps within the Revertive Procedure Flow Chart below.
Figure 1, Flow-Chart of the Revertive Protection-Switching Procedure – after the Service-Affecting defect clears.
What is the purpose of using this Wait-to-Restore Period?
There are two main reasons why we use the Wait-to-Restore period in a Protection-Switching system.
- To make sure that the condition of the Working Transport entity has stabilized and is not still declaring intermittent defects before we start to pass the Normal Traffic signal through it again.
- And to reduce the number of protection-switching events within a protection group.
How Long Should the Wait-to-Restore period be?
ITU-T G.808.1 recommends that this period be between 5 and 12 minutes.
In Summary
All revertive protection-switching systems must wait through a Wait-to-Restore period (after clearing the defect condition) before executing the revertive switch.
The purpose of waiting through this Wait-to-Restore period is to prevent multiple Protection-Switching events due to intermittent defects within the Working Transport entity.
ITU-T G.808.1 recommends that this Wait-to-Restore period be between 5 and 12 minutes.
This means that the Tail-End circuit must go through this Wait-to-Restore period and declare no defects for the entire 5 to 12-minute period before it can move on to revert its protection-switching.
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