What is the Wait-to-Restore Period?

This post briefly defines and describes the term Wait-to-Restore for Protection-Switching systems.


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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.


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 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 that it is declaring this defect condition.

Whenever a Revertive Protection-Group clears that defect, then 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 for an indefinite period.

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 are inducing a glitch (or a burst of bit-errors) 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 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 set of steps, each time it clears a service-affecting defect, which caused a protection-switching event.

  1. It clears the defect condition.
  2. 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.
  3. If the Tail-End circuit declares another service-affecting defect, while it is waiting for this Wait-to-Restore timer to expire, then it will reset this timer back to zero and will continue to wait.
  4. Once the Wait-to-Restore timer expires, then the Tail-End circuit will proceed to revert the protection-switched configuration into the NORMAL configuration.

I show these same steps within the Revertive Procedure Flow-Chart below.

Revertive Protection Switching Procedure Flow Chart


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 what we use the Wait-to-Restore period in a Protection-Switching system.

  1. To make sure that the condition of the Working Transport entity has stabilized and is not declaring intermittent defects before we start to pass the Normal Traffic signal through it again.
  2. 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 they have cleared the defect condition) before executing the actual revertive switch.

The purpose of waiting through this Wait-to-Restore period is to prevent the occurrence of multiple Protection-Switching events, due to the intermittent occurrence of defects within the Working Transport entity.

ITU-T G.808.1 recommends that this Wait-to-Restore period be between 5 and 12 minutes.

All of 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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Author: Darrell Smith

Darrell Smith has more than 30 years of experience as an Electrical Engineer. He has about 20 years of experience as an Applications Engineer and the remainder of his time was spent in Hardware Design and Product Marketing. He will now be sharing his wealth of knowledge on this blog.

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