OTN – Lesson 12 – Introduction to Linear Protection Switching – Part TWO

This blog post contains a Video that serves as the 2nd Introductory Video to Linear Protection-Switching. This video covers Hold-Off Timers, Wait-to-Restore Timers and reviews Trail Protection.

Lesson 12 – Video 2 – Introduction to Linear Protection Switching – Part TWO

This blog post contains a video that covers the second part of our Introduction to Linear Protection-Switching.

In particular, this video discusses and defines the following topics that pertain to Protection-Switching:

  • Nested Protection-Switching Domains
  • Hold-Off Timers – What are They and How are They Useful in a Protection-Switching Design?
  • Wait-to-Restore (WTR) Timers – What are WTR Timers, and why do We Use them?
  • A Review of Trail Protection – This video discusses what Trail Protection is, and why it is not very popular in Protection-Switching Applications.

Check Out the Video Below

Continue reading “OTN – Lesson 12 – Introduction to Linear Protection Switching – Part TWO”

What is the Wait-to-Restore Period?

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


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 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) 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 steps each time it clears a service-affecting defect, which causes 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 waiting for this Wait-to-Restore timer to expire, then it will reset this timer back to zero and continue to wait.
  4. Once the Wait-to-Restore timer expires, the Tail-End circuit will 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 why 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 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 the 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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What is a Revertive APS System?

This post briefly defines and describes Revertive Protection Switching.

What is a Revertive APS (Automatic Protection-Switching) System?

If an Automatic Protection System is Revertive, then that means that the system will always return to transmitting/accepting the Normal Traffic Signal through the Working Transport Entity anytime the system has recovered from a defect or an external request (for Protection Switching).

An Example of Revertive Switching

Let’s use an example to help define the term revertive.

The Normal/No Defect Case

Let’s consider a 1:2 Protection Switching System shown below in Figure 1.

Linear Protection Switching - 1:2 Protection-Switching Architecture

Figure 1, Illustration of a 1:2 Protection Switching System – West to East Direction

NOTE:  Because the 1:N Protection-Switching pictures are somewhat complicated, I only show the West-to-East Direction of this Protection-Switching system to keep these figures simple.

In Figure 1, all is well.  Both of the Normal Traffic Signals within the figure (e.g., Normal Traffic Signal # 1 and Normal Traffic Signal # 2) flow from their Head-End Nodes to their Tail-End Nodes with no defects or impairments.

The Defect Case

Let’s assume that an impairment occurs within Working Transport Entity # 1 and that the Tail-End circuitry (associated with Working Transport Entity # 1) declares either a Service-Affecting or Signal Degrade Defect (e.g., declares the SF or SD condition).

We show this scenario below in Figure 2.

1:2 Protection Switching Scheme - Defect in Working Transport entity # 1

Figure 2, Illustration of our 1:2 Protection-Switching system (West to East Direction only) with a Service-Affecting Defect occurring in Working Transport Entity # 1 

The Protection-Switch

Whenever the Tail-End circuitry (within Figure 2) declares the service-affecting defect condition, it will (after numerous steps) achieve the Protection-Switching configuration shown below in Figure 3.

1:2 Protection Switching Scheme - Protection Event

Figure 3, Illustration of our 1:2 Protection-Switching system (West to East Direction only) following Protection-Switching.

NOTE:  Check out the post on the APS Protocol (within “THE BEST DARN OTN TRAINING PERIOD” training sessions) to understand the sequence of steps that the Tail-End and the Head-End Nodes had to execute to achieve the configuration we show in Figure 3.

Our 1:2 Protection-Switching system will remain in the condition shown in Figure 3 for the duration that the Tail-End Circuitry declares this defect within Working Transport Entity # 1.

The Defect Clears

Eventually, the Service-Provider will roll trucks (e.g., send repair personnel out to fix the fault condition, causing the service-affecting defect); and the defect will clear.

Once this service-affecting defect clears, the East Network Element will wait some WTR (or Wait-to-Restore) period before it proceeds with the Revertive switch.

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The Revertive-Switch

Once the WTR period expires (with no further defects occurring within Working Transport Entity # 1), our Protection Group will switch and route Normal Traffic Signal # 1 back through Working Transport Entity # 1.

We show the resulting configuration below in Figure 4.

Linear Protection Switching - 1:2 Protection-Switching Architecture

Figure 4, Illustration of our 1:2 Protection-Switching System (West to East Direction ONLY) following Revertive-Switch

The Overall Flow for Revertive Switching

Figure 5 presents a flow-chart diagram that summarizes the Revertive Protection-Switching Procedure.

Revertive Protection Switching Procedure Flow Chart

Figure 5, Flow-Chart Diagram summarizing the Revertive Protection Switching Procedure

Check out the relevant post for more information about the Wait-to-Restore period and Timer.

In Summary

A Revertive Protection-Switching system will always perform a second switching procedure after the defect has cleared.

This second switching procedure will return the Protection Group to the state of having the Normal Traffic Signal flowing through the Working Transport Entity.

A Non-Revertive Protection-Switching system will NOT perform this second switching after the Tail-End Node has cleared the service-affecting defect.

Therefore, in a Non-Revertive Protection-Switching system, the Normal Traffic Signal will continue to flow through the Protection Transport entity indefinitely.

To use a Revertive Protection-Switching System or NOT.

There are advantages and disadvantages to using a Revertive system.

I list some of these advantages and disadvantages below.

Disadvantages of Using a Revertive System

  • Each service-affecting or signal degrade defect (SD or SF) occurrence will result in two Switching Events.  We will disrupt the Normal Traffic Signal twice for each defect condition.
    • The first switching event is in response to the defect condition, and
    • The follow-up Revert Switching event.

We strongly advise that you use Revertive Protection-Switching if:

  • You are using a Shared-Ring Protection-Switching system.
  • If the Bandwidth or Performance Capability of the Protection Transport entity is lower or worse than that for the Working Transport Entity (e.g., has more bit errors, inferior performance)
  • Whenever there is a much more significant delay in the Protection Transport entity (than that for the Working Transport entity)
  • If one needs to track which Protected ports are using the Working Transport entity and which are using the Protection Transport entities
  • Protection Transport entity must be readily available for multiple other Working Transport entities (as in a 1:N Protection Architecture).

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Shared-Ring Protection Switching

This post briefly defines the term: Shared-Ring Protection-Switching


What is Shared-Ring Protection Switching?

A Shared-Ring Protection Switching system is a Protection System that contains at least three (3) Nodes.

Each Node within this Shared-Ring Protection-Switching System (or Ring) is connected to two neighboring nodes.

I show an illustration of a Shared-Ring Protection-Switching System below in Figure 1.

4-Fibre/4-Lambda Shared-Ring Protection-Switching System

Figure 1, Illustration of a Shared-Ring Protection-Switching System

Figure 1 presents a shared-ring protection-switching system that consists of six (6) nodes that are each connected to a shared ring that contains four (4) Optical loops (or rings).

Some optical rings carry traffic that flows in the clockwise direction (through each node).  Other rings carry traffic that flows in the counter-clockwise direction.

In Figure 1, I have labeled some of these optical loops as “Working” or Working Transport Entity loops and others as “Protect” or Protection Transport Entity loops.

What are the Nodes within a Shared-Ring Protection-Switching System?

Each node (on the Shared-Ring Protection-Switching system) is an electrical/optical system that functions similarly to an Add-Drop-MUX.

Some data traveling on an optical loop (within the ring) will pass through these nodes.  These Nodes also can add in and drop-out some of the data traveling on these loops.

I show the Add-, Drop- and Pass-Through capability of these Nodes below in Figure 2.

Add-Drop MUX features of Nodes in Shared-Ring Protection-Switching

Figure 2, Illustration of the Add-, Drop- and Pass-Through capabilities of a given node, sitting on the shared-ring.  

It is also important to note that each Node can function as either a Source (or Head-End) Node, a Sink (or Tail-End) Node, or both.

Types of Shared-Ring Protection-Switching Systems

ITU-T G.873.2 defines the following two types of Shared-Ring Protection-Switching systems.

  • The 2-Fibre/2-Lambda Shared-Ring Protection-Switching system, and
  • The 4-Fibre/4-Lambda Shared-Ring Protection-Switching system.

Please click on the links above to learn more about these Shared-Ring Protection-Switching systems.

Types of Protection-Switching within a Shared-Ring Protection-Switching System

The Shared-Ring Protection-Switching system can support both the following kinds of Protection-Switching.

Click on the above links to learn more about these types of Protection-Switching within a Shared-Ring Protection-Switching system.

Design Variations for Shared-Ring Protection-Switching Systems

Shared-Ring Protection-Switching systems are available in a wide variety of features.  I’ve listed some of these features and their possible variations below.

Shared-Ring Protection-Switching Types

  • 2-Fibre/2-Lambda Shared-Ring Protection-Switching systems
  • 4-Fibre/4-Lambda Shared-Ring Protection-Switching systems.

Architecture Type

All Shared-Ring Protection-Switching is of the 1:N Protection-Switching Architecture.

Switching Type

All Shared-Ring Protection-Switching is Bidirectional.

Operation Type

All Shared-Ring Protection-Switching systems use Revertive Operation.

APS Protocol – Using the APS/PCC Channel

All Shared-Ring Protection-Switching systems use the APS Protocol.

What about other types of Protection-Switching?

Other types of Protection-Switching Systems are not Shared-Ring, such as Linear or Shared-Mesh Protection-Switching.

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