OTN – Lesson 12 – Continuation of Discussion of APS Commands for Protection Switching – Part III

This blog post contains a video that continues the discussion of the APS Commands. This video serves as Video 3 of this series of APS Command videos.

Lesson 12 – Video 10 – Detailed Discussion of APS Commands – Video 3

This blog post continues our discussion of APS Commands and the APS/PCC Communication Protocol. This video serves as Video 3 in this discussion. This blog post discusses completing the Force Switch Commands, and the SF (Signal Fail) Commands using the APS/PCC Communication Protocol.

In particular, this video will discuss the following topics:

  • Executing the Force Switch – Normal Traffic Signal n to Protection (for the 1:N Protection Architecture) – Continued from Video 9
    • Implementing the Force Switch – NULL Test Signal to Protection Command – another way to resume Normal Operation.

Signal Fail APS Commands

  • Executing the Signal Fail – Working Transport Entity (SF_W) Command (for the 1+1 Protection Architecture)
    • How does a given Network Element invoke the SF_W Command?
    • The Protection Group’s operation during SF_W Command execution.
    • How do we Recover from this Command (for a Reverting System)?
      • Use of the WTR (Wait-to-Restore) Command
  • Executing the Signal Fail – Protection Transport Entity (SF_P) Command (for the 1+1 Protection Architecture)
    • How does a given Network Element invoke the SF_P Command?
    • The Protection Group’s operation during SF_P Command execution.
    • How do we Recover from this Command?
  • Executing the Signal Fail – Working Transport Entity (SF_W) Command (for the 1:N Protection Architecture)
    • How does a given Network Element invoke the SF_W Command?
    • The Protection Group’s operation during SF_W Command execution.
    • How do we Recover from this Command (for a Reverting System)?
      • Use of the WTR (Wait-to-Restore) Command.
  • Executing the Signal Fail – Protection Transport Entity (SF_P) Command (for the 1:N Protection Architecture)
    • How does a given Network Element invoke the SF_P Command?
    • The Protection Group’s operation during the SF_P Command execution.
    • How do we Recover from this Command?

Check Out the Video Below

Continue reading “OTN – Lesson 12 – Continuation of Discussion of APS Commands for Protection Switching – Part III”

OTN – Lesson 12 – Continuation of Discussion of APS Commands for Protection Switching – Part II

This blog post presents a video that continues our discussion of APS Commands via the APS/PCC Communication Protocol. This video serves as Part 2 of this discussion.

Lesson 12 – Video 9 – Detailed Discussion of APS Commands – Video 2

This blog post continues our discussion of APS Commands and the APS/PCC Communication Protocol. This video serves as Video 2 in this discussion. This blog discussed implementing the LoP (Lock Out of Protection) and Force Switch Commands using the APS/PCC Communication Protocol.

In particular, this video will discuss the following topics:

  • Executing the LoP (Lock Out of Protection Command) – for both the 1+1 and 1:N Protection Architectures
    • What does the LoP Command do to the Protection Group?
    • How do we Implement this Command?
    • How do we Terminate this Command (to resume Normal Operation)
  • Executing the Force Switch – Normal Traffic Signal to Protection (for the 1+1 Protection Architecture)
    • How does this command’s execution affect the Protection Group’s operation?
    • How do we Implement this Command?
    • Implementing the Force Switch – NULL Test Signal to Protection Command – to resume Normal Operation.
  • Executing the Force Switch – Normal Traffic Signal n to Protection (for the 1:N Protection Architecture)
    • How does this command’s execution affect the Protection Group’s operation?
    • How do we Implement this Command?
    • Implementing the Force Switch – Extra Traffic Signal to Protection Command – to resume Normal Operation.

To Learn More About the LoP (Lock Out of Protection) and Force Switch Commands, Check Out the Video Below.

Continue reading “OTN – Lesson 12 – Continuation of Discussion of APS Commands for Protection Switching – Part II”

OTN – Lesson 12 – Introduction to APS and the APS Communication Protocol for Protection Switching

This blog post presents a video that shows how to implement APS (Automati Protection Switching) both with and without using an APS Communication Protocol.

Lesson 12 – Video 8 – Detailed Discussion of APS without and with the APS Communication Protocol – Video 1

This blog post describes how we can implement APS (Automatic Protection Switching) without using an APS Communication Protocol. Afterward, this blog introduces how to implement APS using the APS Communication Protocol. This video serves as the first of several videos on this topic.

In particular, this video will discuss the following topics:

  • Executing APS without using the APS Communication Protocol
    • Under what conditions can we implement APS without using an APS Communication Protocol?
    • Why can we implement APS (without using an APS protocol) in this case?
    • When not supporting an APS Communication Protocol, the Architecture/Design of the Protection-Switching Controllers.
    • How to implement Automatic Protection Switching – without implementing an APS Communication Protocol.
  • Executing APS with an APS Communication Protocol
    • Under what conditions must we implement APS with an APS Communication Protocol?
    • Why do we need to use an APS Communication Protocol for these cases?
    • How to implement Automatic Protection Switching – while using an APS Communication Protocol
      • Introduction to the APS/PCC Field for OTN/Linear Protection Switching Applications (per ITU-T G.873.1).
      • The Architecture/Design of the Protection-Switching Controllers – 1+1 Protection Architecture
      • The Architecture/Design of the Protection Switching Controllers – 1:N Protection Architecture
      • The NR (No Request) Command

To Learn How to Implement APS, with and without an APS Communication Protocol, Check Out the Video Below.

Continue reading “OTN – Lesson 12 – Introduction to APS and the APS Communication Protocol for Protection Switching”

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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Linear Protection Switching

This post briefly defines the term: Linear Protection Switching. It also briefly defines 1+1, 1:N Protection Architectures.


What is Linear Protection Switching?

A Linear Protection-Switching System is a Protection System (or Protection Group) that contains two nodes:

Each of these two nodes is exchanging normal traffic signals, with each other, over a protected network that consists of both the Working Transport entity and the Protect Transport entity.

I show some simple pictures of Linear Protection Switching Systems below in Figures 1, 2, and 3.

The 1+1 Protection-Switching Architecture

1+1 Linear Protection-Switching System

Figure 1, Illustration of a Linear Protection Switching System (A 1+1 Protection-Switching System)

In Figure 1, I show a simple illustration of a 1+1 Protection-Switching system, which also presents the bidirectional traffic flow between the Head-End and Tail-End Nodes.

If you want to learn more about the 1+1 Protection-Switching Architecture, check out the post on this topic.

The 1:N Protection-Switching Architecture

Linear Protection Switching - 1:2 Protection Switching Architecture

Figure 2, Illustration of a Linear Protection Switching System (A 1:2 Protection-Switching System) – for the East to West Direction

Linear Protection Switching - 1:2 Protection-Switching Architecture

Figure 3, Illustration of a Linear Protection Switching System (A 1:2 Protection-Switching System) – for the West to East Direction

Figures 2 and 3 each present an illustration of a 1:2 (or 1:N) Protection-Switching System.

Please note that the 1:N Protection-Switching Architecture figures are more complicated than that for the 1+1 Protection-Switching Architecture.

Therefore, I needed to show this architecture in the form of two figures. 

One figure shows the traffic flowing from West to East, and the other illustrates the traffic flowing from East to West.

If you want to learn more about the 1:2 (or 1:N) Protection-Switching architecture, check out the post on that topic.

In summary, the 1+1 and the 1:N Protection-Switching schemes are Linear-Protection Protection-Switching systems.

Design Variations for Linear Protection-Switching Systems

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

Architecture

Switching Type

Operation Type

APS Protocol – Using the APS/PCC Channel

Click on any of the links above to learn more about these design variations within a Linear Protection-Switching System.

What about Other Protection-Switching Architectures?

There are other types of Protection Switching systems, which are not Linear, such as Shared-Ring Protection-Switching or Shared-Mesh Protection-Switching.

Please see the relevant posts for more information about those types of Protection-Switching Systems.

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What is 1:N Protection Switching?

This post defines and describes the 1:n Protection Switching architecture.

What is the 1:n Protection Switching Architecture?

ITU-T G.808 defines the “1:n (protection) architecture (n >= 1) as:

A 1:n protection architecture has n normal traffic signals, n working transport entities, and one protection transport entity.  

It may have an extra traffic signal.  

At the source end, a normal traffic signal is either permanently connected to its working transport entity and may be connected to the protection transport entity (in the case of a broadcast bridge) or is connected to either its working or protection transport entity (in the case of a selector bridge).  

The sink-end can select the normal traffic signal from either the working or the protection transport entity.  

An unprotected extra traffic signal can be transported via the protection transport entity whenever the protection transport entity is not used to carry a normal traffic signal.

What Does All This Mean?

As for all Protection Groups, a 1:n Protection Architecture consists of the following elements:

  • n instances of the Head-End (or Source-End)
  • n instances of the Tail-End (or Sink-End)
  • and n separate Normal Traffic Signals
  • n sets of Working Transport entities
  • a single Protect Transport entity
  • a single Extra Traffic Signal
  • Protection Switching Controller (that can detect and declare defects within the Normal Traffic Signals).
  • An APS Communications Link/Protocol (Required)

Figure 1 shows a variation of the 1:n Protection Switching architecture.

In this case, we show a 1:2 Protection Switching Architecture.

Basic Drawing of a 1:2 Protection Switching Scheme

Figure 1, Illustration of a 1:2 Protection Switching Architecture

This figure shows that a Broadcast Bridge realizes each of the two Head-Ends (of this Protection Group).  We realize each of the two Tail-ends by a Selector Switch.

I have designated the Broadcast Bridges with the Blue Overlay Shading in this figure.

Likewise, I have designated the Selector Switches with the Red Overlay shading.

NOTES:  

  1. The user can also opt to realize the Head-ends with a Selector Switch for the 1:n Protection Switching Architecture.
  2. Figure 1 includes some other bells and whistles (in the form of some additional Selector Switches) that I will discuss later in this blog.

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How does the 1:n Protection Switching Architecture Work?

One of the noteworthy features of a 1:n Protection Switching Architecture is that you have a single Protection Transport entity that protects n Normal Traffic Signals.

This means that for the 1:n Protection Switching Architecture, you only need 1/n more bandwidth to transport the n Normal Traffic Signals in a protected manner, from the Head-ends to the Tail-ends (where n is the total number of Normal Traffic Signals that you are transporting via this Protection Group).

In contrast, for a 1+1 Protection Switching Architecture, you have one Protection Transport entity protecting one Normal Traffic Signal.

This means that we are providing the Normal Traffic Signal with twice the bandwidth required to transport this signal from the Head-End to the Tail-End in a protected manner.

For many applications, this is inefficient, expensive, and inconvenient.

Of course, this same ratio would also hold if you used a 1:1 Protection Switching Architecture.

Another Key Characteristic of a 1:N Protection Architecture

Another critical characteristic of a 1:N Protection Architecture is the use of Broadcast Bridges on the Head-End Circuitry.  In contrast to a Permanent Bridge, during No-Defect Operation, there will not be a hardwired connection between the Normal Traffic Signal and the Working and Protection Transport entities.  There is only the connection between the Normal Traffic Signal and the Working Transport Entity.  When we are required to perform Protection-Switching, we will close the Broadcast Bridges and complete the electrical connection between the Normal Traffic Signal and the Protection Transport entity.  

We will discuss how the 1:n Protection Switching Architecture works by examining the following cases/conditions.

  • The Normal (No Defect) Case
  • A service-affect defect occurring Working Transport entity # 1
  • Protection Switching (after the defect has been declared).
  • The Normal (No Defect) Case – also using the Extra Traffic Signal

The Normal (No Defect) Case

Figure 2 shows a drawing of the Normal (No Defect) Case.

In this case, we have two Network Elements that are exchanging data with each other.

One Network Element (which we labeled Network Element West) is transmitting data to another Network Element (which we labeled Network Element East).

In most actual applications, we would also have traffic going in the opposite direction (East to West).

But, to keep these figures simple, we are only showing one direction of traffic in each of the figures in this post.

In Figure 2, Normal Traffic Signal # 1 travels over Working Transport entity # 1.

Likewise, Normal Traffic Signal # 2 travels over Working Transport entity # 2.

Additionally, the Extra Traffic Signal is traveling over the Protection Transport entity.

There are no impairments on any of the Working Transport entities, and everything is expected in this case.

1:2 Protection Switching Scheme - Normal Condition - Extra Traffic Signal

Figure 2, Illustration of the Normal (No Defect) Case 

A Service-Affecting Defect occurs in Working Transport entity # 1

Now, let us assume that an impairment occurs in Working Transport entity # 1, such that some circuitry (sitting within the Tail-End of this Working Transport entity, within Network Element East) is declaring either a service-affecting defect such as SF (Signal Fail) or the signal degrade defect, such as SD (Signal Degrade).

In this case, Normal Traffic Signal # 1 can no longer travel on the Working Transport entity # 1.

Figure 3 shows a drawing of this condition.

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

Figure 3, Illustration of a Service-Affecting Defect Occurring in Working Transport Entity # 1

Protection Switching – After the Defect (in Working Transport Entity # 1) has been declared

Now, since the Tail-End circuitry of Working Transport entity # 1, within Network Element East) has declared this defect condition, it needs to invoke Protection Switching.

In particular, this circuitry needs to perform the following four tasks.

  1. The circuitry within Network Element East needs to switch the local Selector Switch, which I’ve labeled SE1 (at the Tail-End of Working Transport entity # 1), away from this (now failed) Working Transport entity over to selecting the Protection Transport entity.
  2. The Network Element East circuitry also needs to send a command across the Transport entities back to the upstream Network Element (e.g., Network Element West).  In this case, Network Element East will also command Network Element West to invoke Protection Switching (for Working Transport entity # 1).
  3. Next, Network Element West (after it has received this command from Network Element East) then needs to command the Broadcast Switch, which I’ve labeled BBW1 (at the Head-end of Working Transport Entity # 1) to switch such that Normal Traffic Signal # 1 is now also connected to the Protection Transport entity.
  4. Finally, Network Element West needs to pre-empt the Extra Traffic signal by opening the switch that I’ve labeled SWP.  Once this switch is OPEN, the Extra Traffic signal will no longer travel across the Protection Transport entity.

In this case, Normal Traffic Signal # 1 will now travel (from Network Element West to Network Element East) using the Protect Transport entity.

Figure 4 presents the resulting configuration (with Network Elements East and West) after protection switching.

1:2 Protection Switching Scheme - Protection EventFigure 4, Illustration of our 1:2 Protection Switching Protect Group, following Protection Switching

NOTE:  Protection Groups using the 1:n Protection Switching scheme are required to support an APS Communications Channel to command and coordinate Protection Switching activities between the Head-ends and Tail-ends of the Protection Group.

This is how 1:N Protection Switching works.

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Additional Options in the 1:n Protection Switching Architecture

There are several options that users can use when designing a 1:n protection switching scheme.

Some of these options include:

  • Transmitting the NULL Signal during the Normal (No Defect) Case – as the Extra Traffic signal.
  • Transmitting the FDI/AIS signal during Protection Switching
  • Revertive Protection Switching
  • Unidirectional or Bidirectional protection switching

We will briefly discuss each of these options below.

Transmitting the NULL Signal during the Normal (No Defect) Case – as the Extra Traffic Signal.

In some cases, the user can transmit the NULL signal as the Extra Traffic signal (via the Protect Transport entities) anytime each of the n Working Transport entities is defect-free and is functioning properly.

In the Protection Group (discussed in this post), we could close the Switch labeled SN and open the Switch labeled SWP (within Network Element West).

This configuration setting would allow the NULL signal (that originates from Network Element West) to flow through the Protect Transport entity, as shown in Figure 5.

1:2 Protection Switching Sceme - Normal with NULL Signal

Figure 5, Transmitting the NULL Signal via the Protect Transport Entity, during Standby Times.  

Transmitting the FDI/AIS signal during Protection Switching

In many cases, the user will transmit the FDI/AIS signal towards the circuitry downstream from Network Element East by switching the switch, which I’ve labeled SEP (within Network Element East), away from the Protect Transport entity towards the FDI/AIS signal source.

Figure 4 (above) shows Network Element East transmitting the FDI/AIS indicator towards downstream traffic during Protection Switching.

The user would typically only do this whenever the Extra Traffic Signal (e.g., the NULL signal or some other low-priority signal) has been pre-empted due to a Protection Switching event.

The purpose of transmitting this FDI/AIS signal is to alert downstream equipment of a service-affecting defect condition within one of the Working Transport entities between Network Elements East and West.

NOTE:  For OTN applications, the Network Element will transmit the ODUk-AIS indicator during these protection switching events.

Revertive Protection Switching

Some Protection Groups will support Revertive operations, and others will not.

Suppose you designed a Protection Group to support Revertive operations.  In that case, the Protection Group will automatically reroute the affected Normal Traffic Signal back through its Working Transport entity shortly after the servicing-affecting defect (which caused the protection switching event in the first place) has cleared.

1:n Protection Switching systems typically support Revertive operations, whereas 1+1 Protection Switching systems may NOT support Revertive operations.

If a 1:n Protection Switching system was to support Revertive operations, then the Network Element that first declared (and is now clearing) the service-affecting defect; would have to send a command back to the other (remote) Network Element to coordinate revert protection switching activities (between both the Head-Ends and Tail-Ends of the Protection Group).

Please see the post on the Revertive Operation and the Automatic Protection Switching Channel for more details on this topic.

Unidirectional or Bidirectional Protection Switching

A 1:n Protection Switching scheme can support either Unidirectional or Bidirectional Protection Switching.

If the Protection Group supports Unidirectional Protection Switching, then the Network Element (that detects and declares the Service-Affecting defect within one of the Working Transport entities) will need to send the necessary command information (back to the upstream Network Element) to command and coordinate the Unidirectional Protection Switching event.

Conversely, suppose the Protection Group supports Bidirectional Protection Switching.  In that case, the Network Element (that detects and declares the Service-Affecting defect) will need to send the necessary command information (back to the upstream Network Element) to command and coordinate the Bidirectional Protection Switch.

Please see the posts for Unidirectional and Bidirectional Protection Switching for more details on this topic.

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