What is the Tail-End for Protection Switching?

This post defines and describes the term Tail-End for (APS) Automatic Protection Switching applications.


What is the Tail-End within an APS (Automatic Protection Switching) System?

ITU-T G.808 Defines the Tail-End as:

The tail-end of a protection group is the end where the selector process is located.  In the case where traffic is protected in both directions of transmission, the tail-end process is present at every end of the protection group.

NOTE:  Whenever people discuss the tail-end of an APS system, they will sometimes use the term sink node.

ITU-T G.808 Defines the Sink Node (within a Protection Group) as:

The node at the egress of a protected domain, where a normal traffic signal may be selected from either the working transport entity or the protection transport entity.

So What Does All This Mean?

In Figure 1, we present an illustration of a Protection Group.

If we were to look closely at the Normal Traffic Signal (in Figure 1), we would see that the Tail-End is the last component that the Normal Traffic signal passes through as it exits the Protection Group.

Protection Group - 1:N

Figure 1, Illustration of a Protection Group

The Tail-End (or Sink Node) circuitry is responsible for declaring and clearing defects and asserting (or de-asserting) the SF (Signal Fail) and SD (Signal Degrade) indicators.

The Sink Node (or Tail-End) will use these SF and SD indicators to select either the Working or Protect Transport entity as its source for the Normal Traffic Signal.

Figure 2 illustrates a simplified schematic within the Tail-End Circuitry (including the Protection-Switching Controller).  

Tail End Circuitry - Protection-Switching Controller for a 1+1 Protection Architecture

Figure 2, Illustration of the Tail-End Circuitry

Additionally, Figure 2 shows that the Tail-End Circuitry contains circuitry that detects and declares defect conditions and asserts the SF or SD indicators.   This circuitry also includes the Protection-Switching Controller.  I will describe how this circuitry works in another post.  

Another post discusses the SF (Signal Fail) and SD (Signal Degrade) indicators.

As I show in Figure 2, the Tail-End will often use a Selector Switch that connects to the Working or Protection Transport entity.

If the Selector Switch is connected to the Working Transport entity, then the Tail-End circuitry will be using it (the Working Transport entity) as its source of the Normal Traffic signal.

The Normal Traffic signal will then propagate through the Tail-End circuitry as it exits the Protection Group.

But, if the Selector (Switch) is connected to the Protect Transport entity, then the Tail-End circuitry will be using the Protect Transport entity as its source of the Normal Traffic signal.

USING THE SELECTOR IN A 1+1 AND 1:N PROTECTION SWITCHING ARCHITECTURE

Figures 3 and 4 show drawings of the Protection Group using a Selector (Switch) in a 1+1 and 1:2 Protection Switching scheme, respectively

Selector in a 1+1 Protection Switching System

Figure 3, Illustration of a Selector within a 1+1 Protection Switching Scheme

Selector in a 1:2 Protection Switching System

Figure 4, Illustration of a Selector within a 1:2 (1:N) Protection Switching Scheme

In the case of the 1+1 Protection Switching scheme, the Working and Protect Transport entities always carry the Normal Traffic Signal.

However, the Protection Switching controller function within the Tail-End circuitry (not shown in Figure 3) will determine whether to select the Normal Traffic signal from the Working or the Protect Transport entity.

Suppose the Protection Switching controller function was to declare (for example) the SF (Signal Fail) or SD (Signal Degrade) defect condition within the Normal Traffic signal passing through the Working Transport entity.  In that case, it will command the Selector Switch to switch over and connect to the Protect Transport entity.

Whenever the Selector Switch switches from the Working to the Protect Transport entities, we call this action protection switching.

Some Protection Groups support a revert feature, where the Selector Switch automatically switches back to connecting to the Working Transport entity after the SF or SD defect has cleared.

Other Protection Groups are non-reverting systems and do not support this feature.

In other blog posts, we discuss the Protection Switching controller function and the revert operation.

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THE NEED FOR COMMUNICATION BETWEEN THE TAIL-END AND HEAD-END COMPONENTS OF A PROTECTION GROUP

The Selector Switch will connect to either the Working or Protect Transport entity, depending upon whether the Protection Switching controller circuitry declares the SF or SD defect within the Normal Traffic signal traveling through these transport entities.

The circuitry that detects and declares these defects (and commands the Selector to switch) resides on the Tail-End side of the Working/Protect Transport entities.

Therefore, in many Protection Switching architectures (e.g., 1:1, 1:n, and Bidirectional 1+1), the Tail-End will need to communicate and coordinate its protection-switching actions with the Head-End circuitry (within the Protection Group) via a communication link.

We discuss this APS communication link (or protocol) in another blog post.

Summary

The Tail-End (called the Sink Node) is usually the last component that a Normal Traffic Signal will pass through as it leaves the Protection Group.

The Tail-End (or Sink Node) contains circuitry that will determine if it should declare any service-affecting defects with the Working and Protect Transport entities and assert the SF (Signal Fail) signal as appropriate.

This Tail-End circuitry will also check the Working and Protect Transport entities for excessive errors (within the OTUk/ODUk signal) and assert the appropriate SD (Signal Degrade) signal.

The Sink Node (or Tail-End) circuitry will use these SF and SD signals to decide whether the Selector Switch should select the Normal Traffic Signal from the Working or the Protect Transport entities.

For normal (no defect) conditions, the Selector Switch will connect to and accept the Normal Traffic Signal from the Working Transport entity.

In this case, the Normal Traffic signal will travel from the Working Transport entity to and through the Tail-end circuitry as it leaves the Protection Group.

However, suppose the Protection-Switching controller determines that the Normal Traffic signal, when passing through the Working Transport entity, contains service-affecting defects, such as SF or SD.  In that case, it will command the Selector Switch to switch over and connect to the Protect Transport entity.

In this case, the Selector will now obtain the Normal Traffic signal from the Protect Transport entity (instead of the Working Transport entity).

Whenever the Selector switches from the Working to the Protect Transport entity, we call this action Protection Switching.

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What is a Protection Group (APS)

This post defines the term Protection Group, for Automatic Protection Switching (APS) purposes.


What is a Protection Group for APS (Automatic Protection Switching) Purposes?

ITU-T G.808 Defines the Protection Group as:  

The collection of head-end and tail-end functions, 1 to n normal traffic signals, optionally an extra traffic signal, 1 to n working transport entities, and a single protection transport entity used to provide extra reliability for the transport of normal traffic signals.

What does all that mean?

The Protection Group comprises components and connections that work together to enhance the reliability of a transmission/network system for a Normal Traffic Signal by implementing Automatic Protection Switching.

Further, the Protection Group for a transmission/networking system consists of all the following items/entities.

The Protection Group enhances the reliability and protects 1 to N numbers of Normal Traffic Signals.

The Protection Group can either support Unidirectional or Bidirectional protection switching.

Figure 1 presents the Illustration of a 1+1 Protection scheme that I have modified to show the various elements within a Protection Group.

Protection Group - 1+1

Figure 1, Illustration of a 1+1 Protection Scheme that also identifies the Protection Group.

Likewise, Figure 2 illustrates a 1:2 Protection scheme that I have modified to show the various elements within the Protection Group.

Protection Group - 1:N

Figure 2, Illustration of a 1:2 Protection Scheme that also identifies the Protection Group.

In both cases, Figures 1 and 2 show numerous components within a shaded box.

All the items within the shaded box (in each figure) make up the Protection Group.

I discuss the individual components that make up the Protection Group in other posts within this post.

In Summary:

A Protection Group is a system that consists of the following items/entities:

Each of these components works together, using Automatic Protection Switching to enhance the transport path’s reliability for a given Normal Traffic Signal.

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What is the Normal Traffic Signal (APS)

This post presents the definition for the term, Normal Traffic Signal, for APS (Automatic Protection Switching) applications.


What is the Normal Traffic Signal for APS (Automatic Protection Switching) Purposes?

ITU-T G.808 Defines the Normal Traffic Signal as:  
A traffic signal that is protected by two alternative transport entities called working and protection transport entities.  

What does all that mean?

In many applications, some traffic signals are critical to the Networking Service provider (and their financial bottom-line) and the end customers themselves.

Hence, these signals warrant a certain amount of protection in the form of some system redundancy.

In the APS (Automatic Protection Switching) World, we call this critical traffic signal the Normal Traffic Signal.

I recognize the apparent silliness in calling a critical traffic signal worthy of system-level protection a mundane name, such as the Normal Traffic Signal.

But that is the language used by the Standards Committee and the Industry (as a whole).

So who are we to buck the trend?

A Normal Traffic Signal becomes protected (in a system design) whenever the signal path between two Network Elements (that reside at the opposite ends of this particular protection group) provides both a Working and Protection Transport Entity (path) for that Normal Traffic Signal.

Figure 1 illustrates two Network Elements that form a protection group and are connected.

In this figure, we have connected these two Network Elements through two separate paths.

We will call one of these paths the Working Transport Entity and the other path the Protect Transport Entity.

Normal Traffic Signal - No Defect Case

Figure 1, Illustration of Two Adjacent Network Elements connected via a Protection Group (e.g., the Working and Protection Transport Entity).

So How Does All of This Work?

In most network system designs, the Network Elements will (by default) transmit the Normal Traffic Signal over the Working Transport Entity.

This Normal Traffic signal can be of any type (e.g., SONET/SDH, OTN, Ethernet, etc.).

Now, let’s suppose that some impairment were to occur within the traffic signal (that is, traveling from Network Element West to Network Element East).

The Network Element East will likely respond to this event by detecting and declaring a service-affecting or signal degrade defect with this traffic signal (e.g., SD, SF, etc.).

The overall system (consisting of both Network Element West and Network Element East) will respond to this event by redirecting the Normal Traffic Signal through the Protect Transport Entity instead of the (now defective) Working Transport Entity, as we show in Figure 2.

In other words, if the Working Transport entity (path) fails, then we can (and will) use the Protect Transport entity (path) as a Backup.

Normal Traffic Signal - Defect Condition

Figure 2, Illustration of Network Element East declaring a defect (with the traffic it receives from Network Element West) and (in response) invoking Protection Switching.

NOTE:  Figure 2 shows the two Network Elements invoking Bidirectional Protection Switching in response to Network Element East declaring a service-affecting defect condition.

The Network Elements could have also responded to this defect condition by using Unidirectional Protection Switching instead.

Please see Unidirectional and Bidirectional Protection Switching posts to learn more about these protection switching schemes/options.

In Summary

A Normal Traffic Signal is a protected signal.  It is an important signal that we will bear some expense to ensure that it gets from Point A to Point B, even with service-affecting or signal degrade defect conditions in the network.  

This means that as a given Network Element (at one end of a protection group) transmits the Normal Traffic signal to the other Network Element (at the other end of the protection group), it can do so by sending this Normal Traffic signal over one of two possible signal paths:

  • The Working Transport entity, and
  • The Protect Transport entity.

The Network Element will (by default) transmit the Normal Traffic signal over the Working Transport entity.

However, suppose the Network detects and declares a service-affecting or signal degrade defect within this Working Transport entity.  In that case, the Network System will redirect the Normal Traffic Signal through the Protection Transport entity (e.g., the backup signal path) instead.

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