Unidirectional Protection Switching

This post defines the term Unidirectional Protection Switching for APS (Automatic Protection Switching) applications.

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What is Unidirectional Protection Switching (and How is it Different from Bidirectional Protection Switching)?

The Definition of Unidirectional Protection Switching

ITU-T G.780 defines Unidirectional Protection Switching as follows:

A protection switching architecture in which, for a unidirectional failure (i.e., a failure affecting only one direction of transmission), only the affected direction (of the “trail”, or “subnetwork connection”, etc.) is switched to protection.  

What does ALL that Mean?

I will explain all this through three illustrations/cases

  • The Normal/No Defect Case
  • The Defect/Unidirectional Protection Switching Case and
  • The Defect/Bidirectional Protection Switching Case

The Normal/No Defect Case

Figure 1 presents an illustration of a “Normal” (e.g., No Defect) condition.

This illustration shows two network elements that are transmitting and receiving traffic to and from each other.

We will call one of these Network Elements Network Element West and we will call the other Network Element Network Element East.

Normal Traffic/No Defect Condition - Protection Switching

Figure 1, Illustration of two Network Elements exchanging traffic with each other during Normal/No-defect conditions.  

Figure 1 also shows that there is Protection Switching support for the traffic flow between these two Network Elements.

This figure shows that there is a Bidirectional Working Transport Entity between these two Network Elements.

The figure also shows that there is a Bidirectional Protect Transport Entity between these two Network Elements, as well.

In this case, neither Network Element is declaring any sort of defect within the Working Transport Entity and Good/Normal Bi-directional traffic is flowing between each of these two Network Elements.

Finally, this figure also shows that all the traffic is flowing through the Working Transport Entities and that none of the traffic is flowing through the Protect Transport Entities.

Next, we will consider what happens whenever one of the Network Elements declare a service-affecting defect condition within the Working Transport Entity.

Unidirectional Protection Switching Case

Figure 2 presents an illustration of a “Defect” condition.

In this case, there is some impairment that exists within the West to East Working Transport entity, and Network Element East is declaring some service-affecting defect with this traffic signal.

Unidirectional Protection Switching

Figure 2, Illustration of Unidirectional Protection Switching – in response to a Service-Affecting Defect in one direction of traffic

Since this defect exists within the Working Transport entity (in the West-to-East direction), this protection switching scheme will then route all West-to-East traffic through the Protect Transport Entity instead.

Please note this protection switching event only applies to the traffic within the West-to-East Direction.

Since Network Element West is not declaring any defects within the East-to-West Working Transport Entity, the East-to-West traffic will continue to travel via the Working Transport Entity (even though the West-to-East traffic has now been switched over to the Protect Transport Entity.

We call this particular protection scheme Unidirectional Protection Switching because, in this case, we only perform protection switching in the ONE direction that has the service-affecting defect.

We will clarify the difference between Unidirectional and Bidirectional Protection Switching, by now showing the Bidirectional Switching Case.

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Bidirectional Protection Switching Case

Figure 3 presents another illustration of a Defect condition.

In this case, there is (once again) some impairment that exists within the West-to-East Working transport connection, and Network Element East is declaring some service-affecting defect with this traffic signal.

Bidirectional Protection Switching

Figure 3, Illustration of Bidirectional Protection Switching – in response to a Service-Affecting Defect in one direction of traffic

Since this defect exists within the Working Transport connection, this protection switching scheme will then route all “West-to-East” traffic through the Protect Transport entity instead.

Yet, this protection switching scheme does not stop there.  It will also route all East-to-West traffic through the Protect Transport entity as well.

In other words, if we detect a defect within the Working Transport Entity (at all), then we will route the traffic for BOTH Directions from the Working Transport entity to the Protect Transport entity.

We call this particular protection scheme Bidirectional Protection Switching because, in this case, we are performing protection switching in BOTH directions, even if we only detect a service-affecting defect in one direction.

Please see the Bidirectional Protection Switching post for more detailed information on this protection switching scheme.

Summary

In summary, the main difference between Unidirectional and Bidirectional protection switching is as follows.

For Unidirectional Protection Switching, if the Network Elements declare a service-affecting defect in ONLY one direction (of traffic); then the protection scheme will only switch the defected traffic direction from the Working Transport Entity to the Protect Transport Entity.  The other (non-defective) traffic direction will continue to use the Working Transport Entity.

In the case of Bidirectional Protection Switching, if the Network Elements declare a service-affecting defect on ONLY one direction (of traffic); then the protection scheme will switch BOTH directions of traffic from the Working Transport Entity to the Protect Transport Entity.

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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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