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 illustrates a “Normal” (e.g., No Defect) condition.
This illustration shows two network elements transmitting and receiving traffic to and from each other.
We will call one of these Network Elements Network Element West and the other Network Element Network Element East.
Figure 1, Illustration of two Network Elements exchanging traffic during Normal/No-defect conditions.
Figure 1 also shows Protection Switching support for the traffic flow between these two Network Elements.
This figure shows 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 declares any defect within the Working Transport Entity, and Good/Normal Bi-directional traffic flows between these two Network Elements.
Finally, this figure also shows that all the traffic flows through the Working Transport Entities and that none of the traffic flows through the Protect Transport Entities.
Next, we will consider what happens whenever one Network Elements declares a service-affecting defect within the Working Transport Entity.
Unidirectional Protection Switching Case
Figure 2 presents an illustration of a “Defect” condition.
In this case, some impairment exists within the West to East Working Transport entity, and Network Element East is declaring some service-affecting defect with this traffic signal.
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 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 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 showing the Bidirectional Switching Case.
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.
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 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.
In other words, if we detect a defect within the Working Transport Entity (at all), 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 in 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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