i. 2-fiber BLSR
ii. 4-fiber BLSR
This system is also known as two fiber multiplex-section shared protection ring. Here, service traffic flows bi-directionally. Both the fibers carries service and protection channels.
When the protection channels are not required, they can be used to carry extra traffic, but at the time of protection switching, this extra traffic is dropped. Only ring switching is supported by this architecture. At the time of ring switching, those channels carrying service traffic are switched to the channels that carry the protection traffic in the opposite direction.
This system is also known as four-fiber multiplex-section shared protection ring. This is the most robust ring architecture. This is most expensive to implementbecause of the extra optical hardware required.
In this system, bi-directional pairs of fibers are used to connect each span in the ring. One bi-directional pair carries the working channels, while the other pair carries protection channels. 4F-BLSR supports both span switching and ring switching. ( but both not at the same time). Multiple span switches can coexist on the ring. This is because, only the protection channels along one span are used for each span switch.
What triggers a protection?
Protection switching is triggered in following cases.
1. Signal Fail , detected as Loss of Signal (LOS) at receiver input. This may be due to faulty hardware in the upstream network equipment or due to broken fiber.
2.Signal degrade, this is monitored by monitoring B2 bytes.
In next post let us read about : How is protection switching implemented in SDH ( K1 & K2 Bytes)