How Protection Switching is implemented in SDH?

      For protection switching, mainly K1, K2 bytes and B2 bytes in Multiplex Section Overhead of SDH frame are used. Normally K bytes carried in protection fiber are used to carry APS protocol. B2 bytes contain bit interleaved parity check of the previously transmitted MSOH plus the VC-n payload.

K1/K2 Byte strycture:

K1/K2 byte structure is as shown in above diagram. Upto maximum 16 nodes can be supported in a SDH ring with protection. This is because, only 4 bytes are used for source and destination ID. In 4F-rings, APS protocol is only active on the protection fibers. APS protocol is optimised for AU level of operation. Each node in the ring should be configured with a ring map. This ring map contains information about the channels that node handles. Also, Each node in the ring is given a unique Id number within the range 0 to 15. 

 At any point of time, each node will be knowing the current status of the ring ( normal or protected). When the protection switches are not active, each node sends K-bytes in each direction indicating "no bridge request".  At the time of failure in the ring between two adjacent nodes,  two paths may exist for communication. Short path is the one, which directly connects both the nodes. Longer path connects these two nodes via all other nodes on the ring. When a node receives a non-idle K-byte message containing a destination ID of another node on the ring,  that node will change to pass through mode. 

Let us read about types of ring protection in next post

BLSR,Bi-directional Line Switched ring

There are two types of BLSR deployed in various networks.
i. 2-fiber BLSR
ii. 4-fiber BLSR

2-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.

4-Fiber BLSR:

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)

Ring networks - G.841 - Interview notes for UPSR

       Further to Linear protection, let us read about ring protection. ITU-T recommendation covers several types of ring network architectures. Ring protection switching can be implemented at path level or at line level. Rings can  be uni-directional or bi-directional. & they may utilise 2-fiber or 4-fibers.

UPSR : Uni-directional Path Switched Ring : 

 In a uni-directional ring, service traffic flows in one direction. ( clockwise in below diagram). Protection traffic flows in opposite direction  (counter clockwise)

In this example, traffic from C to B travels in clockwise direction via A. Traffic from B to C travels directly in clockwise direction. This configuration is also known as multiplex section dedicated protection ring.  This is because, one fiber carries service traffic, while the other is dedicated to protect the main path.All traffic is added in both directions. Decision as to which to use at drop point (no signaling). Normally non-revertive, so effectively two diversity paths

Main advantage of this configurations are :

 single ended switching, simple to implement and does not require any protocol. Single ended switching is always faster while compared to dual ended switching. Chances of restoring traffic under multiple fail conditions is high. Also, implementation of this architecture is least expensive.

However this arcitecture is Inefficient for core networks. There is no spatial reuse. Node needs to continuously monitor every tributary to be dropped.

In next post let us read about BLSR - Bi-directional line switched ring