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