2 Cuts in SDH 2F-MSP Ring cause Traffic interference
Hi ,
Today I want to share with you a troubleshooting case of a service failure after 2 cut in MSP 2f SDH Ring .
Description
The network consists mainly of OSN 3500 & 7500 pure SDH NEs , the topology is MSP 2F Ring .
The Mobile service provider leased Ethernet over SDH (EOS ) in our network to connect media gateways located in BSCs .
-When first cut happens the ring protect the service normally within 50ms and no traffic got affected
MSP Ring mechanism based on reserving half the optical capacity of the fiber link for protection , so when cut happens the traffic passthrough the reserved capacity (VC4s) around the whole ring to the cut point then proceed normally to the destination
MSP Ring first cut service protection
- The ring MSP scheme uses the K1 and K2 bytes in the MSOH to transfer switching requests and to implement automatic protection switching , As you see in the diagram above the 2 nodes which have the cut between them rise a protection order to all NEs in the Ring , then the cross-connect in the 2 affected nodes convert service going from the cut link to the opposite direction in the VC4s reserved for protection .
- In the middle NEs the Cross-connect boards make the traffic passes through the whole ring in the protection VC4s to the cut nodes .
- When 2 nodes which have the cut between them receives the traffic from the protection VC4s then they matrix traffic to working VC4s and go as normal .
The problem cause
- when second cut happens , there will be 4 nodes have fiber cut between them and so there will be 2 sections , as in the diagram below , the NE-A and NE-C have add-drop between them , because of the 2 cuts the protected traffic for NE-A will be looped to it , also NE-C will get its protected traffic looped back into it ,that because each node has fiber cut will convert traffic coming from the protection VC4s to the main path and consider that it has passed the whole ring until they get to the NE which has the fiber cut , but actually these services passed only a section of the ring not the whole ring , causing a loop for the traffic .
- The TDM switches could detect the loop because they consider the sequence of the timeslots but with routers they could not recognize the signals and caused a very big interfering for the network as costumers calls got forwarded to wrong mobiles numbers.
How to deal with this problem?
For this problem we had to move the traffic into another links that have multi-cut protection like ASON protection ,As this problem is related to the working mechanism of 2F-MSP Ring .
I hope to be useful for you .
