1. Check the SNCP service configurations of stations A and B, and those of the pass-through station (station C) in the intermediate electrical layer.
Stations A and B are configured with a pair of TOM-NS2 dual fed service and NS2-TOM SNCP service (note that the ODUK_SNCP must be configured as SNC/N, that is, non-intrusive monitored subnetwork connection must be enabled). Station C is configured with NS2-NS2 pass-through services. All the service configurations are correct.
2. Perform active/standby switching for the cross-connect boards in stations A, B, and C successively.
The problem persists.
3. Check whether channel-level loopback exists. Insert different TTI bytes into the NS2 board between stations A and B, that between stations B and C, and that between stations C and A by using the TTI insertion and detection function supported by the line card.
It is confirmed that the optical channel between stations A and B is normal: Station A can receive the TTI byte sent by station B through the short path and the received TTI byte is the same as that sent by the relevant NS2 board. Similarly, station B can receive the TTI byte sent by station A through the short path and the TTI byte is the same as that sent by the relevant NS2 board.
When the fiber connections between stations B and C and between stations C and A are being verified, the following problem occurs: The TTI byte sent from station B to station C is detected by the NS2 board between stations C and A. In addition, the TTI byte sent from station A to station C is detected by the NS2 board between stations C and B. For details, see the attachments.
The preceding information shows that crossed pair connection exists between the east-west NS2 board and the east-west M40 board in station C. That is, the OUT port of the NS2 board for C-B transmission is connected to the first-wavelength IN port of the M40 board for C-A transmission, while the OUT port of the NS2 board for C-A transmission is connected to the first-wavelength IN port of the M40 board for C-B transmission.
In this case, the dual fed and selective receiving services of station C are not affected. In the B-C-A long path, however, the services from station B to station A are looped back to station B in station C and the services from station A to station B are looped back to station A in station C. (Station C is the first-wavelength pass-through station in the electrical layer. The first 2 x 2.5 Gbit/s NS2-NS2 path for the first wavelength is configured for pass-through services.) Loopback exists at the WDM side. Though the TOM board does not report alarms, the GE services in the standby channel between stations A and B are looped back and the downstream digital communication device encounters link failures.
Correct the fiber connections between the NS2 board and the M40 board, and then check the fiber connections by using TTI bytes.
4. The fiber connection between stations A and B, that between stations B and C, and that between C and A are correct. The TTI bytes are received in pairs. No loopback of GE services is detected during protection switching upon disconnected fiber in the ring. The problem is solved.