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A Loopback Fault Occurs to the GE-Level ODUK_SNCP Services in the Case of Protection Switching of the OptiX OSN 6800

Publication Date:  2012-07-25 Views:  65 Downloads:  2
Issue Description
In an office, OptiX OSN 6800 devices are used to construct the provincial network backbone. Stations A, B, and C are configured with four GE-level ODUK_SNCP services (2 x GE services between stations A and B and 2 x GE services between stations A and C) of the first wavelength. All the services are deployed in station A. The TOM board is used as the tributary card and the NS2 board is used as the line card. The version of the OptiX OSN 6800 is 5.51.4.13. The fiber between stations A and B is broken one day. As a result, the GE services from stations B and C encounter switching due to signal failure (SF). The switching state is normal and the TOM board does not report alarms. The customer's digital communication engineer, however, reports that loopback exists in the 2 x GE services between stations B and A and that the link fails.
Alarm Information

1.         The NS2 board that stations A uses for receiving packets from station B reports a MUT_LOS alarm. The optical amplifier board that station B uses for receiving packets from station A reports a MUT_LOS alarm.

2.         The SCC units of stations A and B report ODUK_SNCP_PS alarms.

3.         The query result shows that the SNCP services at the OptiX OSN 6800 side of stations A and B are in the SF switching state. The working channel is in the SF state and the standby channel is normal.

Handling Process

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.

Root Cause

Loopback might occur to the ODUK_SNCP services upon protection switching in any of the following cases:

1.         The service configurations in the local station are incorrect.

2.         The cross-connect board or the OptiX OSN 6800 (software or hardware) in the local station is faulty, which causes service loopback upon active/standby switching of the cross-connect board.

3.         The optical path is looped back, which makes the standby channel between stations A and B unavailable.

Suggestions

1.         To ensure success of the protection switching upon fiber disconnection in the pass-through station in the intermediate optical layer, enable SNC/N for the ODUK-SNCP services of the OptiX OSN 6800 when configuring SNCP service pairs.

2.         Use the TTI bytes to check fiber connections if loopback exists upon protection switching of SNCP services.

END