Description of the problem

Figure 1

Background description, the common network topology of the railroad system is shown in Figure 1, the OSN3500 in the transmission room of the large station consists of a large ring with two-fiber bi-directional multiplexed segment sharing protection, such as A-F-G-H-I-J-O-P-Q-R-S-A in the figure, and the OSN2000 equipment in the small station consists of a unidirectional channel protection ring, such as A-B-C-D-E-F-A, J-K-L-M-N-O in the figure. -In this case, the ring capacity of the multiplexing segment is 2.5G (#1-#8 VC4 work, #9-#16 VC4 protection); the capacity of the channel ring is 622M, and the channel ring shares the #1-#4 VC4 of the 2.5G optical ports of the 2.5G multiplexing segment, i.e., the optical ports are the same in the duplicated segments of the channel ring and the multiplexing segment. That is, the channel ring shares the #1-#4 VC4 time slots of 2.5G optical ports of 2.5G multiplexing segments, such as between A-F and O-J, which are identical to the duplicated segments of the multiplexing segments.

Railroad TDCS service signaling is characterized by open services between two two stations, such as A-B, B-C, C-D, D-E, E-F open an E1 service between two, A and F and then each open an E1 circuit to the NC core bureau point (the railroad system called A and F back to the core bureau point of the circuit for the tapped circuit), it can be seen that the services between the OSN2000 are protected by the channel ring, and the services between A and F to the NC are protected by the multiplexing segment. The services between OSN2000 are protected by the channel ring, and the services between A and F to NC are protected by the multiplexing segment ring, and the E1 services in both directions from B to A and C are connected to different protocol converters for Ethernet signals, and the Ethernet signals are then connected to the same router, and the same as A, C, D, E and F are connected to B. In this way, A-B-C-D-E-F forms a "hand-in-hand" circuit, and a set of circuits are connected to the router through the router's protocol converters. A group of circuits, through the routing function of the router, the signals from each site are all transmitted to the NC core router, and at the business level, another layer of protection is formed.

At 1:13 on May 16, the NC network management center found that the optical circuit interruption occurred in the multiplexing segment ring A-F (later checked for theft), and the multiplexing segment ring was normally reversed, and the service was not affected.The original plan for May 16 between O-J was to carry out the fiber optic cable cut, and the staff on duty judged that the optical circuit interruption between A-F did not affect the cut operation, and did not call off the cut, and at 1:55, the fiber optic cable cut was started in the O-J segment, and the start of the cut caused the J station to be cut along the route as shown in the figure. At 1:55, the O-J section started to cut the fiber optic cable, and the cut started, resulting in the interruption of all the TDCS services from station J along the counterclockwise direction as shown in the figure (including KLMN) to station F (including BCDE). However, there is no TU-AIS alarm for the low-order services of NMLKJ and ABCDEF. After analyzing, it is normal that the services cannot be transmitted back to the NC because of double breaks in the multiplexing section of the tap circuit between O-P-Q-R-S-A and OSN2000. Customers believe that JKLMNO and ABCDEF two channel ring business in the O-J, A-F cable between the interruption of the situation, there is still channel protection, the tap circuit can still be passed through the J, F back to the NC, to ensure that the two sections of the "hand in hand" circuit is normal, but the actual situation is interrupted, the need to give the reason.

Alarm message

Optical board R-LOS, multiplexed segment inversion alarm, channel protection ring inversion alarm.

Processing

At 2:20, the fiber core between O and J was opened, and the service was restored.At 4:15, the fiber cable between A and F was opened, and all the inverted alarms disappeared.

Root Cause

Figure 2

As shown in Figure 2, the analysis is carried out by taking the service between C and D as an example. Under normal circumstances, because the protection ring attribute is a unidirectional channel protection ring, C receives D to select and receive signal 2 (blue labeled number 2), D receives C to select and receive signal 1, the VC12 time slot of signal 1 (C→D) is VC4:2:12, and the time slot of signal 2 (D→E→F→A→B→C) is VC4:2:12 in the whole process.

Figure 3

As shown in Figure 3, when the A-F cable is interrupted, because the channel protection ring does not require protocols to achieve rapid inversion, D receives C to remain unchanged, and still selects signal 1, C selects D to quickly invert the selection of signal 3; at the same time, the multiplexing segment inversion, A, F into the bridging, inversion state (the bridge occurs on the R-LOS side, as shown in Figure 3), the rest of the site for the penetration of the state so that in less than 50ms, D to Within 50ms, the working signal route from D to C becomes D→E→F→G→H→I→J→O→P→Q→R→S→A→B→C, in which the time slot of D→E→F is still VC4:2:12, that of F→G→H→I→J→O→P→Q→R→S→A is VC4:10:12, and that of A→B→C is VC4:2:12, and after the default WTR of the channel ring of 600s, the signal of C receiving D is inverted, and the signal of C receiving D is inverted. After 600s of default WTR in the channel ring, the signal from C to D is inverted to the selected signal 5. During the above two inversions, the service is normal and not affected.

Figure 4

The E1 service opened between L and K occupies VC12 time slots in the whole process, and both directions (Signal 6: L→K,Signal 7: K→J→O→N→M→L) are also VC4:2:12.

As in Fig. 4, when the optical path interruption occurs in O-J, O and J will receive the R-LOS alarm, which triggers the inversion of O and J, and changes from the pass-through state to the bridging state, and the signal 5 in Fig. 3 will be transiently disconnected, and the signal of C receiving D is immediately selected to receive the signal 3. The unidirectional service routing from D to C is changed, and the original signal 5 (D→E→F→G→H→I→J→O→P→Q→R→S→A→B→C), because J is bridged, is also VC4:2:12 in both directions (signal 6:L→K,signal 7:K→J→O→N→M→L). C), because of the bridge inversion in J, the VC4:10:12 in the direction of J sending O is bridged back to the VC4:2:12 in the direction of J receiving O, and the time slot happens to be the time slot of the original signal 7, which is sent to the K station via the cross-connection of the J station to the K station to land, so that the unidirectional service signal from D to C becomes D→E→F→G→H→I→J→K, and a complete signal flow is sent to the K station, and so the K station does not report an alarm. station K does not report an alarm. Then, in this case, if station C can not receive the signal should be reported to TU-AIS, we continue to analyze, L station originally sent to the K station of the protection service (pink), the signal is transmitted to the O station, in the O station to do the bridge, VC4:2:12 bridge to VC4:10:12, the signal is along the P-Q-S-R-S-A, in the A station, the signal is bridged from the VC4:10:12 again to the VC4:2:12, which happens to be the time slot of the original signal 4, the signal is sent to C via B, i.e., it is routed to signal 9 (L→M→N→O→P→Q→R→S→A→B→C) in the figure, so that station C also receives a complete signal, and does not report the alarm, and after the channel ring passes through the WTR for 600s, the signal received by C to D is inverted to signal 9, which results in the service interruption, while there is no TU alarm.

According to the analysis, the root cause of the problem is found, because the unidirectional channel protection ring shares the time slot on the multiplexing ring, and when two fiber breaks occur in the multiplexing segment, the two-fiber bi-directional multiplexing segment shared protection ring can not be realized on the ring for the suppression of low-order service level (the current two-fiber bi-directional multiplexing segment ring is very demanding for the suppression of the low-order service level), which generates the phenomenon of time-slot misconnection, and the unidirectional channel protection ring has the other direction, although the whole optical path is normal, but there is no TU alarm at the same time. Although the full optical path is normal, but can not play the role of protection, resulting in two two sites on the channel ring between the business is actually interrupted state, the problem is more hidden.

Suggestions and Summary

This case mainly analyzes the root cause of the service interruption between two two nodes on the whole ring caused by the unidirectional channel ring sharing the time slot of the multiplexing segment, according to which two suggestions are made: 1. Separate the unidirectional channel ring from the multiplexing segment ring and divide it into different optical boards, so that the time slot misconnection phenomenon of the multiplexing segment ring will not affect the unidirectional channel ring; 2. Replace the unidirectional channel protection ring with a bidirectional channel protection ring or an SNCP ring, and set the short path as the working route and the long path as the protection route. One of the above two methods can be used to avoid the problem.

Meanwhile, it is recommended that users strengthen the precautionary measures, and stop the cutting operation of other segments to reduce the risk when there is a fiber-optic cable interruption in one of the multiplexed segment rings.

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