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Method for Locating Multiframe Synchronization Failure When 2M E1 Links Are Interconnected

Publication Date:  2012-07-25 Views:  40 Downloads:  0
Issue Description
When the switching and mobile devices are interconnected through 2 Mbit/s E1 links, an alarm indicating multiframe synchronization failure is sometimes generated. The transmission device is provided by Huawei. Therefore, the customer usually suspects that this fault is caused by Huawei’s transmission device. In this case, it is required to perform judgment according to the alarms on the device on the service side and on the transmission device. At present, the support website does not provide a complete document to describe how to locate multiframe faults on 2 Mbit/s E1 links.
Alarm Information
Alarms indicating multiframe synchronization failure are generated on the switching devices on the service side at both ends but no out-of-frame alarms are reported. This indicates that interconnection fails.
In general, no alarms are generated on the transmission device and no performance exceptions such as bit errors and pointer adjustment occur on the VC-12.
Handling Process

1. It is preliminarily inferred that the configuration of the switching device on the service side is incorrect.

2. In general, the transmission device regards the 2M service as the payload. Check the transmission device. No performance exceptions such as bit errors and pointer adjustment occur on the VC-12. Therefore, it is inferred that the configuration of the switching device on the service side may be incorrect.

3. Connect the 2M BER tester if any to the switching device on the service side, and check the relevant bytes of the 2M frame structure to determine whether the configured 2M frame format is correct. For detailed byte definitions of the multiframe, see "Cause Analysis."

4. Check the E1 configurations of the switching devices at both ends to see whether the settings of the PCM30 and PCM31 formats are the same at both ends. If inconsistent, the out-of-multiframe alarm is generated when the PCM30 format is set at the peer end. The reason is that the PCM31 format does not involve multiframes and thus does not carry the multiframe signal octet on timeslot 16 of F0.

Note: In general, for the switching devices provided by the vendors including Huawei in China, the PCM30 format is adopted if the E1 path transmits channel associated signaling and the PCM30 format is adopted if the E1 path transmits common channel signaling. For the devices provided by overseas vendors including E, the PCM30 and PCM31 formats are not bound to the channel associated signaling or common channel signaling. Instead, an additional flag is available to control whether the PCM31 or PCM30 format is selected. Therefore, you need to check the flag carefully during the interconnection. 
Root Cause
The frame format of the 2 Mbit/s link and the multiframe-related principles are as follows:

The 2 Mbit/s link usually supports five frame formats: PCM30, PCM31, PCM30 CRC, PCM31 CRC, and non-frame structure.

The PCM30 and PCM30CRC formats are usually used in channel associated signaling. In these formats, timeslot 0 serves as the synchronization timeslot, and timeslot 16 serves as the signaling timeslot and is used to transmit multiframe signals. For voice channels, only timeslots 1 to 15 and 17 to 31 can be used. Such a frame format is called the PCM30.

The PCM31 and PCM31CRC formats are usually used in common signaling and involve no multiframes. Therefore, the signaling can be transmitted on the timeslots other than timeslot 16. Timeslots 1 to 31 can be all used for voice channels or signaling. Such a frame format is called the PCM31.

The PCM30 format involves multiframes, which is different from the PCM31 format and the non-frame structure. A multiframe consists of 16 sub frames named F0 to F15. Each sub frame has 32 timeslots named TS0 to TS31. For F0, TS16 is used to transmit multiframe synchronization alarms and out-of-frame alarms. The first four bits (that is, four zeros) indicate the multiframe location signal. Bits 5, 7, and 8 are service bits and are set to 1 when they are not used. Bit 6 indicates an out-of-multiframe alarm. Bit 6 is set to 1 when the multiframe is out of synchronization and to 0 when the multiframe is synchronized.

Therefore, if the out-of-multiframe alarm is generated on the switching device on the service side, the multiframe synchronization octets are usually mismatched at both ends during interconnection.

If the out-of-multiframe alarm and out-of-frame alarm are generated on the switching device on the service side at the same time, the transmission side may be faulty.  
Suggestions

Null.

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