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TDM services dropped due to guranteed AM modulation mode decreased and E1 capacity is changed.

Publication Date:  2012-07-25 Views:  39 Downloads:  0
Issue Description
Customer has got an AM description that is TDM services configured on Hyprid MW link RTN950 V100R002 has by default the highest QOS priorities than the Existing configured Ethernet traffic .
Customer has one Hyprid MW link that use 14MHz channel BW and has AM enabled on the link , the AM use gurantee modulaton scheme as 32-QAM and highest modulation scheme as 128QAM,

The link has been designed to use 16 E1 services for 2G traffic and remaining capacity for 3G traffic. At the existing configuration , only Six of those E1 TDM services was configured for three 2G sites ,each site use 2-E1 services., and one ethernet service has been configured for one 3G site .

Customer planning team has modified the link design to use AM modulation range starting from QPSK to 128QAM , when huawei engineer modify the guranteed modulation scheme at both sites from 32-QAM to QPSK [ only one BTS site services dropped and other two BTS sites has got no effect ] ,

Alarm Information

Handling Process
The follow up of each of above reasons has lead to the solution :-
1- U2000 Log data has been checked to know the exact time when huawei engineer modify the configuration of the Lowest AM modulation scheme , when that has been compared with the KPI performance from M2000, it showes that the 2G site has only dropped at the same time the Modulation changed , so it was transmission

2- After checking Huawei Specification on the existing used Channel Capacity 14-MHz , it was as below
Channel Spacing(MHz) Modulation Mode Air Interface Service Capacity (Mbit/s) Maximum Numberoe E1s in Service
14 QPSK 20 10
14 16QAM 42 20
14 32QAM 51 24
14 64QAM 66 31
14 128QAM 78 37
The above table showes that when guranteed modulation will be changed from 32-QAM to QPSK , that mean that E1 maximum Capacity should be changed from 24 to 10 , but that does not show to the customer why the service was down while the link was planned to carry 16 E1 services , but only 6 services was configured , which still leass than 10 services supported by QPSK.

3- when the log of U2000 was checked we found that huawei engineer has already reconfigure the E1 capacity at each site to use 6 E1 services then he modify the lowest modulation scheme at each site to QPSK , that was logical because if he did not do that , he will not be able to modify the modulation , U2000 will generate message that is the maximum E1 services should be less than10 and the old configured E1 capacity of 16 should be modified.

4- when the two sites Alarm attributes has been checked , we found that most of VC12 alarms has been suppressed , when the suppress action has been disabled , two alarms of TU_AIS appeared , when we check the used VC4:1 VC12 channel by those two services we find the [ VC4-1:12 , VC4-1:13 ] , that shows why those two E1 services goes down.

Solution [ The phenomena has been described to customer as the Six configured E1 services from the begining were using the channels [ 1,2 ,3,4 , 12,13] out of avalible 24 channels because 32-QAM was supporting this number of E1 services at that time , but when the lowest modulation changed to QPSK , only 10 E1 services as maximum is supported , thus the configured last two E1 trails services on channels 12 , 13 should be modified to use channels 5,6 before the modulation and the capacity is to be reconfigured. ]

Root Cause
Cause - 1 - \ Problem may be related to BSS [Wireless]  not Microwave [ Transmission ].
Cause - 2 - \ Since the guranteed TDM capacity is equal to the Maximum Numberoe of E1s inService supported by the Guranteed AM modulation scheme , thus when the lowest modulation scheme degraded from 32-QAM to QPSK , also maximum number of guranteed E1 capacity will be degraded to less number.
Cause - 3- \ Huawei engineer make mistake and forget to re-configure the E1 capacity after he modify the lowest modulation schme , and this mistake could be joined with planning mistake since it need to modify the required total TDM capacity in case Modulation has been degraded.
Cause - 4 - \ No apperance of any alarms on U2000 should be checked to find the reasons.

A- Do not suppress any alarms on an existing used traffic.
B- Be sure to check the used trail slot in each link of the trail before you modify link capacity or modulation.