ROOT CAUSE ***YSIS FOR DL TFB DROP KPI INCREASE ON A BSC

 

Product Name	Version
USN9810	USN9810 V900R013C00
BSC6900	V900R015ENGC00SPH536

 

BSC18 downlink EGPRS TBF drop rate was reported to be increased after swap from other vendor’s SGSN to Huawei SGSN.

                                                                                             Figure1   The downlink EGPRS TBF drop rate

The downlink EGPRS TBF drop rate= A9318/ A9302*100

A9318: Number of Downlink EGPRS Intermit Transfers

A9302: Number of Successful Downlink EGPRS TBF Establishments

 

BSC side ***ysis

BSC18 anlysis

1.         From the OPT log, BSC18 GB interface swap on 1^st Nov

 

                                                                                                                      Figure2   OPT log

2.         The downlink EGPRS TBF drop rate increased after the GB swap:

 

                                                                                             Figure3   The downlink EGPRS TBF drop rate

3.         The TBF drop rate increased due to the A9318 Intermit Transfers increased and the A9302 almost same as before;

 

                                                                                           Figure4   A9302 successful TBF Establishment

 

                                                                                                      Figure5   A9318 Intermit Transfers

4.         The L9901 BSC received FLUSH LL PDUs was increased a lot after GB swap;

 

                                                                                            Figure6   L9901 BSC received FLUSH LL PDUs

5.         The clarification of the A9318 due to the FLUSH LL.

When the MS reselects a new cell, the SGSN initiates the cell updating procedure and sends a FLUSH LL PDU to the BSC. Accordingly, the BSC terminates all the PS services of the MS in the original cell irrespective of the MS status.

When the MS is in transmission state and the downlink EGPRS TBF exists, the BSC releases the downlink EGPRS TBF and deletes the downlink PDU buffers which are soon expired when receiving the FLUSH LL PDU from the SGSN.

Figure A shows the procedure of intermitted downlink EGPRS TBF transfer when the BSC receives the FLUSH LL PDU. When the BSC receives the FLUSH LL PDU message (see measurement point A), the counter is incremented by one.

Figure A Intermitted downlink EGPRS transfer due to FLUSH

 

6.         When the BSC receives the FLUSH LL PDU message, BSC will intermit transfers and stat the A9318, when the A9318 was increased, the TBF drop was increased too, so we need SGSN ***ysis the FLUSH LL increased reason;

Other 10 BSCs anlaysis

7.         L1 cutover others 10 BSCs from HW SGSN to HW south EPC at 4th Nov, the TBF drop almost same as before during the day time;

 

                                                                                             Figure7   The downlink EGPRS TBF drop rate

8.         The L9901 BSC received FLUSH LL PDUs were same as before GB swap;

 

                                                                                            Figure8   L9901 BSC received FLUSH LL PDUs

9.         All the 11 BSCs TBF drop rate were at the same level after GB swap to HW south EPC;

 

                                                                                             Figure9   The downlink EGPRS TBF drop rate

10.     The only different of BSC18 and other 10 BSCs was BSC18 was connected with other vendor SGSN and other BSCs connect with HW SGSN before the GB swap

HW SGSN side ***ysis

The Flush LL mechanism of the HW SGSN

While the SGSN receive the MS message, if the SGSN detect the current cell is different from the cell stored in the SGSN and the TLLI used currently has changed, the SGSN will send the two Flush LL message with Old TLLI and new TLLI to the BSC.

After the cell modification, the MS may change the TLLI when accessing the network, SGSN will send two Flush LL message with the New TLLI and Old TLLI to the BSC to find the MS context in the BSC side.

Then the LLC PDU unsent can be transferred to the new cell and sent to the MS.

The MS use the TLLI M to the service and create the MS context and LLC PDU stored. When entering the new cell and the TLLI N is used within the RAU procedure, the SGSN detect the modification of the cell:

(1).If the SGSN only send the Flush LL message to the BSC with the TLLI N, the BSC cannot find the MS context and the LLC PDU buffered just with the TLLI N. Then the LLC PDU buffered cannot be transferred.

(2).If the SGSN send two Flush LL messages with the TLLI M and TLLI N to the BSC, the BSC can find the MS context, the LLC PDU buffered can be transferred to the new cell.

Two Flush LL examples in the RAU procedure：

 Fig B:

 

 The related Flush LL protocol:

 Fig C:

 Conclusion

SGSN will send two Flush LL message to the BSC side with New TLLI and old TLLI after the cell modification. Then the context of the MS and the LLC PDU stored in the old cell can be found.

Based on the current situation, the DL TBF drop rate in the BSC18 cutover from the ZTE reach 2.5% finally.

Besides, the DL TBF drop rate in the other 10 BSCs cutover from the I9 SGSN also reaches the same level (Before the cutover, the DL TBF drop rate is the same almost).

So we suspect the mechanism in the other vendor SGSN is different, the Flush LL message is only sent with the New TLLI.

After cutover to HW USN, HW SGSN will send the Flush LL to both old and new TLLI, so the Flush LL message increased.