VoLTE troubleshooting

3. Contents

4. Services
• VoLTE RAN Optimization
• VoLTE Border Optimization
• VoLTE Parameter and Configuration Audit
• VoLTE E2E Analysis
Analysis ( RAN & EPC )
• Drive logs
• UETR ( X2, S1,RRC)
• MOS scores
• S11
• Rx & Gx
• SGi
• SIP signaling
• EPC traces
Tools:
• TEMS Discovery
• ITK
• Business Object
• ERTT
• VoLTE E2E Analyzer & NEO
• WIRESHARK
• ECO
Accomplishments:
• Optimized 8000+ sites for AT&T and 1700+ sites
for SingTel
• SRVCC Border optimization
• MT RRC re-establishment Feature workaround
• DL ICIC feature testing performed
• Wining DFW KPI improvement challenge
• E2E Problem identification

5. TEMS Discovery
• Analyze layer 3
messages
• Analyze poor MOS
• Analyze VoLTE
setup failures &
drop calls
Drive data ( 3rd Party tool)
• Identify POOR MOS
calls/clips
• Fetch HOIT info
• Fetch NCM/Scanner
data
• Fetch Network type
info
ECO/ERA
• Used for automatic
tilt analysis /
recommendations
using drive data RF
conditions as input
• PCI confusion
• Parameter Audit
ERTT
• Fetch OSS parameters
• Perform Audit
VoLTE End-2-End Analyzer/
Wireshark
• Analyze UETRs & Cell
traces
• Analyze EPC traces
• Analyze IMS traces
ITK / BO
• Used to fetch counter
based statistics for
analysis and provide
supporting stats for
recommendations.

6. List of case studies
1. VoLTE Drop without RRC drop
2. RRC drop without VoLTE drop
3. QCI 1 Profile not defined resulting in VoLTE setup failure
4. MT RRC Work around to reduce VoLTE/LTE Drop
5. Incorrect MCS assignment Causing VoLTE Drop
6. Improvement in Retainability with DL ICIC feature activation & MT RRC Work around
7. Improvement in Retainability and SRVCC SR with change in parameter
“b2threshold2rscpUtraoffset”.

7. 1
2
4
3
2 3
5

8. 1
4
3
2
UE
2 2
1 1 1
4
Cell Z did not have RLC - UM feature for VoLTE bearer ( QCI 1) activated

9. • RLC mode mismatch in source & target cell
• PDCP SN length for QCI 1 mismatch in source & target cell
• RLC SN length for QCI 1 mismatch in source & target cell
• RoHC feature must be activated on both source & target cell
› In some cases it was observed that RLC mode /PDCP / RLC SN length mismatch was observed in
configured value & value found in layer 3 messages
SOLUTION:
– Site should be locked & unlocked after the parameter changes to take in effect.

10. There is time offset of 6Hrs b/t drive logs & UETR
1
2
1
2
3
3
2
1

11. 4.
5.
6.
7.
4
5
4
7
Old
Cell
New
Cell
6
7

12. • RLF can be UE detected or ENB detected
• If the RLF is UE detected & if UE performs a successful RRC connection
before MME releases the UE context then VOLTE call will continue
• If the RLF is ENB detected then ENB will request MME to release the
bearers resulting in VoLTE drop.
It is important to note that RRC drop alone do not
conclude that the VoLTE call has dropped.
Further analysis on bearer level needs to be performed
to ascertain if VoLTE call dropped.

13. 1.
2.
3.
1
2
3
CAUSE: QCI-1 Profile was not activated in the site.
1
1
1
2
3

14. 1.
2.
3.
4.
5.

15. 1.
2.
UE dropped on first carrier & latched on to second carrier cell
3.
4.
This will always result in VOLTE DROP
1
2
3
PENDING
4
1
5
4
4

16. ›
›
›
›
›
›
›
Parameter Value
connectedModeMobilityPrio -1
voicePrio -1
a5Threshold1Rsrp (cell level) -112
a5Threshold2Rsrp (cell level) -112
a5Threshold1RsrpAnrDelta (cell level) 10
a5Threshold2RsrpAnrDelta (cell level) 1
IsRemove Allowed FALSE

17. 1
1
1
1.
2.
3.
4.
2
2
3
3
4

18. ›
›
›
›
Solution:
ENB software was upgraded to L13B EP14 which
fixed this issue
Green CL
drive stopped.
L13B EP 14 Software rollout

19. ›
›
•
•
•
.
›
›

20. LTE Retainability pm
data
LTE Re-establishment
pm data
VoLTE Retainability
pm data
LTE Throughput pm
data
Observation:
Improvement in LTE retainability~0.02 , VoLTE~1. No. of radio link failures reduced significantly, No significant degradation
in throughput.
MARKET TREND

21. MARKET TREND – DRIVE DATA

22. ›
›
›
›

23. Observation: Counter Based
Retainability shows slight improvement after the changes . SRVCC attempt reduced as expected and SRVCC success
rate improved when compared to same amount of attempts before.

24. 1. Normal RRC connection release need not necessarily mean not end to VoLTE call. Mismatch in the below
parameters will lead to bearer drop.
 rlcMode
 rlcSnLength
 pdcpSnLength
 rohcEnabled
2. It is important to note that RRC drop alone do not conclude that the VoLTE call has dropped. Further
analysis on bearer level needs to be performed to ascertain if VoLTE call dropped.
3. QCI 1 profile must be defined to enable VoLTE services.
4. Multi Target RRC Connection re-establishment will not be successful (L13B) if neighbors are not defined in
both the directions. The problem could be more severe when layering strategy does not allow handover
from one carrier to other(IFHO). A work around is possible to reduce the impact of the limitation.
5. Incorrect MCS assigned by ENB to Rel 8 UE was causing the VoLTE drops in poor SINRs, this was a eNB
software bug and was rectified by EP14 rollout. A significant improvement in retainability was observed.
6. DL ICIC feature shows good improvement in VoLTE Retainability.
7. QCI 1 specific “b2Threshold2RscpUtraOffset” helps to improve VoLTE retainability as well as SRVCC
performance. Offset value of 10dB showed good results.