VOLTE CSSR (Call Setup Success Rate) measures the success rate of establishing VoLTE voice calls, calculated as [L.E-RAB.SuccEst.QCI.1] / [L.E-RAB.AttEst.QCI.1] * 100. Degradation is commonly caused by X2AP procedure conflicts, S1 signaling failures, RRC connection issues, or core network processing delays.
Main Principle
X2AP Procedure Conflicts: During X2-based handovers, E-RAB (QCI 1) setup may conflict with handover procedures, leading to failures (e.g., L.E-RAB.FailEst.X2AP.VoIP). Impact: Increased E-RAB setup delay and CSSR degradation.
S1 Signaling Setup Failures: High S1 interface failures (e.g., Sum of Nr of S1 Signaling Setup Fails) directly reduce CSSR. Often linked to transmission (TX) issues or RAN site instability.
RRC Connection Failures: Weak coverage or distant UEs cause L.RRC.SetupFail.NoReply, reducing CSSR. Adjusting QRXLEVMIN and timers (e.g., T300, T301) mitigates this.
Core Network Delays: Changes in core network processing (e.g., INVITE message timing) disrupt QCI 1 bearer setup.
RRC Connection and Cell Selection Parameters
Signal Threshold Adjustment for Cell Access
Improving cell selection criteria helps edge users maintain stable access. Adjusting QRXLEVMIN raises the minimum required signal level for cell selection and reselection, filtering out weak-signal attempts that could fail.
Increasing QRXLEVMIN from -62 dBm to -58 dBm prevents distant UEs with poor channel quality from initiating connections, thus improving success rates.
MOD CELLSEL: LOCALCELLID=XX, QRXLEVMIN=-58;
MOD CELLRESEL: LOCALCELLID=XX, QRXLEVMIN=-58;RRC Timer Tuning to Handle Delays
Extending RRC timer values allows more time for message exchanges in unstable radio conditions. Increase T300 (RRC connection request timer) from 1000 ms to 1500 ms.
Increase T301 (RRC connection setup completion timer) from 200 ms to 400 ms. This reduces RRC setup failures due to delayed responses, especially for distant UEs.
MOD UETIMERCONST: LOCALCELLID=XXX, T300=MS1500_T300, T301-MS400_T301;Handover and E-RAB Conflict Management
Resolving Procedure Conflicts During Handover
Conflicts between X2-based handover preparation and E-RAB (QCI 1) setup can cause VoLTE call failures.
The parameter HoErabConflProcessingStrat controls how such conflicts are handled. Setting it appropriately avoids premature release of UE context during handover preparation, reducing E-RAB setup failures.
Mobility and QoS Parameter Configuration
Separate Mobility Settings for Voice and Data
Since VoLTE has stricter latency and continuity requirements:
Inter-frequency and inter-RAT mobility parameters should be separately configured for voice (QCI 1) and data services. Default mobility parameters are recommended for voice services to ensure continuity, while preserving optimized data parameters.
Signaling Bearer Consistency
Mobility parameters for QCI 5 (signaling bearer) can be aligned with those of the default data bearer since both remain active in RRC_CONNECTED mode.
Optimization Steps
| Parameter | Before | After | Purpose |
| HoErabConflProcessingStrat | Default | ONLY_X2_HO_ERAB_FL OW_FIRST | Prioritize X2 handover to avoid E-RAB conflicts |
| QRXLEVMIN (Cell Selection/Reselection) | -62/-64 | -58 | Improve coverage for distant UEs |
| T300 (RRC Connection Setup Timer) | 1000ms | 1500ms | Reduce RRC failures |
| T301 (RRC Connection Reconfiguration Timer) | 200ms | 400ms | Enhance stability for weak signal UEs |