Mobility troubleshooting

What the specs define

• T304 is started when reconfigurationWithSync is applied. Its expiry is a reconfiguration-with-sync failure (TS 38.331 5.3.5.8.3).
• The UE then initiates RRC re-establishment and sets reestablishmentCause to handoverFailure.

Where to look in a trace

• RRCReestablishmentRequest with reestablishmentCause handoverFailure.
• The t304 value in the reconfigurationWithSync that was applied.

What the specs define

• T310 starts after N310 consecutive out-of-sync indications and is stopped by N311 consecutive in-sync indications.
• On failure the UE starts T311 and selects a cell for RRC re-establishment (TS 38.331 5.3.7 and 5.3.10).

Where to look in a trace

• The n310, n311 and t310 values configured in the serving cell.
• reestablishmentCause in the following RRCReestablishmentRequest: reconfigurationFailure, handoverFailure or otherFailure.

What the specs define

• If the network answers RRCReestablishmentRequest with RRCSetup, the UE falls back to RRC establishment (TS 38.331 5.3.7.8).
• T311 expiry (5.3.7.6) and T301 expiry (5.3.7.7) have their own defined UE actions.
• A gNB can fetch the UE context from the old node with the XnAP Retrieve UE Context procedure (TS 38.423 8.2.4).

Where to look in a trace

• Whether RRCReestablishment or RRCSetup came back.
• The XnAP Retrieve UE Context request and its response between the two nodes.

What the specs define

• 5GMM cause values in TS 24.501: #3 Illegal UE, #7 5GS services not allowed, #9 UE identity cannot be derived by the network, #10 Implicitly de-registered, #11 PLMN not allowed, #12 Tracking area not allowed, #13 Roaming not allowed in this tracking area, #15 No suitable cells in tracking area, #22 Congestion.
• TS 24.301 defines EMM causes #11, #12, #13 and #15 with the same names.

Where to look in a trace

• The 5GMM or EMM cause IE in the Registration Reject or Tracking Area Update Reject.
• The UE state after the reject (TS 24.501 5.5.1.3, TS 24.301 5.5.3.2).

What the specs define

• T3512 (5GS) and T3412 (EPS) both have a default of 54 minutes, and the network can provide other values.
• TS 24.301 also defines a T3412 extended value IE.

Where to look in a trace

• The timer values in Registration Accept or Attach / TAU Accept.

What the specs define

• The UE runs T430 for ntn-UlSyncValidityDuration from epochTime. TS 38.331 states the UE should try to re-acquire SIB19 before the duration ends, and 5.2.2.6 covers T430 expiry.
• ntn-UlSyncValidityDuration takes values from 5 s to 900 s.

Where to look in a trace

• epochTime and ntn-UlSyncValidityDuration in the SIB19 the UE last acquired.
• The time the UE re-read SIB19, relative to the validity duration.

What the specs define

• CHO configured with only condEventD1, condEventD2 or condEventT1 and no RRM measurement event leaves cell detection to UE implementation (TS 38.331 note).
• condEventD1 and condEventT1 are Rel-17; condEventD2 is Rel-18.

Where to look in a trace

• condTriggerConfig contents and which events are configured.
• UE capability for the NTN event types.

What the specs define

• A UE in PSM is not immediately reachable (TS 23.682).
• With eDRX the UE monitors paging only during the Paging Time Window (TS 36.304).

Where to look in a trace

• T3324 and T3412 values in the Attach or TAU Accept.
• The eDRX cycle and PTW granted to the UE.

What the specs define

• Neighbouring cell measurements and measurement reporting do not apply to NB-IoT (TS 36.331).
• RRCConnectionReconfiguration-NB carries no mobilityControlInfo.

Where to look in a trace

• Whether the trace uses the -NB message variants.