5G E2E Protocol Testing Interview Q&A Scenario Based

5G Attach & Registration Scenarios

1. Scenario: UE is unable to attach to the 5G network.

Q: What could be the possible reasons?

A: 

  • Check PLMN selection and cell barring.
  • Verify RRC Connection Setup success.
  • Analyze NAS Registration Request & AMF response.
  • Ensure correct authentication & security mode completion.

2. Scenario: UE attaches to the network but does not get an IP address.

Q: What could be wrong?
A:

  • Check PDU Session Establishment Request/Accept messages.
  • Verify SMF & UPF communication.
  • Ensure APN/DNN is configured correctly in the UE & Core Network.
  • Debug DHCP or IPv6 SLAAC process.

🔹 RRC & NAS Layer Issues

3. Scenario: UE fails during RRC Connection Setup.

Q: How will you debug?
A:

  • Check SIB1 broadcasting.
  • Verify RRC Setup Request & RRC Setup response in logs.
  • Analyze RACH procedure (Msg1-Msg4 failures).
  • Ensure UE RLF (Radio Link Failure) is not occurring.

4. Scenario: UE fails in RRC Connection Reconfiguration.

Q: What could be the reason?
A:

  • Incorrect DL/UL resource allocation.
  • UE not supporting certain bearer configurations.
  • Inter-cell interference degrading UE reception.

🔹 Core Network & NAS Issues

5. Scenario: UE registration fails at the AMF level.

Q: How will you troubleshoot?
A:

  • Check Registration Request & Registration Reject reasons.
  • Verify SUPI/SUCI encryption.
  • Ensure 5G AKA authentication completes successfully.

6. Scenario: PDU session establishment fails.

Q: What are possible causes?
A:

  • Check SMF response (PDU Session Reject means failure).
  • Verify DNN/APN configuration in SMF.
  • Ensure UPF & DN connectivity.

🔹 Handover & Mobility Testing

7. Scenario: UE fails during handover from one gNB to another.

Q: What could be the reason?
A:

  • Check Handover Request & Handover Command messages.
  • Verify correct Xn interface setup for inter-gNB handover.
  • Ensure proper mobility event trigger (A3/A5 event reporting).

8. Scenario: Handover from 5G to LTE (EPS fallback) fails.

Q: What will you check?
A:

  • Check NAS Service Request & TAU update in LTE.
  • Verify VoLTE/IMS registration before fallback.
  • Ensure Xn/S1 handover interfaces are configured.

🔹 Data & Throughput Performance

9. Scenario: UE is experiencing low downlink throughput.

Q: How will you analyze this?
A:

  • Check DL MCS & TBS (Transport Block Size).
  • Analyze PHY Layer Modulation (64QAM/256QAM support).
  • Verify SRS & CSI-RS reports for UE scheduling.
  • Check DL buffer occupancy & HARQ retransmissions.

10. Scenario: UE experiences high latency in data transmission.

Q: What could be the cause?
A:

  • Check RTT measurements using ping.
  • Verify UPF processing latency.
  • Ensure TCP optimization settings are correct.

🔹 Security & Authentication Testing

11. Scenario: Authentication fails during UE registration.

Q: What will you verify?
A:

  • Ensure correct SUPI, SUCI encryption & keys in UDM.
  • Check 5G-AKA authentication failure messages.
  • Debug Security Mode Command responses.

12. Scenario: Integrity check fails during NAS Security Mode Setup.

Q: What could be the reasons?
A:

  • Incorrect NIA/NEA algorithm selection.
  • UE rejecting Integrity Protection activation.

🔹 Network Slicing & MEC

13. Scenario: UE is unable to get the correct network slice.

Q: What will you check?
A:

  • Verify S-NSSAI configuration in AMF & NSSF.
  • Ensure correct slice mapping at SMF level.

14. Scenario: MEC latency is high.

Q: How will you debug?
A:

  • Check UPF routing to local MEC servers.
  • Ensure Edge Computing selection policies are correct.

🔹 NSA vs SA Issues

15. Scenario: UE fails to switch from LTE to 5G in NSA mode.

Q: What are possible causes?
A:

  • Check RRC Reconfiguration for SCG Addition.
  • Verify EN-DC configuration in LTE/eNB.

16. Scenario: NSA UE is unable to utilize NR carrier for data.

Q: What could be wrong?
A:

  • Check NR-PDCP setup failure in logs.
  • Ensure NR SCG bearer activation in Core.

🔹 VoNR & IMS Testing

17. Scenario: VoNR call fails after SIP INVITE.

Q: What will you analyze?
A:

  • Check IMS registration status.
  • Verify SIP INVITE & 183 Session Progress messages.
  • Ensure QoS flow (QCI-5) is properly assigned.

🔹 5G KPI & QoS Issues

18. Scenario: Packet loss is high in 5G network.

Q: How will you debug?
A:

  • Check RLC retransmissions & MAC HARQ failures.
  • Analyze gNB buffer status reports (BSR logs).

🔹 Testing Tools & Automation

19. Scenario: You need to automate 5G test cases for regression testing.

Q: What tools will you use?
A:

  • Use Keysight UeSIM, Spirent Landslide, Viavi TeraVM.
  • Automate using Robot Framework & Python.

20. Scenario: You need to analyze NAS, RRC, and NGAP logs.

Q: What tools will you use?
A:

gNB logs for RRC & NGAP debugging.

Wireshark for NAS & NGAP.

QXDM/QCAT for UE logs.

🔹 21. Scenario: UE is unable to roam on another PLMN

Q: What could be the possible reasons, and how will you debug?
A:

  • Check if manual/automatic PLMN selection is enabled on the UE.
  • Ensure roaming agreements exist between home and visited PLMNs.
  • Analyze NAS Registration Request in logs to see if the UE sends the correct S-NSSAI and PLMN ID.
  • Verify UDM/HSS database to ensure the UE is allowed to roam.
  • Debug PDU session establishment to confirm the UE gets the correct APN/DNN.

🔹 22. Scenario: Paging is not reaching the UE

Q: What are the possible causes, and how will you troubleshoot?
A:

  • Check NGAP Paging message at the AMF level to see if the paging request is sent.
  • Verify T3513 (Paging Timer) to ensure retries are happening.
  • Ensure UE is in RRC_IDLE or RRC_INACTIVE to receive paging.
  • Debug TA (Tracking Area) updates to ensure the UE is in the correct area.
  • Check 5G DRX (Discontinuous Reception) settings in RRC messages.

🔹 23. Scenario: 5G NR PDU session is lost intermittently

Q: What will you check to debug this issue?
A:

  • Analyze NAS deregistration reason in logs (Cause: Network Failure, Re-Registration Required).
  • Check RRC connection re-establishment attempts to see if the UE tries to reconnect.
  • Verify UPF log traces to see if the session is being released due to inactivity.
  • Ensure SMF session continuity settings are configured correctly.
  • Debug gNB-AMF communication logs (N2 messages).

🔹 24. Scenario: UE is not switching to power-saving mode (C-DRX)

Q: What could be the reason, and how will you resolve it?
A:

  • Check if C-DRX (Connected Mode DRX) is enabled in RRC configuration.
  • Verify UE capability information to confirm support for DRX.
  • Ensure that eDRX (Extended DRX) and PSM (Power Saving Mode) are enabled for IoT devices.
  • Analyze DRX configuration in RRC Connection Setup Complete message.
  • Debug UE battery saving logs to ensure correct behavior.

🔹 25. Scenario: 5G SA call drops after 10 minutes of inactivity

Q: How will you debug this issue?
A:

  • Check if Idle Mode Timer (T3512) is expiring and causing UE deregistration.
  • Verify if Session Management (SM) timers in the SMF are expiring, leading to PDU session release.
  • Ensure gNB does not trigger RRC Release prematurely.
  • Debug UPF session timeout settings to prevent automatic disconnection.
  • Analyze NAS deregistration logs for UE-Initiated or Network-Initiated Release.

🔹 26. Scenario: 5G multi-carrier aggregation is not working

Q: What could be the reasons for CA (Carrier Aggregation) failure?
A:

  • Ensure UE supports the required CA bands by checking UE Capability Information.
  • Verify RRC Connection Reconfiguration includes additional carriers.
  • Check SCell (Secondary Cell) activation in MAC scheduler logs.
  • Ensure gNB supports CA configuration in EN-DC (NSA) or NR-CA (SA) mode.
  • Debug MIMO configuration & SRS (Sounding Reference Signal) reports.

🔹 27. Scenario: gNB fails to establish an N2 connection with AMF

Q: What are the possible issues, and how will you troubleshoot?
A:

  • Verify SCTP (Stream Control Transmission Protocol) connection between gNB and AMF.
  • Check NG Setup Request & Response messages in Wireshark.
  • Ensure correct PLMN ID & TAC (Tracking Area Code) is configured in gNB.
  • Debug gNB & AMF logs for Cause: Network Failure or Cause: Invalid Configuration.
  • Restart NG setup and analyze N2 interface logs.

🔹 28. Scenario: TCP throughput is lower than UDP in 5G network

Q: What could be the reason, and how will you improve TCP performance?
A:

  • Check RLC layer retransmissions & HARQ failures.
  • Analyze TCP window size and congestion control algorithms.
  • Ensure UPF supports TCP optimization features (like proxy, MSS adjustment).
  • Reduce packet loss in the RLC/MAC layers by optimizing MCS selection.
  • Debug Wireshark logs for TCP retransmissions, delays, and Zero Window events.

🔹 29. Scenario: High interference detected on 5G mid-band spectrum

Q: What are the possible causes, and how will you mitigate interference?
A:

  • Analyze RSRP, SINR, and RSRQ values to detect interference levels.
  • Check Neighboring cell overlap & PCI conflicts.
  • Adjust gNB power levels and beamforming parameters.
  • Ensure Dynamic TDD switching (for 5G TDD bands) is properly configured.
  • Use ICIC (Inter-cell Interference Coordination) techniques to reduce interference.

🔹 30. Scenario: Dynamic Spectrum Sharing (DSS) between LTE & 5G is not working

Q: What could be the reasons, and how will you troubleshoot DSS failure?
A:

  • Ensure gNB and eNB support DSS technology (MBSFN subframes, NR-PDCCH, NR-PDSCH multiplexing).
  • Verify LTE and 5G share the same carrier frequency in DSS mode.
  • Check LTE PDCCH power boosting configuration to avoid NR signal loss.
  • Debug UE logs to see if LTE and NR scheduling are handled correctly.
  • Ensure correct bandwidth allocation between LTE and NR in shared spectrum.