5G NAS Interview Questions – Expert Scenario Based (20 Q&A)
This page provides expert-level 5G NAS interview questions covering registration, authentication, security, mobility management, PDU session handling, real-field failures and lab troubleshooting. These questions are designed for Core Network, Integration, Protocol Testing and L2/L3 Support roles.
Conceptual & Architecture Questions
Q1. What is the primary role of NAS in 5G?
NAS manages signaling between UE and the 5G Core, including registration, authentication, security, mobility management and PDU session control.
Q2. How is NAS different from RRC?
NAS handles core network procedures and terminates at AMF, while
RRC
handles radio access signaling between UE and gNB.
Q3. Which network function terminates NAS?
NAS signaling is terminated at the AMF and transparently carried over RRC and NGAP.
Q4. Why is NAS independent of radio technology?
NAS allows service continuity across different access technologies without re-establishing core services.
Registration & Authentication Scenarios
Q5. What happens during 5G registration?
UE sends Registration Request, followed by authentication, security setup and context establishment with the AMF.
Q6. Why does registration fail in live networks?
Authentication rejection, subscription mismatch, AMF selection failure or security negotiation issues.
Q7. How is 5G authentication different from LTE?
5G introduces SUCI/SUPI concealment and a unified authentication framework for stronger privacy.
Q8. Why does UE repeatedly attempt authentication?
Incorrect credentials, synchronization failure or AMF–UDM communication issues.
Security & Mobility Management
Q9. When is NAS security activated?
After successful authentication, before sensitive NAS messages are exchanged.
Q10. What causes NAS security mode failure?
Key mismatch, algorithm incompatibility or COUNT desynchronization.
Q11. How does NAS support mobility?
NAS manages registration updates, AMF relocation and session continuity during mobility events.
Q12. Why does UE re-register frequently?
Unstable coverage, AMF change, timer misconfiguration or network optimization issues.
PDU Session & Field Scenarios
Q13. What is a PDU session?
A PDU session provides IP or non-IP connectivity between UE and data network with defined QoS and slice.
Q14. Why does PDU session establishment fail?
Slice unavailability, SMF rejection, policy mismatch or UPF connectivity issues.
Q15. Data stops but UE remains registered – why?
PDU session may be released, UPF path broken or policy enforcement blocking user-plane traffic.
Q16. How do you analyze NAS issues in logs?
Inspect NAS message sequence, cause codes, security headers and interaction with
RRC
and NGAP.
LAB & Hands-On Scenarios
Q17. UE registers but no data flows – lab analysis?
Check PDU session establishment, DNN/APN configuration and UPF routing.
Q18. How do you test NAS security in lab?
Trigger authentication, security mode procedures and verify encryption and integrity behavior.
Q19. Lab shows frequent PDU session release – debugging?
Review SMF policies, inactivity timers, slice availability and UE behavior.
Q20. How do you design a NAS stress test?
Trigger mass registrations, authentication attempts and PDU session setups while monitoring AMF and SMF load.
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