5G PDCP Layer Interview Questions – Expert Scenario Based (20 Q&A)
This page provides expert-level 5G PDCP layer interview questions covering security, header compression, sequence handling, real-field failures and lab troubleshooting. These questions are designed for protocol testing, security validation, core–RAN integration and L2/L3 support roles.
Conceptual & Design-Level Questions
Q1. Why is PDCP layer critical in 5G?
PDCP ensures security, header compression and in-order delivery. It protects user and control data before delivery to upper layers.
Q2. What are the primary functions of PDCP?
Ciphering, deciphering, integrity protection, ROHC header compression, sequence number management and reordering.
Q3. Difference between control-plane and user-plane PDCP?
Control-plane PDCP supports ciphering and integrity protection, while user-plane PDCP supports ciphering and ROHC compression.
Q4. Why does PDCP sit above RLC?
PDCP requires fully reassembled data from the
RLC layer
to apply security and header compression correctly.
Sequence Number & HFN Handling
Q5. What is the role of PDCP sequence number?
It ensures in-order delivery, duplicate detection and correct deciphering of packets.
Q6. What is HFN and why is it required?
HFN (Hyper Frame Number) extends the PDCP sequence number space to maintain ciphering synchronization over long sessions.
Q7. What happens when PDCP SN wraps around?
HFN is incremented to keep the COUNT value unique for ciphering and deciphering.
Q8. How is PDCP COUNT value calculated?
COUNT is derived from HFN and PDCP SN and is used as input to ciphering and integrity algorithms.
Ciphering & Integrity Scenarios
Q9. When is PDCP ciphering activated?
Ciphering is activated after successful security mode command triggered by
RRC procedures.
Q10. What causes PDCP deciphering failure?
COUNT mismatch, HFN desynchronization, packet loss or incorrect key configuration.
Q11. How does integrity protection work?
Integrity protection ensures authenticity of control-plane messages using integrity keys and message authentication codes.
Q12. How do you debug integrity failures?
Verify integrity keys, COUNT synchronization, message sequence and security activation timing.
ROHC & Performance Scenarios
Q13. Why is ROHC used in PDCP?
ROHC reduces IP header overhead, improving spectral efficiency and throughput for real-time services.
Q14. What happens if ROHC context is lost?
Decompression fails and full headers must be sent again to rebuild context.
Q15. How does PDCP handle out-of-order packets?
PDCP uses reordering windows and timers to reorder packets before delivery to upper layers.
Q16. Can PDCP increase latency?
Yes. Reordering, ciphering processing and ROHC recovery can add latency if not optimized.
Real Field & LAB Scenarios
Q17. Packets drop after handover – PDCP role?
COUNT mismatch or reordering timer expiry during handover may cause packet drops.
Q18. How do you identify PDCP issues in logs?
Check PDCP SN continuity, ciphering flags, ROHC status and reordering behavior.
Q19. Lab shows deciphering failure – debugging steps?
Verify HFN, COUNT, key consistency and packet ordering across UE and gNB logs.
Q20. How do you design a PDCP stress test?
Introduce packet loss, reordering and handover events while monitoring security stability and throughput.
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