5G RLC Layer Interview Questions – Expert Scenario Based (20 Q&A)
This page provides expert-level 5G RLC layer interview questions ranging from conceptual understanding to real-field troubleshooting and lab-based scenarios. These questions are designed for RAN, protocol testing, integration and L2/L3 support roles.
Conceptual & Design-Level Questions
Q1. Why is RLC layer critical in 5G architecture?
RLC ensures reliable data transfer, segmentation, reassembly and in-order delivery. It complements error recovery beyond
MAC HARQ.
Q2. Difference between RLC AM and RLC UM?
RLC AM provides reliability using retransmissions and status reporting, while RLC UM supports low-latency services without retransmissions.
Q3. Why is RLC AM needed when HARQ exists?
HARQ provides limited PHY-level recovery, while RLC AM ensures end-to-end reliability across scheduling delays and deep fades.
Q4. How is RLC mode selected?
RRC selects RLC mode based on service type, QoS requirements and latency sensitivity.
Segmentation & Data Flow Scenarios
Q5. Why does RLC perform segmentation?
When PDCP SDU size exceeds the available transport block size provided by
MAC, RLC segments data.
Q6. How does RLC reassembly work?
RLC uses sequence numbers and segmentation indicators to reassemble PDUs before forwarding to
PDCP.
Q7. What happens if a segment is lost?
In AM mode, missing segments trigger status reports and retransmissions. In UM mode, data loss occurs without recovery.
Q8. How does RLC prevent buffer overflow?
RLC manages transmission windows, buffer thresholds and flow control mechanisms.
Retransmission & Timer Scenarios
Q9. What triggers RLC AM retransmission?
Retransmissions are triggered by STATUS reports indicating missing sequence numbers or timer expiry.
Q10. Role of t-PollRetransmit timer?
This timer ensures the receiver is periodically polled for status, preventing deadlock.
Q11. Impact of incorrect RLC timer configuration?
Incorrect timers can cause excessive retransmissions, increased latency or premature radio link failure.
Q12. How does RLC avoid infinite retransmissions?
Maximum retransmission limits trigger failure handling and notify
RRC.
Real Field & Log Analysis Scenarios
Q13. Throughput is low though MAC scheduling is good – why?
RLC window stalling, retransmissions or segmentation inefficiencies can limit throughput.
Q14. How do you identify RLC retransmissions in logs?
Repeated sequence numbers, STATUS PDUs and retransmission counters indicate RLC retransmissions.
Q15. What causes RLC window stalling?
Delayed acknowledgments or lost STATUS messages can stall the transmission window.
Q16. How does MAC congestion affect RLC?
MAC congestion delays transmission opportunities, causing RLC buffer buildup and retransmission probability.
LAB & Hands-On Scenarios
Q17. In lab, packet loss observed – how to isolate RLC issue?
Compare transmitter and receiver sequence continuity and retransmission counters.
Q18. How do you test RLC AM vs UM behavior?
Introduce packet loss and observe retransmissions in AM and packet drops in UM.
Q19. Lab shows high latency – RLC role?
Excessive retransmissions, large buffers or conservative timers increase latency.
Q20. How do you design an RLC stress test?
Generate high traffic, induce packet loss and monitor window movement, retransmissions and latency KPIs.
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