RLC
Radio Link Control: a Layer 2 sublayer that provides segmentation/reassembly, duplicate detection, and ARQ retransmission in acknowledged mode for reliable data transfer.
RLC sits between PDCP and MAC and adapts data to what the radio can actually carry, chiefly through segmentation and reassembly — breaking PDCP packets into pieces sized for the transport blocks the scheduler grants, and stitching them back together at the far end. NR simplified RLC from its LTE form: concatenation moved out, and reassembly was streamlined to cut processing latency.
The part that defines its behaviour is the mode. RLC runs in one of three:
- Transparent Mode (TM) — no overhead at all, used for broadcast and some control channels.
- Unacknowledged Mode (UM) — segmentation and reassembly but no retransmission, for delay-sensitive traffic like voice where a late packet is useless anyway.
- Acknowledged Mode (AM) — adds ARQ retransmission for reliable delivery, used for most user data.
AM is the interesting one: it provides a slower, second line of error recovery behind MAC's fast HARQ. HARQ catches most failures quickly; the rare ones it misses fall through to RLC ARQ, which retransmits based on status reports. Two loops, different speeds, complementary jobs.
Related terms
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