RLC — Radio link control — segmentation and ARQ
The protocol stack
Where it sits
What it is
What it is. The sublayer that owns segmentation and reliable delivery over the radio link. TS 38.322. It sits between PDCP and MAC and offers three different levels of care.
| Mode | Clause | What it does | Used for |
|---|---|---|---|
| TM — transparent | 4.2.1.1 | Nothing. No header, no segmentation, no retransmission. | Broadcast and paging — where there is nobody to retransmit to |
| UM — unacknowledged | 4.2.1.2 | Segments and reassembles, numbers the pieces, but never retransmits. | Voice and real-time video — a late packet is worse than a lost one |
| AM — acknowledged | 4.2.1.3 | Segments, numbers, and retransmits what was not acknowledged, using status reports. | Anything that must arrive — signalling, file transfer, web |
Why a second retransmission mechanism, when HARQ already exists? Because they fail differently, and each covers the other's blind spot.
HARQ is fast and fallible. It settles in hundreds of microseconds (PDSCH §12.3) but its feedback is a single bit sent over a noisy uplink. A NACK misread as an ACK loses the block silently, and the design expects to fail one time in ten (PDSCH §10.1).
RLC AM is slow and thorough. Its status reports carry sequence numbers, so a gap is unambiguous. It catches what HARQ dropped, at the cost of a round trip measured in milliseconds.
Two layers, two timescales. HARQ handles the common case cheaply; RLC handles the rare residual case correctly. Neither alone would do.
Read on
This concept was first written up in ref-stack, which reads the whole group as one argument.
Before this concept, the hierarchy says to learn the following — the full chain, in order: