PDCP — Ciphering, integrity, ordering, duplicate removal
The protocol stack
Where it sits
What it is
What it is. The sublayer that owns security, ordering and header size. TS 38.323. It is the highest of the three user-plane sublayers, and the first that a packet meets on the way down after SDAP.
The functions, from TS 38.323 clause 4.4 — abridged to the ones that matter here:
Transfer of data, user plane or control plane · maintenance of PDCP sequence numbers · header compression using ROHC or EHC · ciphering and deciphering · integrity protection and verification · timer-based SDU discard · duplication · reordering and in-order delivery · out-of-order delivery · duplicate discarding.
Security lives here, and only here. Ciphering and integrity protection are PDCP functions — neither RLC nor MAC nor the physical layer has any part in them. The scrambling in PDSCH §6.1 is not encryption, and that note says so; this is where the actual encryption is.
Reordering is needed because the layers below reorder. HARQ retransmits asynchronously (PDSCH §12.1), so a repaired block can arrive after blocks that were sent later. PDCP sequence numbers put them back in order before anything above notices.
Duplication is reliability bought with spectrum. The same PDCP PDU is sent over two carriers or two legs; whichever arrives first wins. It is the layer-2 counterpart of the URLLC configuration in PDSCH §12.3 — reliability by repetition rather than by care.
Header compression is why small packets are affordable. A voice packet's IP/UDP/RTP headers are 40 bytes on a 32-byte payload; ROHC takes them to a handful.
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: