PDCP
Packet Data Convergence Protocol: a Layer 2 sublayer providing header compression, ciphering, integrity protection, and in-sequence delivery for both user and control plane data.
PDCP sits at the top of Layer 2 (just below SDAP in the user plane) and bundles several jobs that are easy to overlook until one of them breaks. It does ROHC header compression to shrink the IP/UDP/RTP overhead on small packets like voice. It applies ciphering and integrity protection — and in NR, integrity protection extends to the user plane, not just signalling. It handles in-sequence delivery and duplicate detection, and it reorders packets that arrive out of order from the layer below.
Its quiet importance shows up in mobility and reliability. During handover, PDCP is where data forwarding and the sequence-number continuity happen so packets aren't lost or duplicated across the switch. In dual connectivity, PDCP is the split point — one entity feeds two RLC legs (on the master and secondary nodes) and reassembles the result. It also supports PDCP duplication, sending the same packet down two legs for ultra-reliable delivery. So when you debug a handover that drops packets or a duplication feature that isn't helping, PDCP is the layer in frame.
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