What is NGAP, and which radio protocols (RRC, PDCP, RLC, SDAP) carry traffic before it reaches it?
NGAP (NG Application Protocol) is the central control-plane signalling protocol between the gNB and the AMF, carrying NAS messages from the RAN into the core (over SCTP/IP). Before NGAP, the radio protocols handle the air interface: RRC (radio connection control), PDCP (security & ordering), RLC (reliable transfer), and — on the user plane only — SDAP (QoS mapping).
NGAP (NG Application Protocol):
- The central signalling protocol between gNB and AMF (i.e., between RAN and core)
- Transports the NAS messages from the gNB to the AMF, running over SCTP/IP
- Functions: registration processes, handover signalling, UE context information
- The NG interface is an open, point-to-point interface (multi-vendor interoperability), split into NG-C (control signalling via NGAP) and NG-U (user-data transport)
The radio protocols of the Access Stratum (before NGAP, over the air):
| Protocol | Role |
|---|---|
| RRC | controls and manages the radio connection between UE and gNB |
| PDCP | secure, reliable transfer — encryption and packet-order control |
| RLC | reliable transfer over the air — segmentation, error correction, buffering |
| SDAP | (user plane only) maps data flows to the right QoS classes — top of the U-plane stack |
The stack split (key fact): PHY ↔ MAC ↔ RLC ↔ PDCP is common to both planes. Above that they diverge: the control plane has RRC + NAS on top (NAS → AMF), while the user plane has SDAP on top (→ UPF). In the core, the radio protocols (RRC/PDCP/RLC) are replaced by IP/SCTP/NGAP; user data is tunnelled to the UPF over GTP-U.
Memory anchor: Remember the divergence — common base (PHY/MAC/RLC/PDCP), then C-plane = RRC+NAS->AMF, U-plane = SDAP->UPF. SDAP exists only on the user plane because its job is QoS-flow mapping for user traffic.
Go deeper:
5G NGAP in detail (ShareTechnote) — the gNB↔AMF control protocol message-by-message: registration, UE context, and PDU-session setup procedures over SCTP, with the call-flow diagrams.- 5G Radio Protocol Stack Architecture (ShareTechnote) — the full SDAP/PDCP/RLC/MAC/PHY stack laid out per plane, showing exactly where each layer sits relative to NGAP and the core.