5G NR Architecture: Nodes, Interfaces and the Protocol Stack on One Page

Most confusion about 5G is a location problem. Someone asks where QoS flows are mapped, or why a PDCP problem shows up in a different box from an RLC problem, or which node a NAS message is actually addressed to — and the answer is always a matter of knowing which layer lives where.

This page is the map. Every item on it has its own article; the point here is to see how they fit before reading any of them.

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Three domains, two boundaries

The system has three parts: the UE, the radio access network — NG-RAN — and the 5G Core.

Between the UE and the RAN is the air interface, NR-Uu. Between the RAN and the core is NG, which splits into a control-plane half towards the AMF and a user-plane half towards the UPF.

NG-RAN itself contains two kinds of node. A gNB serves NR. An ng-eNB serves LTE but connects to the 5G Core. A third name, en-gNB, is an NR node attached to the LTE core as a secondary node — which is what the 5G side of a non-standalone network actually is, and it is not part of NG-RAN at all.

The interfaces at a glance

The last two only exist inside a disaggregated gNB. The gNB article covers them properly, including why F1-U needs flow control that its LTE equivalent never did.

The user-plane stack, layer by layer

Five layers, top to bottom. Each has one job worth remembering.

SDAP maps QoS flows from the core onto data radio bearers, and marks each packet with its flow identifier. It is new in NR — LTE had no need for it, because LTE's QoS model was bearer-based all the way through.

PDCP is the security and sequencing layer: ciphering, integrity protection, header compression, reordering and duplicate discard. It also does packet duplication, sending the same PDU over two paths for reliability.

RLC handles segmentation, and in acknowledged mode, ARQ. It runs in one of three modes — transparent, unacknowledged, acknowledged — chosen per bearer. NR's RLC is leaner than LTE's: concatenation is gone, and reordering moved up to PDCP.

MAC multiplexes logical channels into transport blocks, runs HARQ, and hosts the scheduler. Random access, buffer status and power headroom reporting live here too.

PHY does coding, modulation, the physical channels and the reference signals — everything from the transport block down to the waveform.

The control plane adds two

RRC terminates in the gNB. It owns the connection: setup, reconfiguration, measurement configuration, mobility and the RRC states.

NAS terminates in the AMF, not in the gNB. It rides inside RRC over the air and inside NGAP over N2, and the gNB carries it without interpreting it. Registration, authentication and session management are NAS procedures, which is why a registration failure is diagnosed in the core rather than the RAN.

Three kinds of channel

Data changes name as it moves down the stack, and the boundaries between names are the boundaries between layers.

Logical channels sit between RLC and MAC and describe what the data is — control or traffic, dedicated or common.

Transport channels sit between MAC and PHY and describe how it is carried — its format and characteristics.

Physical channels are the actual resource-element allocations: PDSCH and PDCCH down, PUSCH and PUCCH up, plus PBCH and PRACH. How the three map onto each other in the downlink is covered in the channel mappings article.

Where the layers go when the gNB is split

A gNB can be one box, but it is specified as a logical node that may be split. The standard split point, F1, falls between PDCP and RLC.

Central unit: RRC, SDAP and PDCP. The CU itself divides again into a control-plane half and a user-plane half, joined by E1.

Distributed unit: RLC, MAC and PHY.

The logic is timing. Everything that must react within a slot — HARQ, scheduling, the physical layer — stays in the DU, close to the radio. Everything that tolerates a few milliseconds of transport delay can be centralised and pooled. Knowing that one rule predicts most of the split.

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The mental model

UE, NG-RAN, 5G Core. Air interface between the first two, NG between the last two.

SDAP maps flows, PDCP secures, RLC segments, MAC schedules, PHY transmits. One job each.

NAS terminates in the AMF. The gNB carries it without reading it.

Logical, transport, physical — the names change exactly where the layers do.

The split follows timing. Slot-level reactions stay in the DU; everything else can move to the CU.

Further reading

• 3GPP TS 38.300 — NR and NG-RAN overall description
• 3GPP TS 38.401 — NG-RAN architecture, including the CU/DU split
• 3GPP TS 37.324, 38.323, 38.322 and 38.321 — SDAP, PDCP, RLC and MAC specifications
• 3GPP TS 38.331 — RRC protocol specification
• 5G core architecture, for what sits on the other side of NG