The 5G architecture, every piece explained.
RAN splits, CU-CP / CU-UP / DU / RU breakdown, every interface (NG-C, NG-U, F1-C, F1-U, E1, Xn, X2, S1, NAS, N1–N6), full protocol stack, and radio architecture. Vendor-neutral. 3GPP TS 38.401, 38.410, 38.473, 38.463 aligned.
Four ways to physically deploy the same gNB
Every gNB is logically the same. The choice is where the functions physically live — and which fiber/transport you light between them.
Distributed RAN
All baseband functions (CU + DU + RU) live in one box at the cell site. The classic deployment — simple, no fronthaul.
Centralized RAN
RU at the cell site, BBU (CU+DU) pooled in a central office. Fronthaul is CPRI/eCPRI fiber. Vendor-locked.
Virtualised RAN
CU and DU run as software on commodity x86/ARM servers (OpenShift / K8s). Still single-vendor stack, but on COTS hardware.
Open RAN (7.2x split)
Open standardised interfaces (Open Fronthaul 7.2x, F1, E1, A1, E2). Multi-vendor: pick best-of-breed RU, DU, CU, RIC.
Every box, what it does, where it lives
Each card is a real 3GPP network function — defined in TS 38.401 (RAN) and TS 23.501 (Core).
gNB-CU-CP
Terminates RRC toward the UE, PDCP control plane, NGAP to AMF. Holds UE context. Speaks F1-C to DU, E1 to CU-UP. TS 38.401
gNB-CU-UP
QoS-flow → DRB mapping (SDAP), PDCP user plane, ciphering. Speaks F1-U to DU and NG-U to UPF. TS 38.401
gNB-DU
Lower layers: RLC ARQ, MAC scheduler, PHY high (coding, modulation, layer mapping). Speaks F1 to CU, FH to RU. TS 38.401
O-RU / RRU
iFFT/FFT, beamforming weight application, DAC/ADC, PA/LNA, antenna feed. O-RAN 7.2x split definition. O-RAN WG4 CUS
Access & Mobility Function
UE registration, idle/connected mobility, authentication anchor, NGAP termination to gNB. TS 23.501 · 24.501
Session Management Function
PDU session establishment, IP address allocation, QoS rules to UPF via N4 PFCP. TS 23.501 · 29.502
User Plane Function
The data-plane workhorse — GTP-U termination, packet inspection, traffic steering, QoS enforcement (QER), usage reporting. TS 29.244
NRF · UDM · UDR · AUSF · PCF · NSSF · SEPP · NEF · BSF · CHF · NWDAF · SCP
Discovery, subscriber data, authentication, policy, slice selection, border gateway, exposure, charging, analytics — every NF talks over HTTP/2 + JSON.
Every wire between every box
Filter by plane. Each row is a 3GPP-defined reference point with its protocol stack and spec.
| Interface | Endpoints | Protocol Stack | Carries | Spec |
|---|---|---|---|---|
| F1-C CP | gNB-DU ↔ gNB-CU-CP | F1AP / SCTP / IP | UE-context setup, RRC transfer, paging, error indication | TS 38.473 |
| F1-U UP | gNB-DU ↔ gNB-CU-UP | NR-U / GTP-U / UDP / IP | User-plane PDUs between DU and CU-UP (DRB data) | TS 38.425 |
| E1 CP | gNB-CU-CP ↔ gNB-CU-UP | E1AP / SCTP / IP | Bearer-context create/modify, security keys, GTP-U TEIDs | TS 38.463 |
| Open FH 7.2x · M-Plane MP | O-DU ↔ O-RU | NETCONF / SSH / TCP | RU config (cells, carriers, MTU, syncE), operations | O-RAN MP.0 |
| Open FH 7.2x · CUS-Plane UP | O-DU ↔ O-RU | eCPRI / Ethernet | IQ samples, beamforming weights, U-plane & C-plane & S-plane | O-RAN CUS.0 |
| NG-C CP | gNB ↔ AMF | NGAP / SCTP / IP | UE-context, PDU-session resource setup, paging, handover | TS 38.413 |
| NG-U UP | gNB ↔ UPF | GTP-U / UDP / IP | User-plane PDUs between RAN and UPF (QFI marking) | TS 29.281 |
| Xn-C CP | gNB ↔ gNB | XnAP / SCTP / IP | Inter-gNB handover, dual connectivity, secondary node info | TS 38.423 |
| Xn-U UP | gNB ↔ gNB | GTP-U / UDP / IP | User-plane forwarding during inter-gNB handover | TS 38.424 |
| N1 CP | UE ↔ AMF (over Uu+NG-C) | NAS-5GS / RRC / PDCP / RLC / MAC / PHY | Registration, authentication, PDU-session NAS messages | TS 24.501 |
| N2 CP | gNB ↔ AMF (same as NG-C) | NGAP / SCTP / IP | Logical reference point name for NG-C in core view | TS 23.501 |
| N3 UP | gNB ↔ UPF (same as NG-U) | GTP-U / UDP / IP | Logical reference point name for NG-U in core view | TS 23.501 |
| N4 CP | SMF ↔ UPF | PFCP / UDP / IP | PDR / FAR / QER / URR rule installation, session-report | TS 29.244 |
| N6 DN | UPF ↔ Data Network | IPv4 / IPv6 / Ethernet | Internet / enterprise data network reach-out | TS 23.501 |
| N9 UP | UPF ↔ UPF | GTP-U / UDP / IP | Inter-UPF (home-routed roaming, anchor / I-UPF chaining) | TS 23.501 |
| X2-C CP | eNB ↔ eNB / eNB ↔ en-gNB | X2AP / SCTP / IP | LTE inter-eNB handover, EN-DC secondary-node modification | TS 36.423 |
| X2-U UP | eNB ↔ eNB | GTP-U / UDP / IP | User-plane forwarding during LTE handover or split bearer | TS 36.425 |
| S1-MME CP | eNB ↔ MME | S1AP / SCTP / IP | LTE control plane: attach, bearer setup, paging, handover | TS 36.413 |
| S1-U UP | eNB ↔ S-GW | GTP-U / UDP / IP | LTE user-plane PDUs | TS 36.414 |
| S6a MP | MME ↔ HSS | Diameter / SCTP / IP | Authentication vectors (AKA), subscriber profile, AVPs | TS 29.272 |
| A1 MP | Non-RT RIC ↔ Near-RT RIC | HTTPS / REST / JSON | Policy & ML model deployment from rApp → xApp | O-RAN A1 |
| E2 MP | Near-RT RIC ↔ E2 Node (gNB/eNB) | E2AP / SCTP · KPM, RC, NI service models | RAN metrics, control commands (handover override, slicing) | O-RAN WG3 |
| O1 MP | SMO ↔ O-RAN managed elements | NETCONF / YANG | FCAPS — config, fault, performance, security | O-RAN O1 |
Control plane & user plane, layer by layer
Click any layer to see what it does. Aligned to 3GPP TS 38.300 §6.
Control Plane UE ↔ AMF
User Plane UE ↔ UPF
From digital baseband to the antenna feed
The physical side of 5G — what actually hits the air.
Antenna & Massive MIMO RF
32T32R, 64T64R, 128T128R cross-pol arrays. Antenna elements at λ/2 spacing.
- Active antenna array (AAA / AAS)
- Beamforming weights at the RU
- Digital + analog hybrid beamforming
- SRS-based reciprocity at TDD
RF chain (PA · LNA · ADC) PHY-low
Power amp, low-noise amp, mixer, filter, ADC/DAC. Defines EIRP, noise figure, ACLR.
- PA backoff for OFDM PAPR
- Digital pre-distortion (DPD)
- Calibration loop · temperature comp
- Output power class (FR1 200W · FR2 25dBm/element)
OFDM & numerology PHY
μ = 0…6 → 15 / 30 / 60 / 120 / 240 / 480 / 960 kHz SCS. CP-OFDM down, CP-OFDM or DFT-s-OFDM up.
- FR1 sub-6 (μ=0 or 1)
- FR2 mmWave (μ=2 or 3)
- FR2-2 52.6–71 GHz (μ=5,6)
- Numerology drives slot duration & latency
Channel coding L1
Data channels use LDPC; control channels use Polar. Both are rate-matched, soft-buffered, HARQ-combined.
- LDPC for PDSCH / PUSCH
- Polar for PDCCH / PUCCH long
- RM (Reed-Muller) for short control
- HARQ-IR with up to 16 processes
Synchronization & sync sources PHY
Tight time-frequency sync end-to-end. eCPRI fronthaul needs sub-µs alignment.
- GNSS (GPS / GLONASS) at RU
- SyncE for frequency
- PTP IEEE 1588 for time-phase
- SSB-based UE downlink sync
Spectrum & bands FR
n1 / n3 / n7 / n28 / n41 / n77 / n78 / n79 / n257 / n258 / n260 / n261 — 3GPP-defined NR bands.
- FR1: 410 MHz – 7.125 GHz
- FR2: 24.25 – 52.6 GHz
- FR2-2: 52.6 – 71 GHz (Rel-17)
- TDD or FDD per band, BWP-based
Now run it live.
Apply this architecture inside our hands-on 5G Core Lab and the cinematic 5G Advanced Core video course.