The Tower Is
Not the Network.
The invisible nervous system of 5G, mapped in one hour. The gNB decomposed exactly as TS 38.401 V19.2.0 wrote it — the CU hosting RRC/SDAP/PDCP, the DU's one-cell law, the CU-CP spider commanding CU-UPs and DUs over E1 and F1-C. The three transport segments and their three orders of magnitude of tolerance. TR 38.801's latency ladder quoted rung by rung — 10 ms of luxury down to the 250-microsecond floor — and the 614.4 Mbps rule that walls off raw-sample fronthaul at 12.8 terabits per sector. Then the treaty: Option 2 becomes F1, Option 7-2x becomes the open fronthaul, and everything between them becomes your career. Every definition verbatim from TS 38.401 V19.2.0 / TS 38.470 V19.1.0 / TR 38.801 V14.0.0.
Fingertips, spinal cords, brains — and the roads between them
The stage is a live transport canvas: packets race a stadium's streams across three glowing segments, the 2005 cabinet explodes into CU, DU and RU, the 20-km leash strangles C-RAN, the latency ladder's floor vanishes below the scheduler, the 614.4 wall rises to 12.8 terabits — and one URLLC packet rides home through every interface you just learned. Karaoke subtitles in English + Persian, four subtitle modes, fullscreen.
Every structure of the architecture
Each structure the video teaches — where it lives in TS 38.401 / TS 38.470 / TR 38.801, what it holds, and what to read from it. Click any row to open its full breakdown. Filter to find any of them.
| Message | Interface | Dir | What it does | Clause |
|---|
The architecture, as the spec wrote it
Everything the video teaches, as a reference you can scan — the TS 38.401 node definitions and connectivity rules, the TR 38.801 Annex A transport table, the F1 protocol stacks, all verbatim from TS 38.401 V19.2.0 / TS 38.470 V19.1.0 / TR 38.801 V14.0.0 — plus a live xHaul dimensioning playground.
| Node | Definition (verbatim essence) | Terminates |
|---|---|---|
| gNB-CU | logical node hosting RRC, SDAP and PDCP · controls the operation of one or more gNB-DUs | F1 |
| gNB-DU | logical node hosting RLC, MAC and PHY · partly controlled by gNB-CU · one or multiple cells · one cell → only one gNB-DU | F1 |
| gNB-CU-CP | hosts RRC + control-plane part of PDCP | E1 · F1-C |
| gNB-CU-UP | hosts user-plane part of PDCP + SDAP | E1 · F1-U |
The spine
- a gNB = one CU-CP + multiple CU-UPs + multiple DUs
- one DU → only one CU-CP · one CU-UP → only one CU-CP
- NG and Xn-C terminate in the CU
Many-to-many below
- one DU ↔ multiple CU-UPs · one CU-UP ↔ multiple DUs
- …only under the same CU-CP
- CU-CP wires them via bearer context management (E1AP)
Resiliency notes
- DU / CU-UP MAY reach multiple CU-CPs "by appropriate implementation"
- CU-UP ↔ CU-UP forwarding may ride Xn-U
- CU + DUs visible to the world only as a gNB
TR 38.801's own numbers, reactive — raw-sample bitrate per §11.1.4.2, delay at fiber's 5 μs/km, verdicts per Annex A Table A-1.
Bitrate: Table 11.1.4.2-1 scale (≈0.5 Gbps per port per 10 MHz, rounded) and the §11.1.4.2 theoretical maximum (614.4 Mbps per 10 MHz per port). Delay: 5 μs/km one-way + 10 μs engineering allowance per switch hop. Verdicts: Annex A Table A-1 one-way latency limits.
| Split | Required bandwidth (DL / UL) | Max one-way latency | Note |
|---|---|---|---|
| Option 1 | 4 Gb/s / 3 Gb/s | 10 ms | entire user plane distributed |
| Option 2 → F1 | 4016 / 3024 Mb/s | 1.5 ~ 10 ms | 16 / 24 Mbps assumed signalling |
| Option 3 | lower than option 2 | 1.5 ~ 10 ms | centralized ARQ |
| Option 4 | 4000 / 3000 Mb/s | ≈100 μs | we crossed the scheduler |
| Option 5 | 4000 / 3000 Mb/s | 100s of μs | intra-MAC |
| Option 6 | 4133 / 5640 Mb/s | 250 μs | UL grew past DL — soft bits fly up |
| Option 7a | 10.1–22.2 / 16.6–21.6 Gb/s | 250 μs | intra-PHY family · 7-2 → O-RAN 7-2x |
| Option 7b | 37.8–86.1 / 53.8–86.1 Gb/s | 250 μs | |
| Option 8 | 157.3 / 157.3 Gb/s | 250 μs | raw time-domain samples |
Assumptions (Table A-2): 100 MHz channel · 256QAM · 8 MIMO layers · 32 antenna ports · IQ bitwidth 2×(7–16) bit. LTE-reference values, replaced by NR when available — a 2026 massive-MIMO site is heavier.
F1-C — signaling
- F1AP → SCTP → IP → (any L2) → (any L1)
- SCTP streams: one loss stalls one lane only
- multi-homing: two paths, survives a fiber cut
F1-U — user data
- GTP-U → UDP → IP — same language as NG-U
- 32-bit TEID · UDP port 2152
- §6.3: TNL addresses are sealed envelopes to the RNL
All at the site
- F1 = a patch cable
- zero pooling gain · truck-roll upgrades
Metro mainstream
- DU ≤ 10–20 km from RUs (100 μs ÷ 5 μs/km)
- CU pooled 40–100 km upstream
Virtualized
- CU/DU as cloud workloads
- transport = the gNB's backplane
Filter — NODE · INTERFACE · NUMBER · RULE.
| Element | Family | Value / formula | Clause | Meaning |
|---|
The one sentence for this module: a base station became three logical nodes and five interfaces, the study priced every possible cut in gigabits and microseconds, the pragmatic cut won the midhaul and the precoding cut won the fronthaul — and everything between them became your transport network.
CT-XHAUL-1000 · Module 1 · xHaul Foundations & the RAN SplitFoundations mastery check
Questions and answers reshuffle every load. 70%+ before module two — every later module stands on this map.