Open the pink line.
Down to the last bit.
In Chapter 1 the Open Fronthaul was one pink cable. Here we split it open. Where you cut the protocol stack decides your radio's cost, its power, and the fiber you rent — so we climb the split ladder, prove why 7-2x won, place every PHY block inside the O-RU, decode an eAxC ID bit-by-bit, and compute the real gigabits. Built verbatim from 3GPP TR 38.801 §11 and O-RAN WG4 CUS.0 v21.
Functional splits & the 7-2x fronthaul — the full story
The player screen is a live animation stage — a tower rises, the fronthaul lights up, the split ladder assembles, IQ streams flow down the fiber and the bandwidth pipe fills, exactly as the narration reaches each idea. Karaoke subtitles in English + Persian, four subtitle modes, fullscreen.
Split & bandwidth simulator
Dial a radio and watch its Open-Fronthaul line rate. The formula: occupied subcarriers × symbol rate × 2 (I+Q) × bits/sample × streams, then link overhead. Category A ships antenna streams; Category B ships data layers.
Eight ways to cut a base station — slide the cut, watch the trade-off
One horizontal line across the stack. Cut high → cheap link, smart radio, relaxed latency. Cut low → fat link, simple radio, brutal latency. Pick an option: the stack splits into O-DU / O-RU, the fronthaul re-sizes, and the three meters move against each other. O-RAN lives at 7-2x.
In the 7-2x split the O-DU holds High-PHY (channel coding, rate matching, scrambling, modulation, layer mapping, and — for Category A — precoding), while the O-RU holds Low-PHY: (i)FFT, cyclic-prefix add/remove, PRACH filtering, digital beamforming (Cat B) and the RF front end. The fronthaul therefore carries frequency-domain IQ already mapped to subcarriers — the key reason its rate is far below CPRI's raw time-domain stream.
Every flow's postal code — 16 bits, four fields
The eAxC (extended Antenna-Carrier) ID rides in every C-plane and U-plane message. Its 16 bits are partitioned into four sub-fields whose widths are configured per deployment and must sum to 16.
| Sub-field | Names | Typical width | Meaning |
|---|---|---|---|
| DU_Port_ID | which O-DU processing port / spatial stream | 2–4 bits | ties the flow to a DU pipeline |
| BandSector_ID | band + sector | 3–6 bits | which physical sector on the tower |
| CC_ID | component carrier | 1–4 bits | which carrier in CA |
| RU_Port_ID | spatial stream / antenna at the O-RU | 4–8 bits | which stream inside the radio |
The O-DU and O-RU must agree on the bit-widths of each sub-field (o-ran-uplane-conf). If the O-DU packs [4|4|4|4] and the O-RU expects [2|6|2|6], every eAxC decodes to the wrong stream and the carrier never keys up — a classic first-integration failure you will meet in M6/M9.
Wrapping IQ for a switched network
| Encapsulation | Stack | When | O-RAN status |
|---|---|---|---|
| eCPRI over Ethernet | Eth ▸ (VLAN) ▸ eCPRI ▸ eAxC ▸ IQ | the default L2 fronthaul | dominant ✓ |
| IEEE 1914.3 (RoE) | Eth ▸ RoE ▸ IQ | radio-over-Ethernet mapper | permitted |
| eCPRI / IQ over UDP/IP | Eth ▸ IP ▸ UDP ▸ eCPRI ▸ IQ | routed / L3 fronthaul | permitted |
The C-, U- and S-planes share the switch, so they are separated by VLAN and prioritized by 802.1Q PCP / DSCP — S-plane (PTP) and C-plane must never queue behind bulk U-plane IQ. Because one slot of IQ can exceed an MTU, CUS.0 §5.5 defines fragmentation (eCPRI concatenation/segmentation, or IP fragmentation) so large transfers split and reassemble in order. Get VLAN + PCP wrong → intermittent timing alarms that look like a bad radio.
🛠 Engineer detail block — what you'll actually meet in the field
- 25 GbE is not always enough. A 3-sector 64T64R site sending naive antenna streams wants >100 Gb/s per sector. Category B precoding + 9-bit BFP is what makes it fit on 25G optics — the bandwidth math chooses your radio.
- Category is the first purchase question. Cat A = precoding in the O-DU (≤ 8 streams, simple macro). Cat B = precoding in the O-RU (mMIMO). Ask it before latency, before price.
- eAxC bit-plan is a joint config. DU_Port / BandSector / CC / RU_Port widths must match on both ends. Mismatch = carrier never activates, and it looks like an RF fault.
- Frequency-domain, not time-domain. 7-2x ships subcarrier IQ, which is why it beats CPRI (option 8). If someone says "CPRI fronthaul", they do not mean Open Fronthaul.
- S-plane priority is sacred. The single most common fronthaul rookie error is letting U-plane bulk starve PTP. VLAN + PCP first, always.