Disaggregating the base station means choosing where to cut the protocol stack — and the O-RAN 7.2x split cuts inside the physical layer. Where that line falls decides the fronthaul bandwidth, the timing budget and whether the radio and baseband can come from different vendors.
A traditional base station did everything in one box. Open, disaggregated RAN breaks it into three — the radio unit (RU), the distributed unit (DU) and the central unit (CU) — so they can come from different vendors and sit in different places. The question is where to cut the protocol stack between them, and there is no single right answer: every split trades fronthaul bandwidth against how much intelligence the radio needs.
Cut high in the stack and the radio is complex but the link between them is cheap; cut low and the radio is simple but the link must carry enormous, constant bandwidth. The O-RAN 7.2x split is the industry's chosen compromise for the lower layers.
The 7.2x split cuts inside the physical layer. The RU keeps the radio-facing low-PHY — the FFT/iFFT, cyclic prefix and, crucially, beamforming — plus the RF. Everything above, the high-PHY (forward error correction, modulation, and in some variants precoding), the MAC and the RLC, lives in the DU. The CU holds PDCP and RRC above that, reached over the F1 interface.
Keeping beamforming in the RU is the whole point for massive MIMO: it means the fronthaul carries beamformed streams, not per-element samples, which is dramatically less data. The boundary of what the RU does is split into category A and category B, differing in whether precoding sits in the RU or the DU.
The legacy split (option 8, CPRI) cut below the PHY entirely: the RU was a dumb radio and all the PHY ran in the DU, so the fronthaul carried raw time-domain samples. That bandwidth is fixed and huge — it does not care whether one user or a thousand are active — and it scales terribly with massive MIMO's antenna count.
7.2x moves the low-PHY into the RU, so the fronthaul carries frequency-domain, beamformed data that scales with actual traffic. It needs far less bandwidth, which is what makes fibre fronthaul to many radios affordable — and it is carried over eCPRI (Ethernet-based) rather than CPRI, with block floating-point compression to shrink it further.
Splitting the PHY across a network link means the two halves must agree on time to a tiny fraction of a symbol. The 7.2x fronthaul needs sub-microsecond synchronisation across every radio, delivered by PTP (IEEE 1588) and SyncE. If the timing drifts, uplink and downlink collide in TDD and beamforming falls apart.
That is why a real deployment is as much about the transport and timing as the radio: the fronthaul has a strict latency budget, a fronthaul gateway often aggregates many RUs onto shared fibre, and the whole cluster trusts one timing source. Get the split and the timing right and the reward is a radio and baseband that can be sourced, placed and upgraded independently.
The O-RAN lower-layer functional split that divides the physical layer between the radio unit and the distributed unit. The RU keeps low-PHY (FFT/iFFT, cyclic prefix, beamforming) and RF; the DU takes high-PHY, MAC and RLC. It is carried over an eCPRI fronthaul.
Because the split trades fronthaul bandwidth against radio complexity. Cutting high makes the radio complex but the link cheap; cutting low (old CPRI) makes the radio simple but the link carry huge, fixed bandwidth. 7.2x is the compromise that keeps beamforming in the radio while moving the rest to the DU.
Option 8 (CPRI) cut below the PHY, so the RU was a dumb radio and the fronthaul carried raw time-domain samples at fixed, huge bandwidth. 7.2x cuts inside the PHY, so the fronthaul carries beamformed frequency-domain data that scales with traffic and needs far less bandwidth, over eCPRI.
Two variants of the 7.2x split differing in where precoding sits. In category A the DU does precoding; in category B the RU does, which can further reduce fronthaul for massive MIMO at the cost of more processing in the radio.
Splitting the PHY across a network link means both halves must share time to a fraction of a symbol. The fronthaul needs sub-microsecond synchronisation via PTP and SyncE; if it drifts, TDD transmissions collide and beamforming breaks.
eCPRI — an Ethernet-based evolution of CPRI — usually over fibre, with block floating-point compression to reduce the payload, and often through a fronthaul gateway that aggregates many radios onto shared transport.
The 7.2x fronthaul, BFP compression and sub-microsecond PTP/SyncE timing between RU and DU are where a disaggregated radio is won. Tell us the split and the transport.