The BBU is the baseband brain of a base station — the digital processing that O-RAN later pulled apart into the distributed and central units. Pooled in a C-RAN hub it serves many radios from shared compute; connected over fronthaul it drives remote radio heads and O-RUs. Understanding the BBU is understanding what the DU and CU decompose.
The BBU is the whole baseband of a base station. O-RAN decomposes that baseband into two functions with an open interface between them: the O-DU (real-time high-PHY, MAC, RLC) and the O-CU (non-real-time PDCP, RRC, SDAP). So the DU and CU together do what the BBU did, but disaggregated and multi-vendor.
Centralised RAN pools the BBUs from many cell sites into one hub, so baseband compute is shared rather than duplicated at every tower. It improves utilisation and lets cells coordinate, at the cost of needing high-capacity, low-latency fronthaul back to the radios.
Classic BBU-to-RRH fronthaul uses CPRI, carrying time-domain IQ at very high, fixed bandwidth. O-RAN’s 7.2x split uses eCPRI, which is packet-based and carries frequency-domain IQ, cutting the fronthaul bandwidth and running over standard Ethernet transport.
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