In a 4G Centralized/Cloud Radio Access Network (C-RAN) architecture, the BBU physically resides in a distant central office or data center.
Also in 5G the 5G RAN will evolve from the traditional BBU and Remote Radio Head (RRH) architecture used in 4G networks to a Distributed Unit (DU), Centralized Unit (CU), and Active Antenna Unit (AAU) architecture.
And as we see in the grah, These are links that are connecting the RU (Radio Unit), DU (Distributed Unit), CU (Centralized Unit) and the core network together.
In fact, Fronthaul refers to the network that connects remote radio heads to BBUs many kilometers away, while Midhaul refers to the link between the DU and CU, and finally the Backhaul is representing the link between the CU and the core network.
By disaggregating the BBU, some of its functionalities like PHY, MAC, and RLC sublayers will reside in the DU, while remaining sublayers like PDCP, SDAP, and RRC will form the CU.
Also, it is important to underlign that the 4G fronthaul interface will evolve from the existing closed and propriety Common Public Radio Interface (CPRI) to a new and open standards-based fronthaul interface, based on the O-RAN 7.2x fronthaul specification.