The 5G BTS named gNB can be divided into two physical entities named CU (Centralized Unit) and DU (Distributed Unit). The question often arises, what is CU and DU in 5G?
CU provides support for the higher layers of the protocol stack such as SDAP, PDCP and RRC. Meanwhile, DU provides support for the lower layers of the protocol stack such as RLC, MAC and Physical layer. Also, note that SDAP layer will not be present if the CU is connected to a 4G Core network. As a result, we should have 5G core network to support SDAP. To clarify, what is CU and DU in 5G is essential for understanding these protocol layers.
Practically speaking, there is a single CU for each gNB. However, one CU controls multiple DUs. For example, more than 100 DUs can be connected to one CU. In summary, what is CU and DU in 5G refers to their functional division and relationship with each other.
Each DU is able to support one or more cells, so one gNB can control hundreds of cells unlike the 4G BTS. Also, note that the interface between CU and DU is named F1 and as per 3GPP, it should be an open interface, so you connect one CU from vendor X to another DU from vendor Y. 
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