In practical deployments, a 5G base station or gNB is not a monolith located at the cell site. It’s often disaggregated into Radio Unit (RU), Distributed Unit (DU) and Centralized Unit (CU). 5G allows multiple ways in which gNB functions can be split across RU, DU and CU.
One way is to split MAC sublayer into two parts: High-MAC and Low-MAC. Such a split is called Intra-MAC Split . With this split, High-MAC, RLC, PDCP and RRC will be in the CU. Low-MAC will be in the DU. The DU-CU interface is commonly called midhaul.
Multiplexing/demultiplexing are done in High-MAC. High-level scheduling decisions are part of High-MAC. Inter-cell interference coordination as needed in CoMP can be done in High-MAC in a more centralized manner. Time-critical processing such as HARQ, random access control, scheduling-related information processing and reporting are in Low-MAC.
