CU-DU Split
Centralized Unit (CU) & Distributed Unit (DU) Functional Split
The disaggregation of a 5G gNodeB into a Centralized Unit (RRC/SDAP/PDCP) and Distributed Unit (RLC/MAC/High-PHY) over the F1 interface.
Technical Explanation
3GPP TS 38.401 standardizes the functional decomposition of the gNodeB base station: • Centralized Unit (CU): Hosts higher protocol layers. Divided into CU-CP (RRC and PDCP-C) and CU-UP (SDAP and PDCP-U) interconnected by the E1 interface. Can be centralized in regional data centers. • Distributed Unit (DU): Hosts time-critical lower protocol layers (RLC, MAC, and High-PHY). Deployed closer to cell towers to satisfy strict hybrid ARQ timing. • Radio Unit (RU): Handles RF conversion, Low-PHY, and antenna processing, connected to DU via open fronthaul.
Key Functions
- Separation of latency-critical lower layer processing (DU) from centralized control (CU)
- Independent scaling of user plane (CU-UP) and control plane (CU-CP) via E1 interface
- Standardized F1-C (F1AP) and F1-U (GTP-U) interfaces enabling multi-vendor DU/CU pairings
- Facilitating pool-gain and energy savings in Cloud RAN (C-RAN)