Centralized Unit – Distributed Unit (CU-DU) Split Base Station | 5G Systems Base Station Architecture


​There are different base station architecture types specified in 5G Systems (5GS).

  1. Standalone Base Station
  2. Non-Standalone Base Station
  3. CU-DU Split Base Station

​In this post, we will talk about Centralized Unit (CU) - Distributed Unit (DU) Split Base station architecture. 

​Centralized Unit (CU) and Distributed  Unit (DU) Split Base Station Architecture

​General Structure

​The general structure of CU-DU split architecture is shown in figure above.

Centralized Unit (CU) Supports following protocol layers

  • SDAP (Only in caes of 5G, not present in 4G LTE)
  • ​RRC
  • PDCP

Distributed Unit (DU) Supports the following

  • RLC 
  • MAC
  • Physical Layer ​​​

​Centralized  Unit (CU)

  1. ​​​​Provides support for higher layers of the protocol stack
  2. There is a single CU for each gNodeB
  3. ​CUs belonging to multiple gNodeB can be implemented using a shared hardware platform.
  4. Benefit of shared hardware platform is that cloud computing and Network Function Virtualization (NFV) can provide benefits when deploying the population of Centralized Unit (CU).

​Distributed Unit (DU)

  • ​Provides support for lower layers of the protocol stack. 
  • There can be multiple DU connected to each Centralized Unit (CU).  As an example there can be more than 100 distributed units connected to a specific centralized unit.
  • Distributed Unit (DU) includes both baseband processing and RF functions
  • Each DU is able to support one or more cells, while each cell is able to support one or more beams.
  • DU can support a range of mobility scenarios
    1. intra-gNodeB
    2. intra-DU
    3. intra-cell
    4. inter-beam
    5. intra-gNodeB
    6. intra-DU
    7. inter-cell
    8. ​intra-gNodeB
    9. inter-DU
    10. inter-gNodeB
  • ​​​DU is responsible for allocating C-RNTI to a UE during Random Access procedure
  • DU also allocates the C-RNTI during an incoming Handover procedure

​Control plane Signalling and User Plane Data in CU-DU

​Control Plane Signalling

  • ​F 1 interface connects the CU to each DU. Control plane signalling is transferred across F 1 interface.  
  • Control plane signalling is based upon F 1 Application Protocol (F1AP) as listed in 3GPP standards.
  • ​F 1AP uses SCTP over IP and supports Interface Management, UE Context Management and RRC Message Transfer procedures

​​​User Plane Data

  • ​User plane data is transferred across F 1 interface using GTP-U over UDP over IP.
  • A GTP-U tunnel is setup across the F 1 interface for each Data Radio Bearer (DRB).
  • Each tunnel is identified using the combination of source TEID, destination TEID, source IP address and destination IP address​​​​


  1. ​3GPP TS 38.473
  2. 3GPP TS 38.425
  3. 5G NR in Bullets by Chris Johnson​​​