There are different base station architecture types specified in 5G Systems (5GS).
- Standalone Base Station
- Non-Standalone Base Station
- CU-DU Split Base Station
In this post, we will talk about Centralized Unit (CU) - Distributed Unit (DU) Split Base station architecture.
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)
Distributed Unit (DU) Supports the following
- Physical Layer
Centralized Unit (CU)
- Provides support for higher layers of the protocol stack
- There is a single CU for each gNodeB
- CUs belonging to multiple gNodeB can be implemented using a shared hardware platform.
- 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
- 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
- 3GPP TS 38.473
- 3GPP TS 38.425
- 5G NR in Bullets by Chris Johnson