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
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