The interface between DU and RU is known as Fronthaul. When this interface allows to connect any vendor DU to any vendor RU, know as Open Fronthaul. To enable this multi vendor DU and RU interconnection some signaling formats and control messaging is required are detailed by Open Standard i.e. O-RAN Alliance as part of O-RAN fronthaul specification.
These specifications anticipated different scenarios about Distributed Unit (DU) and Radio Unit (RU) interaction, what the underlying 5G will demand, time synchronization issues between two endpoints, and it make it deployment ready for Service Providers. By addressing these issues, the O-RAN standard is ensuring inter-op between DU and RU vendors.
O-RAN Fronthaul defines following planes of operations:
- C-Plane (Control Plane): Control plane messages define the scheduling, coordination required for data transfer, beam-forming etc.
- Scheduling and beam-forming commands
- DL precoding configuration
- Mixed numerology and PRACH handling
- U-Plane (User Plane): User plane messages for efficient data transfer within the strict time limits of 5G numerologies.
- Support Data Compression
- I/Q data transfer
- DL data precoding
- S-Plane (Synchronization Plane) : Synchronization plane is responsible for the timing and sync aspects between the O-DU and O-RU. In Cloud RAN deployments, a high accurate synchronization is required between O-DU and O-RUs to achieve controlled linking for inter-O-RU sync operation for TDD, Carrier Aggregation using multiple O-RUs, MIMO, and similar processes. Using S-Plane, O-RAN fronthaul specifications support protocols such as PTP and SyncE to achieve high-accuracy synchronization on the O-RU side by synchronizing with the clock high-performance available at O-DU side.
- Synchronization Typologies
- PTP and SyncE profiles for Synchronization
- Time and Frequency Sync guidelines
- M-plane (Management Plane) : Management plane messages are used to manage the radio unit. M-Plane provides a variety of O-RU management functions to set parameters on the O-RU side as required by the C/U-Plane and S-Plane , e.g. manage O-RU software, perform fault management, etc. O-RAN fronthaul specification for M-Plane provides various parameters as data models to FCAPS functions. This data models eliminates dependence on each O-RU vendorʼs implementation and makes a real multi-vendor Open RAN possible
- Support Hierarchical/Hybrid Model
- C/U Plane IP and Delay management
- FCAPS including sync configuration and status
Protocol Stack for O-RAN Fronthaul
The O-RAN fronthaul specifications protocol stack of each above mentioned plane is shown in below picture.
- C/U-Plane, the O-RAN fronthaul specifications support a protocol stack that transmits data used by eCPRI or Radio over Ethernet (RoE) directly over Ethernet and an optional protocol stack that transmits the signals over UDP/IP
- S-Plane in O-RAN fronthaul support a protocol stack that transmits data used in Precision Time Protocol (PTP) and SyncE over Ethernet
- M-Plane support a protocol stack that transmits signals used in NETCONF over Ethernet with IP transported using TCP with Secure SHell (SSH)
- Open RAN (O-RAN) Reference Architecture
- What is eCPRI, how it contributes to 5G and Open RAN?
- O-RAN Fronthaul Spilt Option 7-2x