Terminology » O-RAN C-Plane & U-Plane Packet Fields: Structure & Function

O-RAN C-Plane & U-Plane Packet Fields: Structure & Function

5g open ran o-ran packet c-plane u-plane

Introduction: In the O-RAN fronthaul, the C-Plane (Control Plane) and U-Plane (User Plane) packet fields orchestrate how instructions and IQ data flow between O-DU and O-RU. The C-Plane delivers scheduling, beamforming and symbol timing commands, while the U-Plane carries the actual IQ samples for user data. This page explores their packet structures, fields and mapping functions in O-RAN context.

O-RAN C Plane Packet

C-Plane carries control/configuration/scheduling/beamforming etc. instructions from O-DU to O-RU. It informs O-RU how to map, process, transmit or receive IQ sample data (U-Plane).
C-Plane messages are typically carried over eCPRI encapsulated in ethernet frames (or optionally UDP/IP) between O-DU and O-RU.
Following are major fields in O-RAN C-Plane (Control Plane) packet and their purposes.

Ethernet/IP/UDP header
eCPRI header
Radio Application Common Header
- Fields : {dataDirection, payloadVersion, filterIndex, frameID/subframeId/slotId/symbolId}
Sections : Each of the sections after common header has its own header plus fields that give details on how to process specific subset of U-Plane data.
- Fields : {sectionId, sectionType, startprbc/numPrbc, numSymbol/startSymbolId, reMask, beamId, frequencyOffset}
Section extension fields (Optional)
Other control fields
- Includes filterIndex, frameStructure fields for mixed numerology
- PRACH when ‘Section Type’ corresponds to PRACH.

O-RAN U Plane Packet

In the O-RAN architecture, the User Plane (U-Plane) is responsible for transporting user data, specifically the in-phase and quadrature (IQ) samples, between the Open Radio Unit (O-RU) and the Open Distributed Unit (O-DU).
These IQ samples are encapsulated in packets that follow structured format to ensure efficient and synchronized data transmission.
U-Plane packet contents are carried over eCPRI and consists of following key components.

Ethernet Header
eCPRI header
Radio Application Common Header
Data Section Application Data Header
Data Payload

Major fields and their purposes

Following table mentions major fields of O-RAN U-Plane (User Plane) packet and their purposes.

U-Plane Packet Field	Purpose
dataDirection	Indicates the direction of data (e.g., Downlink or Uplink).
payloadVersion	Specifies the version of the payload format.
filterIndex	Identifies the filter configuration applied to the data.
frameId	Specifies the frame number for synchronization.
subframeId	Indicates the subframe within the frame.
slotId	Identifies the slot within the subframe.
symbolId	Specifies the symbol within the slot.
sectionId	Identifies the section within the data.
sectionType	Specifies the type of data section (e.g., normal PRBs, PRACH, mixed numerology).
startPrbc	Indicates the starting Physical Resource Block (PRB) for the section.
numPrbc	Specifies the number of PRBs in the section.
startSymbolId	Specifies the starting symbol ID for the section.
reMask	Resource Element mask: indicates which resource elements are used.
beamId	Identifies the beam or antenna configuration for the section.
frequencyOffset	Indicates the frequency offset for the section (e.g., for PRACH).
udCompHdr	User Data Compression Header: contains compression related information.
udSample	Represents the IQ samples for the section.