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Protocol Stack

Splitting control-plane processing from user-plane processing allows for more flexible and efficient network deployment. The protocol stack defines the functional entities standardized by the O-RAN Alliance, and these components—O-CU-CP, O-CU-UP, and O-DU—are collectively referred to as E2 nodes. Their roles and definitions are specified by the O-RAN Alliance to ensure interoperability and modularity across vendors.

The PHY is split between the O-RU (lower PHY for RF processing) and O-DU (higher PHY), allowing fast signal processing near the radio and centralizing less time-sensitive functions.

All O-RAN components are connected to the 5G core network (IP core). TheO-CU-CP connects to the AMF (Access and Mobility Management Function) via the N2 interface, allowing the AMF to handle UE registration, authentication, mobility management, and session management. Meanwhile, the O-CU-UP connects to the UPF (User Plane Function) via the N3 interface, handling user data routing, forwarding, and QoS enforcement.

Furthermore, the O-CU-CP, O-CU-UP, and O-DU are connected to the Near-Real-Time RIC using an E2 Interface, as described in the architectural overview.

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However, the above-mentioned protocols only offer a coarse-grained overview.
To enable a more fine-grained analysis, the protocol stack is further structured as presented in the subsequent figure.

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Within the PDCP (Packet Data Convergence Protocol) layer, the processing is further subdivided into five distinct stages:

  • Retransmission Buffer: Stores PDCP Protocol Data Units (PDUs) for potential retransmission, ensuring reliability in acknowledged modes.
  • Numbering: Assigns sequence numbers to PDUs to support in-order delivery and duplicate detection.
  • Header Compression: Reduces protocol overhead (e.g., IP headers) using schemes such as Robust Header Compression (ROHC), thereby improving spectral efficiency.
  • Ciphering: Applies encryption to protect the confidentiality of user plane and control plane data.
  • Add PDCP Header: Appends the PDCP-specific header to each PDU, carrying information such as sequence numbers and control fields.

The RLC (Radio Link Control) layer is subdivided into a High RLC and a Low RLC part.
Together, these sublayers host the following functional components:

  • Transmission Buffer: Stores incoming PDCP SDUs awaiting segmentation and transmission.
  • Segmentation: Splits larger PDCP SDUs into smaller RLC PDUs that can be efficiently transmitted over the MAC layer.
  • Add RLC Header: Appends RLC-specific headers carrying sequence numbers and control information to each PDU.
  • RLC Control: Manages operational modes (Acknowledged, Unacknowledged, Transparent), sequence numbering, and delivery guarantees.
  • Retransmission Buffer: Retains transmitted PDUs for potential retransmission in Acknowledged Mode (AM) to ensure reliability.