central-unit

Abstract

This module implements the gNB Central Unit - Control Plane (CU-CP) as defined in 3GPP TS 38.401 for 5G NR Open RAN architectures. The CU-CP serves as the control-plane anchor for the gNB, orchestrating radio resource control (RRC) procedures across F1-C, E1, and N2 interfaces.

The implementation follows an event-driven architecture with a central RRC task coordinating protocol handlers for NGAP (AMF interface), F1AP (DU interface), and E1AP (CU-UP interface), mirroring the ITTI message-passing paradigm established in OpenAirInterface (OAI).

System Architecture

                    ┌─────────────────────────────────────────┐
                    │              5G Core Network            │
                    │                   AMF                   │
                    └─────────────────┬───────────────────────┘
                                      │ N2 (NGAP)
                                      │ SCTP PPID=60
                    ┌─────────────────▼───────────────────────┐
                    │                                       │
                    │           CU-CP (this module)         │
                    │         ┌───────────────────┐         │
                    │         │    RRC Task       │         │
                    │         │  (orchestration)  │         │
                    │         └────────┬──────────┘         │
                    │                  │                     │
                    │    ┌─────────────┼─────────────┐      │
                    │    │             │             │      │
                    │ ┌──▼──┐      ┌───▼───┐     ┌───▼───┐  │
                    │ │NGAP │      │ F1AP  │     │ E1AP  │  │
                    │ └─────┘      └───────┘     └───────┘  │
                    └───┬──────────────┬─────────────┬──────┘
                        │              │             │
                        │ F1-C         │ E1          │
                        │ PPID=62      │ PPID=62     │
                    ┌───▼───┐      ┌───▼───┐         │
                    │  DU   │      │ CU-UP │         │
                    └───────┘      └───────┘         │

Key Architectural Components:

Component	Responsibility
RRC Task	Central orchestrator for all control-plane procedures
NGAP Handler	N2 interface toward AMF (UE context, NAS transport)
F1AP Handler	F1-C interface toward DU (RRC transfer, F1 setup)
E1AP Handler	E1 interface toward CU-UP (bearer management)
Context Manager	UE, DU, and AMF state maintenance
Transport Layer	SCTP connection management (PPID filtering)

Prerequisites

Operating System Requirements

This implementation requires SCTP (Stream Control Transmission Protocol) kernel support:

Platform	Support Status
Linux (kernel 2.6+)	Full support
FreeBSD	Full support
macOS	Partial (limitations apply)
Windows	Not supported

Dependencies

Ubuntu/Debian:

sudo apt-get update
sudo apt-get install -y libsctp-dev lksctp-tools

Verify SCTP module availability:

lsmod | grep sctp
# If not loaded:
sudo modprobe sctp

Runtime Requirements

• Go: 1.24.4 or later
• Network: IP connectivity to AMF and DU endpoints

Quick Start

Build

git clone <repository-url>
cd central-unit
go mod download
go build -o cucp ./cmd/main.go

Configuration

Create a minimal configuration file (see Configuration Reference for complete schema):

cucp:
  node_id: "0001"
  node_name: "gNB-CU-CP"
  plmn:
    mcc: "999"
    mnc: "70"
    mnc_length: 2

f1ap:
  local_address: "192.168.1.10"
  local_port: 38472

ngap:
  gnb_id: "000001"
  amf_address: "192.168.1.15"
  amf_port: 38412

logging:
  level: "info"
  format: "json"

Execute

# Using default configuration path
./cucp -config config/config.yml

# Alternative: direct execution
go run ./cmd/main.go -config config/config.yml

Expected Behavior

Upon successful initialization, the CU-CP will:

1. Load and validate configuration parameters
2. Initialize the logging subsystem
3. Instantiate the CU-CP context (UE/DU/AMF maps)
4. Bind F1AP server on configured endpoint (awaiting DU connections)
5. Establish NGAP association with AMF
6. Enter event-processing loop

Graceful shutdown is triggered via SIGINT (Ctrl+C) or SIGTERM.

Project Structure

central-unit/
├── cmd/main.go                 # Entry point, signal handling
├── config/config.yml           # Default configuration
├── internal/
│   ├── app/                    # Application lifecycle management
│   ├── common/                 # Shared utilities (FSM, logger, ASN.1)
│   ├── context/                # Protocol orchestration core
│   │   ├── protocol_*.go       # F1AP, NGAP, RRC handlers
│   │   ├── handle_*.go         # AMF, DU, UE event dispatchers
│   │   ├── amfcontext/         # AMF connection state
│   │   ├── du/                 # DU context and F1AP encoding
│   │   └── uecontext/          # UE state machine, security context
│   └── transport/              # SCTP server/client implementation
├── pkg/
│   ├── config/                 # Configuration parsing and validation
│   └── model/                  # Shared type definitions
└── docs/                       # Extended documentation

Documentation

Document	Description
Architecture	Detailed system architecture, protocol flows, and implementation rationale
Configuration Reference	Complete configuration schema with parameter descriptions
Development Roadmap	Current implementation status and planned features

Standards Compliance

This implementation references the following 3GPP specifications:

• TS 38.401: NG-RAN Architecture Description
• TS 38.413: NG Application Protocol (NGAP)
• TS 38.473: F1 Application Protocol (F1AP)
• TS 38.463: E1 Application Protocol (E1AP)
• TS 38.331: NR Radio Resource Control (RRC)

License

MIT License. See LICENSE for details.