O-RU Start-Up Procedure in O-RAN- From Power-On to Active State

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In the Open RAN (O-RAN) deployments, the O-RU (O-RAN Radio Unit) plays a critical role as the front-end element that connects with mobile devices over the air. Before it can transmit or receive, the O-RU must go through a carefully orchestrated start-up sequence, as defined by O-RAN Alliance  Working Group 4 (WG4) specifications.

The start-up sequence also referred as O-RU Management Plane (M-Plane) procedure where it defines how the O-RU initializes, discovers the O-DU (Distributed Unit), establishes a secure communication, and transitions into an operational, ready-for-service state.

WG4 has standardized this call flow to ensures multi-vendor interoperability, efficient onboarding, and reliable performance monitoring for the .

In this article, we breaks down the O-RU start-up process step by step, from power-on to operational state which is basically carrier active and transmitting state. At high level the complete start procedure is consist of eight steps sequence shown in following figure.

complete start procedure is consist of eight steps sequence

ORU M-Plane Connectivity

ORAN specification has defined two configuration model for O-RU management namely Hierarchical model and Hybrid model. In O-RU M-Plane, the O-DU and NMS are used to manage the O-RUs using NETCONF protocol. O-DU, NMS acts ad NETCONF clients while O-RUs acts as NETCONF servers as shown in following diagram. 

two configuration model for O-RU management namely Hierarchical model and Hybrid model

ORU Pre-condition for Startup Procedure

  • Power-ON for O-RU/NETCONF Server or O-RU restart operation.
  • Power-ON for O-RU controller/NETCONF Client(s) and/or pnfRegistration event-collector
  • Physical interfaces are connected

ORU Startup Procedure Details

Overall Start up procedure of an O-RU as per WG4 specification is shown in following picture and explained below in details.

Overall Start up procedure of an O-RU

1. ORU Power-On and Transport Initialization

The procedure begins when the O-RU is powered on or newly connected to the O-DU (O-RAN Distributed Unit). The device initializes its Ethernet interface and starts scanning for the appropriate VLANs configured for O-RAN management traffic. There are following two methods available to initialize the O-RU M-Plane Transport Layer:

  1. Manual Configuration (alternative)
  2. Automatic Configuration (alternative)
    1. Stateful DHCP (alternative)
      1. IPv6 (option)
    2. Stateless DHCPv6 (alternative)

At this stage, the O-RU sends out a DHCP DISCOVER message using its MAC address. This step is critical because the O-RU does not yet know its management IP or the location of the controller (SMO or O-DU).

Manual Configuration and Automatic Configuration of ORU M-Plane Transport Transport

2. IP Address Assignment (DHCP)

The DHCP server responds with a set of configuration parameters. These configuration parameters typically includes following information:

  • Management-plane IP address for the O-RU
  • Default gateway and subnet mask
  • DNS server details
  • Vendor class identifier (to identify the device type)
  • O-DU/SMO address or FQDN (via DHCP option fields)

This process make sure that the O-RU becomes a known entity on the network and can establish higher-layer communication. O-RU shall identify itself to DHCP servers by using DHCP option(s) using the vendor-class-data string within the o-ran dhcp YANG model.

DHCPv4 Vendor Class Option:

  • Vendor Class Identifier Option 60: string

The format of the vendor class string shall be configured to one of the following three options:

  1. ““o-ran-ru2/<vendor> , e.g., “o-ran-ru2/vendorA”
  2. “o-ran-ru2/<vendor>/<product-code>”, e.g., “o-ran-ru2/vendorA/ORUAA100”
  3. “o-ran-ru2/<vendor>/<product-code>/<serial number> ”,e.g. , “o-ran ru2/vendorA/ORUAA100/FR1918010111”

3. NETCONF over SSH Session Establishment

When IP address in place with O-RU, it sets up a secure NETCONF over SSH session to the O-DU or SMO, acting as the M-plane controller. This procedure also know and NETCONF Call home given RFC standard RFC8071 .

the O-RU sets up a secure NETCONF over SSH session to the O-DU or SMO, acting as the M-plane controller.

In this communication, authentication is handled using X.509 certificates. Mutual authentication is mandatory—ensuring that both the O-RU and the controller can trust each other. This step provides security and prevents unauthorized devices from connecting to the O-RAN network.

4. O-RU Capability Exchange

Once the secure communication channel is established, the O-RU shares its capability set using standardized O-RAN YANG models. This capability set includes following information:

  • Supported frequency bands and channel bandwidths
  • Transmit power range and antenna configuration
  • Beamforming support (if available)
  • Timing options (PTP, SyncE)
  • Software and hardware version details

The O-DU/SMO uses this information to determine how best to configure the O-RU within the broader RAN setup.

5. O-DU Configuration of O-RU

Next, the O-DU sends down the required configuration via NETCONF. This includes:

  • Transport parameters – VLAN IDs, IP addresses, and UDP ports for C/U/S-plane traffic
  • Security settings – trust anchors, TLS/SSH keys
  • Radio settings – carrier frequency, channel bandwidth, PCI, and other cell parameters
  • Synchronization Details – clocking source, PTP profiles, SyncE configurations

This stage essentially programs the O-RU with the operational blueprint it needs to function in harmony with the O-DU.

6. Synchronization Setup

Synchronization is a most important part of mobile networks. The O-RU now locks to the synchronization source provided by the O-DU or external timing system: It typically uses

  • PTP (IEEE 1588v2) for phase and time sync
  • SyncE (Synchronous Ethernet) for frequency sync

Only once the O-RU achieves stable synchronization can it move toward activating its radio carriers.

7. Control-Plane and User-Plane Setup

With configuration and synchronization in place, the O-RU sets up C-plane (control) and U-plane (user data) sessions with the O-DU. These sessions often use eCPRI (enhanced Common Public Radio Interface) or RoE (Radio over Ethernet), depending on deployment.

This step ensures that signaling, scheduling, and actual user data can flow between the O-DU and the O-RU.

8. Operational or Carrier Active State

Finally, the O-RU transitions to the Operational/Carrier Active state. The O-DU can now schedule downlink transmissions, receive uplink signals, and begin actual RAN operations. At this point, the O-RU is fully integrated into the O-RAN ecosystem.

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