Navigating the open ONU landscape: Empowering fiber operators

April 11, 2024
As service providers leverage fiber in their last-mile networks, they face the challenge of providing differentiation and keeping costs in check.

By Janet Bean / Harmonic

Today’s broadband industry is racing toward fiber to retain and attract subscribers that, in many cases, perceive fiber broadband services as more reliable, faster and future-forward than cable. As the number of fiber access network deployments continues to rise, broadband service providers must differentiate and stay competitive, all while keeping costs contained.

In passive optical networks (PONs), the device at the subscriber site is called an optical network unit (ONU) or optical network terminal (ONT). PON utilizes a single optical line terminal (OLT) in the access network to connect to multiple ONUs as a point-to-multipoint fiber access technology. The ONU is responsible for transporting network traffic between the subscriber and the OLT.  Choosing the right in-premises device at the subscriber site is critical for operators because the ONU impacts the total cost of ownership and subscriber experience, which influences the operator’s bottom line.

Network operators that are flexible regarding ONU optionality profit from enhanced operational effectiveness and quality of experience for subscribers.

Current State of PON Interoperability

PON standards are defined and available to operators; therefore, they are generally considered open. Current standards have effectively achieved optical layer and physical layer interoperability between OLTs and ONUs. Yet, there are some insufficiencies in the operations, administration, and maintenance (OAM) layer and additional higher software layers.

The OAM layer manages messages between the OLT and ONU and is used to provision, operate, and alarm the ONU. The ONU software interacts with the OAM layer messaging system to provide the desired broadband services, which depend upon effectively combining and sequencing certain messages. Today, the industry lacks standardized implementation details for specific use cases, resulting in an ecosystem where each combination of OLT and ONU resembles a proprietary system.

Some standards organizations have recognized interoperability challenges for ITU-T XGS-PON and attempted to resolve issues by certifying ONUs, thereby creating the BBF.247 certification. This certification supports a subset of the more than 300 managed entities defined by the ONU management control interface (OMCI) standard. While it assures a high level of implementation to that subset, it is insufficient to guarantee interoperability with any supplier’s OLT.

In parallel, another standards body developed the DOCSIS provisioning of EPON (DPoE) standards, which are focused on specific use cases to ensure interoperability. The standards define a much smaller set of OAM messages (more than 100); however, they only support IEEE 10G-EPON today.

Since true interoperability requires significant effort, most major OLT suppliers only support their proprietary ONUs. Imposing specific ONUs on operators simplifies implementation for the vendor, considering they own both sides of the PON. However, this approach creates vendor lock-in and limits operators from balancing costs, increasing network agility and providing best-in-class service. 

What is the Open ONU Strategy?

While an Open ONU strategy is still based on the current standards for service provider use cases, the key difference is that the Open ONU approach puts the responsibility on the suppliers to ensure interoperability, effectively removing the burden from the operator. For the Open ONU strategy to work, the OLT supplier must interact directly with a broad range of individual ONU suppliers to ensure interoperability with an extensive range of ONUs available in the market. An Open ONU strategy offers operators considerable freedom and flexibility while maintaining the comfort and ease of a single hand to shake.

Flexibility, Operational Improvements and Cost Benefits

When service providers are free to choose from many different suppliers in the marketplace, they gain the flexibility to balance functionality against cost. With an Open ONU strategy, best-of-breed solutions can give operators a competitive edge when differentiating their services. For example, operators can leverage integrated home gateways with the latest Wi-Fi technology, offer user interfaces capable of negotiating up to 10 Gbps or 2.5 Gbps connectivity, or support RF video interfaces. Operators can also differentiate themselves from the competition by building a portfolio of services and tiers that cater to different subscriber use cases, all enabled by the vast landscape of ONUs.

A service provider can potentially be first to market or significantly reduce time to market by selecting the next-in-breed ONU, eliminating the need to wait for a sole vendor to develop and deliver new features or technologies. Operators also benefit from operational improvements when multiple ONU suppliers are available. If one supplier’s product availability is delayed, an alternative ONU from another vendor could be used. With an Open ONU approach, deployment is faster and easier, while subscriber service is enhanced.

Of course, financial considerations stem from the Open ONU approach that favors service providers. The service provider benefits from lower ONU costs when suppliers compete to win the service provider’s business.

The following table illustrates a generalized example of how an operator could spend roughly an additional $2.5 million for a 100,000-subscriber deployment using proprietary ONUs. The lower price for the Open ONU approach reflects the assumed impact of competitive price pressure.


Open ONU

Proprietary ONU






Quantity of ONUs per PON




Total ONU cost per PON




Total deployment size (# subs)




Total ONU cost





penalty for proprietary ONU

Open ONU also opens the door to Open OLT. When OLTs support ONUs from multiple suppliers, service providers can select an alternative or new OLT supplier without impacting the equipment used at the subscriber sites.

Open ONU Considerations
Once a service provider decides to pursue Open ONU, there are a few things to consider. Regarding suppliers, including the OLT and the ONU suppliers, each would need to work together to ensure successful integrations and deployments, as well as long-term effectiveness and upgradability. To achieve success, service providers should ask suppliers the right questions.

For example, does the OLT supplier have a committed integration and sustaining engineering program for third-party ONUs? This is important for achieving interoperability and for assuring continued services and operations in the network and across future OLT upgrades.

Also, does the ONU supplier have an integration and sustaining integration process with OLTs? In the same scope, is the ONU supplier committed to assuring continued services and operations across future ONU firmware upgrades?

Service providers deploying fiber networks must find ways to differentiate their offerings to stay competitive and attract and maintain subscribers. An Open ONU strategy provides the flexibility to select and deploy equipment that balances multiple factors regarding functionality, cost, and availability.

Best-in-breed and next-in-breed devices facilitate service differentiation, which can attract and retain subscribers. Taking advantage of ONU competition can mitigate costs. Employing multiple ONUs that support similar services can diminish operational risks such as supply chain availability. Ultimately, an Open ONU strategy offers significant benefits to empower operators navigating a competitive broadband landscape.

Janet Bean is the PON solutions architect at Harmonic.

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