ROADMs can change the destinations of optical signals with different wavelengths, and many of them are being implemented in optical communication networks, where they act as key devices.
ROADM Technologies and Advantages:
ROADMs simplify network design, support bandwidth change requests, and improve operational efficiency through remote reconfiguration. A typical ROADM consists of multiple wavelength selective switches (WSSs), which in turn consist of a combination of multiplexers/demuxes, optical switches, variable optical attenuators (VOAs), and control modules. The functionality of ROADMs, which change the destination of optical signals without electrical conversion, gives operators the ability to optimize route design.
ROADMs enable dynamic adjustment of wavelength bandwidth based on data transmission requirements. They facilitate remote management of optical signal routing for individual wavelengths, effectively eliminating the need for manual on-site intervention by network technicians. In networks using ROADM technology, software-based automation optimizes connection configurations, ensuring efficient communication. This approach minimizes the risk of network outages caused by human error and significantly reduces the operational workload associated with network management.
ROADMs allow network operators to add, remove, or pass wavelengths as needed anywhere to ensure optimal network efficiency, since traffic routing and services can be modified without physical changes. They are easily scalable, enabling the rapid addition of new wavelengths, services, or network nodes as bandwidth demand increases. Multi-grade ROADMs, which allow connections between multiple fiber paths, can also be used to redirect traffic in the event of fiber cuts or hardware failures, providing greater network resilience and redundancy. ROADM has become a key technology in the development of high-capacity, flexible, and agile optical networks, such as 400G WDM, for demanding use cases in the cloud, 5G, and data centers.
Low-latency edge computing:
There are several architectures that network designers can implement. One example is CDC (Colorless, Directionless, and Contentionless), which enables non-blocking mesh configurations. "Colorless" adds or removes any wavelength or color on any port, "directionless" adds or removes wavelengths in any direction, and "contentionless" adds or removes wavelengths without interference. Multi-degree ROADMs and CDC are essential for enabling fast routing and switching that ensure low latency. This type of remote dynamic routing allows edge services, such as AI workloads, real-time analytics, and 5G use cases, to maintain optimal, low-latency direct paths even under high congestion or when a link fails.
ROADMs not only provide automation and maximize spectral efficiency, but they can also keep data in the optical domain for longer by passing wavelengths optically without optical-electrical-optical (OEO) regeneration. These ROADMs reduce equipment costs and decrease latency at edge nodes and data center interconnects.
Promoting Open Networks:
Since the 2010s, the adoption of ROADM technology has gained significant momentum, leading to new technical challenges. Historically, the design and production of ROADM equipment was handled by a small number of specialized hardware vendors. This lack of diversity resulted in minimal interoperability between devices from different manufacturers. Consequently, ROADM-related systems had limited ability to be flexibly integrated based on specific functional requirements or cost considerations.
The growing demand for flexible, scalable, and autonomously operable optical networks has necessitated significant efforts to "open up optical networks." These efforts encompass establishing standardized technical specifications and improving interoperability among devices from multiple vendors. Such initiatives are being actively pursued within the framework of the OpenROADM project, launched in 2015 with AT&T and other industry leaders as founding members. The project currently has more than 30 member organizations.
Figure 2: Optical network systems become open
OpenROADM focuses on standardizing optical interfaces and their specifications to ensure seamless interoperability between ROADM systems supplied by various manufacturers. This initiative also emphasizes the development of integrated management functionalities facilitated through multi-vendor software-defined network (SDN) controllers.
Among the current challenges facing ROADM systems is the limited interoperability between equipment from different vendors. Variations in network configurations between vendors often result in increased costs and operational complexity when integrating new components into existing systems.
Furthermore, reliance on a single vendor for network equipment limits competition, reduces innovation, and decreases system flexibility. This vendor dependency, commonly known as "vendor reliance," restricts overall interoperability within the network ecosystem. OpenROADM aims to address these limitations by providing a standardized framework that improves vendor-independent interoperability.
OpenROADM Specifications:
OpenROADM encompasses five types of hardware with optical interfaces, including pluggable optical modules, transponders, inline optical amplifiers, transponder/switches, and the ROADMs themselves. Combined with software-based controllers, these devices can be managed via SDN controllers that utilize a common data model and application interface (API). The open APIs allow operators to develop customized network applications, enabling features such as low latency and high reliability. By adopting the YANG language for data modeling and control methods, OpenROADM ensures compatibility and seamless integration across different vendors.
Furthermore, the non-contention, non-directive switching capabilities of ROADM architectures, when orchestrated using SDN, effectively resolve wavelength conflicts and automatically reassign paths across the network. This enhances redundancy and ensures uninterrupted data continuity, even in the event of multiple failures. Integrating Optical Performance Monitoring (OPM) with SDN control allows the controller to optimize resource allocation and avoid deploying unfeasible optical paths. Additionally, SDN provides a unified control layer, orchestrating protection and restoration schemes at both the optical layer and higher network layers, thereby maximizing network resilience and operational efficiency.
Emergence of the Open ZR+ Standard:
Although large data centers are distributed across multiple locations, the communication distance between them (data center interconnections, DCI) is kept relatively short (between 80 and 120 km) to minimize latency. Furthermore, point-to-point connections are used between data centers. The Optical Internetworking Forum (OIF) has developed 400ZR as an interface specification specifically for this DCI application. 400ZR also defines a transmission method using WDM, enabling the implementation of the Internet Protocol (IP over DWDM) through high-density multiplexed modulation.
On the other hand, the OpenROADM optical interface uses a communication protocol called the Optical Transport Network (OTN), which has been standardized by the ITU-T and can support data rates from 100 to 400 Gbps. However, the cost is also high. Although the two standards have different starting points, both use a digital coherent scheme suitable for WDM. For this reason, the hardware specifications of the corresponding optical transceivers have many elements in common, and a movement to integrate the standards has emerged. This led to the creation of the "OpenZR+" standard. This standard inherits the basic specifications of 400ZR while enabling the creation of DCI networks and longer-distance communications (around 480 km).
In December 2023, 600 Gbit/s and 800 Gbit/sa specifications were added to the OpenROADM optical interface, while the OIF published the 800ZR standard in October 2024. Specifications for 800 Gbit/sa OpenZR+ are expected to be added soon.
The Need for ROADM Testing:
To ensure interoperability between devices from different vendors, maintaining communication quality at both individual devices and the network as a whole is essential. Rigorous testing is required to guarantee communication stability, prevent errors, and improve reliability. Furthermore, real-time configuration adjustments and rapid recovery mechanisms are critical to maintaining network flexibility in the event of failures. To keep pace with rapid technological advancements, it is necessary to adopt testing solutions that can seamlessly adapt to emerging technologies and specifications.
Anritsu and OpenROADM
Figure 3: Demonstration systems and the role of Anritsu products at OFC 2024
Anritsu, in collaboration with the University of Texas at Dallas, demonstrated advancements in OpenROADM/IP over DWDM (IPoDWDM) orchestration systems at OFC 2024 and SC24. Using the YANG model—a vendor-independent network control methodology developed by OpenROADM and the IETF—their demonstration showcased integrated networks with an orchestration system managed by the University of Texas at Dallas. Two 400G ports on Anritsu’s MT1040A Network Master Pro (400G tester), equipped with integrated 400G OpenZR+ transceivers, were connected via the Add/Drop line of an OpenROADM system. This configuration allowed the orchestration system to configure channel settings while monitoring critical network performance metrics, such as bit error rate, throughput, and latency, through the MT1040A. The unified system « , facilitated by real-time assessment of ROADM route changes based on performance quality data from « , provides a robust framework for monitoring and adaptation.
Conclusion
This article has presented the ROADM, a switching device for optical communication networks, and OpenROADM, which defines interfaces to achieve interconnection between multiple providers.
The pursuit of flexibility, automation, and cost-effectiveness for the future of ROADM technology will continue to drive its evolution. Photonic integrated circuits (PICs), co-packaged optics, and advanced materials are fundamental to miniaturizing and optimizing ROADMs to meet future network demands.
A notable recent trend is the push toward higher transmission speeds. In January 2024, the OIF launched a project focused on the 1600ZR+ specification in response to market demand for improved ZR+ mode performance. Naturally, this development is expected to impact both the OpenZR+ standards and the OpenROADM specifications. Amid these advancements, Anritsu, as a provider of measurement equipment, is actively involved in OpenROADM and remains committed to offering products that contribute to the quality control and monitoring of optical communication networks.
Author: Kazuichi Ichikawa, Assistant Manager, Anritsu Corporation
