The pressure is on for the industry to deliver more open and interoperable optical networks. Projects including the Open ROADM Multi-Source Agreement, the Telecom Infra Project’s Open Optical Packet Transport Group, the Open Networking Foundation (ONF) and Optical Interworking Forum (OIF) are exploring the way forward. However, the question remains whether network operators are ready and willing to trade off performance for openness and interoperability.
What’s an ‘open’ optical network, anyway?
Optical networks include hardware elements (transponders/terminals and line system equipment), software operating systems and management/control software. We define a fully open optical network as one that includes open hardware supporting open application programming interfaces (APIs) that can interact with and be managed by open source software.
Open source software is software code developed and contributed by any interested parties, and it is freely available and shared. Production ‘distributions’ of the code may also be curated, tested, supported and licensed. Open source software relating to optical networking includes ONOS, OpenConfig, Open Daylight (ODL) and many others.
Open APIs are a group of APIs that are defined in an open forum and made available for anyone to implement into their hardware and software. Examples in the optical networking domain include the OIF’s T-API and NETCONF/YANG.
Open hardware is a bit more open to interpretation in today’s market. One approach is optical equipment meeting specifications set by industry standards bodies and/or multi-source agreement (MSA) projects such as AT&T’s Open ROADM project or the Telecom Infra Project’s (TIP) Voyageur platform. This approach is designed with equipment interoperability in mind, enabling network operators to have more freedom in purchasing equipment from different vendors. It does, however, result in some tradeoffs from the performance and applications perspectives. Optimizations that lead to increased spectral efficiency, especially in longer-reach applications, are the result of fine tuning in coherent digital signal processing (DSP) capabilities, forward error correction (FEC) algorithms, net coding gain and other areas. These optimizations are typically only available in a single-vendor environment. While Open ROADM has put in place a strong foundation defining what is required to build open and interoperable optical transponders and line systems equipment, the initial specifications are limited to 100G implementations for shorter metro reaches. More work will be required to broaden the application scope to other applications.
Another approach, which attempts to balance the desire for increased interoperability while maintaining performance, is to ‘open’ up proprietary optical equipment through support for open APIs that then enable the hardware to be controlled and managed by third-party (open) management and/or software-defined networking (SDN) controllers.
Where does optical hardware disaggregation fit in?
Fully disaggregated optical system architecture separates the various hardware elements used in a transmission network (ROADMs, amplifiers, passives, line cards, switches and chassis) as well as the related software (management, control and applications), with the end goal of enabling ‘best-of-breed’ network elements to be purchased from individual vendors or for the network to be custom built to a specific company’s requirements.
From an open optical networks perspective, while equipment disaggregation is typically undertaken within an open network context, disaggregated hardware is not a necessary condition for open optical networks. The base requirement is for hardware to be accessible and controllable via open APIs — with the fundamental objective of being able to operate in an open and multi-vendor environment. Of the many ways in which optical networks could be put together with full equipment and software disaggregation, the approach that is garnering the most interest as a viable first step forward is the separation of transponders from the line systems — otherwise known as open line systems (OLS). Being able to purchase and deploy transponders separately from the line system provides more flexibility in vendor selection and allows the network operator to more easily take advantage of technology upgrades on the transponder side, which typically evolves more quickly than the underlying line system.
What’s driving the shift to more open optical networks?
Open line systems are one of the more advanced use cases for open optical networks, and many of the key drivers for the shift to OLS are similar to what we are hearing as key drivers for the overall shift to more open optical networks.
In a recent IHS Markit 100G+ and ROADM Strategies Survey, our service provider respondents rated reducing their reliance on a single vendor as their number-one driver for deploying an OLS, followed by less capex and better performance.
It’s early days
Even with strong endorsements from major service providers, there remain many questions and some important barriers to be addressed before open optical networking becomes more widely accepted. Key concerns include the loss of spectral efficiency gains, system integration and maintenance, lack of operational tools to manage disaggregated networks and slow or disparate standards development.
Again looking at the survey data from OLS as an indicator of expectations for open optical networks, an IHS Markit survey conducted in 2016 showed over two-thirds of respondent operators were considering OLS technology, and the rest were undecided. At this point, OLS are still in their infancy, but the level of interest is still higher than many in the industry would expect.
There are many ways to start down the path to open optical networks, from introducing new equipment with open APIs operating under open, multivendor SDN control to fully disaggregated optical equipment — and many combinations in between. The key building blocks are starting to come into place, but much work remains to overcome the challenges ahead on the path to moving forward with open optical networks.
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