In a previous blog, “Building Blocks and Innovations Enabling ROADM Evolution,” I discussed the key innovation enablers of ROADM evolution. These innovations include component-level developments related to the wavelength selective switch (WSS), amplifiers, optical supervisory channel (OSC), optical channel monitoring (OCM), and optical time-domain reflectometer (OTDR). Other innovations relate to ROADM form factors, shelf form factors, and link control software. But what are the benefits of these innovations for operators of optical networks? In this blog, I will describe how these innovations have enabled, and continue to enable, ROADMs to evolve along the following seven vectors:
1. Enhanced Coherent Transceiver Wavelength Capacity-Reach
ROADM performance is evolving to help enable maximized coherent transceiver wavelength capacity-reach with corresponding reductions in transceiver cost per bit, power consumption, and footprint. Flexible-grid ROADMs, first with 12.5 GHz granularity and more recently 6.25 GHz granularity, together with gridless/colorless add/drop and wide-passband fixed filters, are enabling ever higher baud rates. A second evolution is improved ROADM cascadeability, enabling more WSS in the wavelength’s path. A third evolution is the improvements in amplifier gain and noise enabled by both amplifier technology evolution and ROADM-/node-on-a-blade architectures. Reduced amplifier noise enables higher-order modulation for the same reach requirement. An additional key enabler of maximized transceiver performance is the evolution to automated generalized optical signal-to-noise ratio (GOSNR)-based link control. Further performance enhancements come from the addition of dynamic gain equalization (DGE) for wavelength power optimization in in-line amplifiers (ILAs).
2. Increased Fiber Capacity
In addition to improvements in transceiver spectral efficiency enabled by the improvements in amplifier performance, cascadeability, and link control described previously, ROADMs are also evolving to maximize total fiber capacity. The spectrum available on each fiber has evolved to 4,800 GHz with the extended C-band, 6,000 GHz with the super C-band, and 9,600 GHz with C+L, increasing to 11,200 GHz with super-C + super-L.
3. More Add/Drop and Degree Flexibility
ROADMs have also become more flexible in terms of both add/drop and the number of degrees. Add/drop has evolved to colorless-directionless (CD) with contentioned MxN WSSs. Colorless-directionless-contentionless (CDC) has evolved with low-port-count unamplified multicast switches (MCSs), high-port-count amplified MCSs, and contentionless MxN WSSs, each providing different options for CDC cost and scalability. High-baud-rate fixed add/drop is also an option with the recent availability of wide-passband fixed filters. WSS port counts have evolved to 30+, with 48 ports and even 60 ports likely to be available in the future. This provides the option to support nodes with large numbers of links/adjacent nodes at hub sites and it provides another option for scaling capacity with multiple parallel fiber links. High-port-count WSSs also enable CD and CDC add/drop scalability.
4. Reduced Footprint
ROADM footprint has shrunk dramatically. This has been enabled by the reduced size of key ROADM components including WSSs and amplifiers, and the availability of functions such as OSC and OTDR as SFP pluggables. It has also been enabled by the evolution to twin and quad WSSs on a single unit, as well as the move to ROADM-on-a-blade architectures. Furthermore, it has been enabled by the availability of compact modular and 600-mm-deep platforms. For example, in ~2005-2010, long-haul ROADMs typically required around 6RU per degree, while metro ROADMs were around 3RU per degree. Modern compact modular platforms can now deliver two ROADM degrees in 1RU. Node-on-a-blade architectures, with two or more ROADM degrees on a single module, will enable even denser configurations, with four ROADM degrees in 1RU.
5. Increased Openness
With benefits that include accelerated innovation, optimized networks, and transformed economics, many network operators are embracing open optical networking. ROADMs are therefore becoming more open, with integrated OCMs, WSS-based attenuation, and alien wavelength-friendly link control simplifying support for third-party wavelengths, while flexible-grid capabilities also provide a path to spectrum services. Management interfaces such as TL1 and SNMP have evolved to open APIs (NETCONF, RESTCONF, gRPC, gNMI) with Open ROADM, and in the future OpenConfig, YANG data models. The Open ROADM MSA has provided standards for line interoperability between CD metro ROADMs from different vendors, with specifications covering the SDN domain controller, link control, power levels, laser safety, performance monitoring, fault detection, and OSC (1 GbE, LLDP, Ethernet auto-negotiation).
6. Enhanced Operations and Manageability
A number of innovations address the need for reduced operational cost and enhanced manageability. These include the adoption of multi-haul compact modular platforms for all applications from metro edge to long-haul and submarine. Installation has been simplified with zero-touch provisioning, cabling verification and auto discovery, the auto-discovery of transponder/muxponder and router modules, and switchable gain amplifiers. High-performance link control no longer requires complex planning and configuration. Coherent probes enable wavelength paths to be characterized and validated before transceivers are installed and provisioned. Other management enhancements include streaming telemetry, high-resolution coherent OCMs, and integrated OTDR, including coherent OTDR.
7. Higher Network Availability
Optical restoration times are evolving from minutes to seconds with faster power balancing enabled by enhanced link control. Integrated OTDR has enabled the fast location of fiber cuts, with coherent OTDR extending this to long repeatered submarine fibers, while also providing the potential for advance warning of terrestrial fiber cuts. Amplified spontaneous emission (ASE) noise sources enable more resilient C+L networks, including faster recovery after failures. Cable verification can also significantly reduce cabling errors. For protected ring and mesh scenarios that require full degree separation, compact modular and ROADM-on-a-blade reduce the footprint penalty for putting each ROADM degree in its own shelf. Node-on-a-blade can also increase network availability in unprotected linear chain scenarios by reducing the number of components at each intermediate node, therefore reducing the risk of a service-impacting component failure.
So, to summarize, driven by the requirements of coherent evolution and the trend toward open optical networking, ROADM technology continues to evolve along these seven vectors that are together enabling significant reductions in optical network total cost of ownership. To learn more about this important topic, download the Infinera white paper “The Seven Vectors of ROADM Evolution.”