A Brief Introduction of ROADM for 400G WDM Transmission

As global optical networks advance, there is an increasing necessity for new technologies such as 400G that meet the demands of network operators. Video streaming, surging data volumes, 5G network, remote working, and ever-growing business necessities create extreme bandwidth demands.

Network operators and data centers are also embracing WDM transmission to boost data transfer speed, increase bandwidth and enhance a better user experience. And to solve some of the common 400G WDM transmission problems, such as reduced transmission reach, ROADMs are being deployed. Below, we have discussed more about ROADM definition, ROADM technology, ROADM network and other important things for 400G WDM transmission.

ROADM Definition and Its Technology

A Reconfigurable Optical Add-Drop Multiplexer (ROADM) is a device with access to all wavelengths on a fiber line. Introduced in the early 2000s, ROADM allows for remote configuration and reconfiguration of A-Z lightpaths. ROADM network standard enables blocking, adding, redirecting, or passing visible light beams and modulated infrared (IR) in the fiber-optic network based on the particular wavelength.

ROADMs are employed in systems utilizing wavelength division multiplexing (WDM). They support more than two directions at sites for optical mesh-based networking. Unlike its predecessor, the OADM, ROADM can adjust the add/drop versus pass-through configuration whenever traffic patterns change in the wdm network

As a result, the operations are simplified by automating the connections through an intermediate site. This implies that it's unnecessary to deploy technicians to perform manual patches in response to a new wavelength or alter a wavelength's path. The results are optimized network traffic where bandwidth demands are met without incurring extra costs.

Overview of Open ROADM

Open ROADM is a 400G pluggable solution that champions cross-vendor interoperability for optical equipment, including ROADMs, transponders, and pluggable optics. This solution defines some optical interoperability requirements for ROADM in optical communication and comprises hardware devices that manage and routes traffic over the fiber optic lines.

Initially, Open ROADM was designed to address the rise in data traffic on wireless networks experienced between 2007 and 2015. The major components of Open ROADM – ROADM switch, pluggable optics, and transponder – are controllable via an open standards-based API accessible through an SDN Controller.

One of the main objectives of Open ROADM is to ensure network operators and vendors devise a universal approach to designing networks that are flexible, scalable, and cost-effective. It also offers a standard model to streamline the management of multi-vendor optical network infrastructure.

400G and WDM Transmission

WDM transmission is a multiplexing technique of several optical carrier signals through a single optical fiber channel by varying the wavelength of the laser lights. The wavelength division multiplexing technology allows different data streams to travel in both directions over a fiber network, increasing bandwidth and reducing the number of fibers used in the primary network or transmission line.

With 400G technology seeing widespread adoption in various industries, there's a need for optical fiber networking systems to adapt and support the increasing data speeds and capacity. WDM transmission technique offers this convenience and is considered a technology of choice for transmitting larger amounts of data across networks/sites. WDM-based networks can also hold various data traffic at different speeds over an optical channel, allowing for increased flexibility.

400G WDM still faces a number of challenges. For instance, the high symbol rate stresses the DAC/ADC in terms of bandwidth, while the high-order quadrature amplitude modulation (QAM) stresses the DAC/ADC in terms of its ENOB (effective number of bits.)

As far as transmission performance is concerned, the high-order QAM requires more optical signal-to-noise ratio (OSNR) at the receiver side, which reduces the transmission reach. Additionally, it’s more sensitive to the accumulation of linear and non-linear phase noise. Most of these constraints can be solved with the use of ROADM network. We’ve discussed more below.

Open ROADM MSA and the ROADM Architecture for 400G WDM

The Open ROADM MSA defines some interoperability specifications for ROADM switches, pluggable optics, and transponders. Most ROADMs in the market are proprietary devices built by specific suppliers making interoperability a bit challenging. The Open ROADM MSA, therefore, seeks to provide the technical foundation to deploy networks with increased flexibility.

In other words, Open ROADM in optical communication aims at disaggregating the data network by allowing for the coexistence of multiple transponders and ROADM vendors with a few restrictions. This can be quite helpful for the 400G WDM network, especially when lead-time and inventory issues arise, as the ability to mix & match can help eliminate delays.

By leveraging WDM network for fiber gain as well as optical line systems with ROADMs, network operators can design virtual fiber paths between two points over some complex fiber topologies. That is, ROADMs introduce a logical transport underlay of single-hop router connections that can be optimized to suit the IP traffic topology. These aspects play a critical role in enhancing 400G adoption that offers the much-needed capacity-reach, flexibility, and efficiency for network operators.

That said, ROADMs have evolved over the years to support flexile-grid WSS technology. One of the basic ROADM network uses fixed filters for add/drop, while the other architectures offer flexibility in wavelength assignment/color or the option to freely route wavelengths in any direction with little to no restriction. This means you can implement multi-degree networking with multiple fiber paths for every node connecting to different sites. The benefit is that you can move traffic along another path if one fiber path isn’t working.

Conclusion

As data centers and network operators work on minimizing overall IP-optical network cost, there's a push to implement robust, flexible, and optimized IP topologies. So by utilizing 400GbE client interfaces, ROADMs for 400G can satisfy the ever-growing volume requirements of DCI and cloud operators. Similarly, deploying pluggable modules and tapping into the WDM transmission technique increases network capacity and significantly reduces power consumption while simplifying maintenance and support.