The optical line system is undergoing a revolution.

First, some background: An optical line system consists of optical amplifiers, multiplexers/demultiplexers and ROADMs. Historically, the line system has been tightly coupled with the terminal systems (i.e., transponders) with both line and terminal supplied by the same vendor. The end-to-end optical network has historically been a closed and proprietary system. Lately, though, some web-scale providers have concluded that this model stifles innovation and are bringing the concept of disaggregation to optical networking in the form of the open line system (OLS).

An open line system decouples the optical line elements from the terminals to create several potential benefits for service providers, with rapid technology adoption being one of the biggest drivers. Coherent detection and photonic integration (including silicon photonics) have spurred rapid innovation in transponders, while line systems are evolving more slowly. Decoupling the line from the terminals allows service providers to advance through several generations of transponder technologies without having to change the line systems.

Second, An OLS naturally eliminates vendor lock-in, as service providers can purchase line systems and transponders from different suppliers, share a single line among several transponder suppliers and even source different line components (such as ROADMs) from multiple suppliers. This speeds industry innovation and lets service providers select best-of-breed technology.

Third, open application programming interfaces (APIs) on OLSs allow service providers to break from proprietary control/management systems and provide a path to true SDN control in the optical domain in which northbound APIs from the elements connect to network orchestrator software -- which can either be built in-house or purchased from an orchestration/SDN software supplier. Thus, an OLS with open APIs under a common network orchestrator allows for end-to-end control of multi-vendor networks.

Web-scale companies are leading adoption of disaggregation and open line systems. Google has been public about its plans for OLS, including disaggregated ROADMs and ILAs. Google's goals for OLS are very much in-line with the OLS benefits cited above. Google also requires a high-end optical performance comparable to proprietary systems. Microsoft has also spoken publicly about its plans for OLS. Initially developed for a point-to-point application, Microsoft has extended the concept to include colorless, directional ROADM architecture with hybrid Raman/EDFA amplification, with the system being monitored and configured via Microsoft's SDN controller.

On a recent Light Reading webinar, Facebook's Director of Optical Engineering Matthew Mitchell presented an overview of Facebook's OpenTransport architecture, introduced at OFC 2018, which includes open transponders, a metro line system, in-line amplifiers and ROADMs. Uniform API models across all hardware elements will be key to tying elements together.

Significantly, traditional network operators share many of the requirements with web-scales and are also showing interest in OLS. The AT&T-led Open ROADM MSA is the highest profile of the operator open line initiatives, but it is not the only one. TIP is most widely recognized today for its white-box DWDM transponder shelf project, Voyager, but the open source group is also building an OLS within its Open Packet Optical Transport Working Group. The stated mission of the OLS subgroup is to define open models to automate the configuration and operation of optical line systems and to validate interoperability of optical transponders from different vendors with OLS equipment.

The ONF Open and Disaggregated Transport Network (ODTN) open source group is an operator-led initiative to build data center interconnects using disaggregated optical equipment, open and common standards and open source software. Components include an OLS, disaggregated multi-vendor transponders and SDN controller, with applications to span metro and long haul.

Despite progress, challenges exist for open line systems (OLS), and these challenges appear to be holding back traditional network operators more than web-scales -- at least for now. Charter is one network operator that is interested in OLS on paper but sees challenges that must be addressed for commercial adoption. During our webinar, Charter Director of Advanced Engineering Chris Ellefsen outlined a set of advanced technologies that must be included in its line systems, including colorless, directionless, contentionless, Flexgrid and L band. These are high-end functions that are costly and would be absent from any type of generic line architecture. Ellefsen also raised concerns over the slow pace of open standards initiatives that won't meet Charter's time frames and could also produce the lowest common denominator technology that Charter needs to avoid.

There are surprisingly few ROADMs hardware suppliers in the market today, but Nistica (now owned by Molex) is among them and is actively engaged with web-scales and telecom operators in developing OLS architectures with the advanced feature sets that are required.

The optical needs of web-scales and traditional operators seem to be converging more than diverging and we expect that, over time, telecom operators will increasingly join the OLS trend that the web-scales are pioneering.

For additional information on this topic, see Heavy Reading's white paper "Disaggregation in Optical Networks With Open Line Systems" or view the archived webinar.

This blog is sponsored by Nistica (Molex LLC)

— Sterling Perrin, Principal Analyst, Optical Networking & Transport, Heavy Reading