New cloud services, the internet of things and the evolution of mobile networks towards 5G are driving capacity needs in the metro to unprecedented levels. In an effort to overcome bandwidth bottlenecks, reduce cost and ensure scalability, more and more network operators are now turning to flex-coherent technologies. Total capacity and lowest cost per bit are two critical metrics in the metro. That's why software-defined, flex-coherent optics - which can efficiently carry 100G, 200G and even up to 600G of capacity on a single wavelength - are so important for metro applications.

While bandwidth demand from connectivity services - such as wholesale, mobile backhaul and enterprise connectivity - is growing quickly, service rates themselves are not increasing as fast. In fact, 100Gbit/s services are still the exception; most applications can still be served by 1Gbit/s and 10Gbit/s services. What's driving the massive additional capacity demand in the metro is predominantly the growth of end points and connectivity applications, rather than capacity per service.

This imposes some significant challenges. On the one hand, metro network operators need to provide more capacity at the lowest possible cost. On the other, the need for granularity increases significantly when moving to flex-coherent technology and launching wavelengths carrying 100s of gigabits.

One solution to this problem is introducing sub-wavelength granularity based on optical transport network (OTN) architecture. As a technology, OTN is now mature, comprehensive and able to support for the full range of expected service types. OTN multiplexing and cross-connect systems can be deployed across the network to increase granularity and satisfy bandwidth needs - and this has already started.

Deploying OTN cross-connect technology in the way it was originally designed, however, is costly. It also creates inflexibility. That's because traditional OTN cross-connects are closed systems optimized for certain capacities. They come at significant first-in cost and scale only to a limited and predefined amount of total capacity. Lifetime is therefore rather limited if significant overprovisioning is out of question, and the entire architecture needs to be replaced when scalability limits have been exceeded.

Another challenge with traditional OTN cross-connect technology arises from fixed slot-capacity assignment and proprietary architectures. These don't support the openness that metro network operators are looking for today to scale their networks in the future. The solution to this problem is deploying disaggregated open OTN cross-connect designs that remove scalability barriers and proprietary architectures. By disaggregating the architecture and introducing open and standardized interfaces, scalability is guaranteed for many years, while modules deployed on day one don't become non-interoperable.

The disaggregated design also ensures lowest first-in costs, an important metric in times of unpredictable capacity demand. Furthermore, disaggregated open OTN cross-connects deployed as on-ramp aggregators at the edge of the network enable switching technologies deployed in the core to scale and operate more efficiently. Loading wavelengths at the edge of the network with as much data traffic as possible avoids presenting low traffic volumes to expensive high-capacity core switch interfaces Switch port utilization in the core is a significant challenge, especially when moving to flex-coherent technologies at moderate network loads.

Disaggregation and open OTN cross-connect architectures are a critical component for the evolution of metro networks. The wider spread between wavelengths capacity and service bandwidth needs to be bridged by an aggregation and switching technology that follows an open design and is ready to scale for years to come. Physical disaggregation and open interface design are key elements to guarantee lowest first-in cost and, at the same time, scalability beyond closed system limitations.

Open and disaggregated designs also allow for new interface types and functions to be selectively introduced in the future. And, for the most demanding operators, SDN control technologies can also be harnessed, enabling rapid service activation, comprehensive monitoring and efficient protection and restoration capabilities.

The time is now to consider deploying open cross-connect architecture as on-ramp service aggregation technology in your network. Moving to flex-coherent technology is one thing, but utilizing it efficiently and keeping overall costs low from day one will be key.