In recent years, mass migration to cloud-based services has driven rapid expansion in global data centers with ever larger facilities being built.
Lately, however, this trend has reached a limit and internet content and cloud service providers have begun to focus on creating regional multi-data center clusters. This strategy enables ICPs to maximize the efficiency of their operations and offer superior disaster recovery, business continuity, and digital media delivery. What it relies on, though, is a new level of power and space efficiency in metro data center interconnect (DCI) networks.
To avoid DCI infrastructures becoming bottlenecks, network designers are having to adapt fast and embrace the latest innovations in optical transmission technologies. They need to deploy single-span DWDM links optimized for distances up to 80km with Tbit/s capacity.
So what solutions fit the bill? Well, when it comes to DCI, there are four crucial consideration: cost, simplicity, power, and space. Everything comes down to spending and any investment in new technology needs to eventually lead to cost reductions. Solutions need to be easy to install and operate as DCI operating teams are often small and overstretched. Reducing unnecessary Watts per bit and keeping energy usage low is key for long-term sustainability of the business and environment. And, with demand growing quicker than available space, compact solutions with configurations that take up little rack space are also essential.
Maximizing the synergy of these four variables is essential. That’s why the great debate right now is about the choice between direct detection and coherent detection technologies when it comes to building tomorrow’s DCI networks.
Coherent detection optical transport has transformed long-haul DWDM networks, enabling 100Gbit/s transport on a single wavelength over thousands of kilometers. The technology involves a local oscillator at the receiver tracking the phase of the optical transmitter. These work in conjunction with a coherent digital signal processor (DSP) to encode and decode the DWDM signal.
This technology can enhance spectral efficiency and performance in metro DCI infrastructure. That’s great news for long-haul networks but it does entail increased cost and energy consumption, which makes it less attractive for shorter links. Its installation simplicity, on the other hand, is a highly desirable feature for over-burdened ICP operations teams.
Meanwhile, recent technological advances have increased the appeal of direct modulations for metro DCI networks. Direct detection appliances can now transport 100Gbit/s and, by leveraging PAM4 (pulse-amplitude modulation 4) technology, they can comfortably reach distances of up to 80km while complying with requirements for optical signal-to-noise ratio and chromatic dispersion compensation.
Coherent Versus Direct Detection
The two modulations differ significantly in terms of size, power, cost, fiber capacity and installation simplicity. Coherent detection offers more in terms of spectral efficiency, however, the reduced capacity of direct detect is usually sufficient to connect data center clusters locally. Direct detect networks are naturally less complex, which means they are smaller and more cost- and energy-efficient. Additionally, smart optical line systems, deployed to support 100Gbit/s direct detect, are ensuring that the installation simplicity of coherent and direct detect solutions is becoming more similar.
Direct detect solutions may not be able to compete with coherent solutions over long distances but with the increase in regional data center clusters, direct detect has its path cut out. Low on energy consumption, cost-efficient and easy to apply, direct detect solutions for distances under 80km are an obvious choice. What’s more, the advances that have been made with solutions such as PAM4 with 50Gbaud technology or PAM8 ensure that direct detect technologies will remain attractive in the future.
And, of course, the introduction of 400ZR solutions in 2020, which will drive reduced-performance coherent technologies into client plug form factors, should make this a highly competitive space for years to come.
While direct detect systems are optimized for shorter distances and point-to-point connections, coherent technology is the perfect fit for long-haul data transport networks.
About the author:
Niall Robinson, VP Global Business Development at ADVA Optical Networking has more than 26 years in the telecommunications industry. A recognized expert on data center interconnect technology and frequently requested speaker at conferences all over the world, Niall’s specialty is moving cutting-edge technology from the lab into the network.