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Data Center Interconnect for the Web-Scale Era

Sterling Perrin

To address the tremendous bandwidth growth in cloud networking, the web-scale Internet companies -- including Google, Apple, Facebook, Microsoft and Amazon -- have largely written their own playbooks in areas including data networking and telecom, hardware and software, and even standardization. In this blog, we discuss three key web-scale trends in data center interconnect (DCI): disaggregation in terminals, open line systems (OLSs) and 100G+ transponders.

Disaggregation is the separation of networking equipment into functional components and allowing each component to be individually deployed. Disaggregation runs counter to the functional convergence trend in optical equipment over the past two decades, in which vendors tightly coupled proprietary hardware and software together in advanced systems (such as packet optical transport). The web-scale-led disaggregation trend is breaking the convergence hold among traditional suppliers and their traditional telecom operator customers.

One prominent example of disaggregation in action is the Telecom Infra Project's (TIP's) Voyager Open Packet DWDM transponder platform, announced in November 2016. Based on merchant hardware and an open software architecture, Voyager includes a Broadcom Tomahawk packet-switching ASIC, Acacia AC400 DSP ASIC and an open software layer that decouples hardware from software, consistent with the principles of SDN. Voyager's trial progress has been rapid, including Equinix (US) Orange (France), MTN (South Africa) and Telia Carrier (pan-Europe). We can expect more major updates at MWC 2018.

Just like they are doing with the end terminals, web-scale providers are changing the way that optical line systems are designed and deployed for DCI. Optical line systems consist of wavelength multiplexers/demultiplexers, optical amplifiers and ROADMs, as well as the control/management of those components. Historically, the line system has been tightly coupled with the terminal systems so that both line and terminal are supplied by the same vendor, and other vendors' transponders don't work over someone else's optical line. The end-to-end optical network is a closed and proprietary system.

Some web-scale providers have concluded that decoupling the line from the terminals can yield major benefits for them moving forward -- thus extending the disaggregation concept of DCI to include the optical line system, as well the terminals. One of the main benefits of an open line system (OLS) is rapid technology adoption. Coherent detection and photonic integration (including silicon photonics) are leading to rapid innovation in transponders, while line systems evolve 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. The OLS also eliminates vendor lock-in, as service providers can buy their line systems and their transponders from different suppliers.

Microsoft has been one of the strongest proponents of the OLS. At OFC 2016, Microsoft presented results of a lab system emulation of an OLS running over 4,000km of fiber and representing its North American backbone network. Colorless, flexgrid ROADMs were used.

With DCI bandwidth continuing its rapid growth, channel capacity is at the heart of transponder/line-side priorities for webscale DCI. Within metro DCI, the weight of service provider spending is rapidly shifting to 100G and higher data rates as those rates become commercially available, all but eliminating the need for 10G in the DCI network.

DCI is not just about more bandwidth, and increasing the flexibility of available capacity is a critical requirement for the line. Web-scale providers' needs for greater flexibility have sped the availability of multiple modulation formats within transponders. Coherent transponders from many vendors now support multiple modulation formats, including BPSK, QPSK, 8QAM and 16QAM, as well as different options for forward error correction (FEC) coding and software programmability for setting functionality.

Coherent transmission provides the greatest capacity and modulation flexibility, but some web-scale providers are driving 100Gbit/s direct detect modulation formats for shorter-reach metro DCI applications. Direct detection lacks coherent's tremendous reach, but, on the plus side, direct detect cuts costs, size and power consumption compared to equivalent 100Gbit/s coherent systems. Given reach limitations, PAM4 is targeted at DCI applications less than 80km. Of special interest is PAM4 modulation (though other direct detect variants will likely surface as this application matures). At OFC 2017, Microsoft, ADVA and Inphi published a technical paper detailing results of a 4Tbit/s commercial system and line successfully delivering full capacity over an 80km link, using PAM4 modules.

Finally, we note the importance of optical testing in DCI, including turn-up and in-service testing. As the cloud becomes the primary model for enterprise applications and as the data center becomes the new central office in telecom, assuring availability and reliability of connections within and between data centers is essential for data center operators and their customers. This includes physical layer testing to ensure properly functioning fibers and connectors and protocol testing up to 400 Gbit/s to cover the proliferating options for modulation and standards. In addition, concepts of disaggregation, open hardware and open software appear first in DCI before spreading to broader applications. Thus, we see white box testing and verification as important for DCI today.

A new Heavy Reading-authored white paper entitled Data Center Interconnect for the Webscale Era provides additional information on this topic.

— Sterling Perrin, Principal Analyst, Heavy Reading

This blog is sponsored by Viavi.

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