DCI requirements drive the next generation of Coherent DWDM

Why are we so infatuated with data center interconnect, or DCI? Sure, it’s the application most responsible for recent growth in optical spending. And yes, it flows from two of the great megatrends shaping the networking world – digital content delivery and enterprise cloud IT. It also doesn’t hurt that some of its most notable practitioners are names we see every day on our TVs, laptops, and mobile devices.

But I’m an engineer. And if you’re reading this, it’s likely that you are, too. So you might agree that DCI grabs our attention because it distills optical networking to its purest form. More than any other application, DCI explores the technical and economic boundaries of transmitting maximum capacity at the lowest possible cost per bit over distances from metro to subsea. If optical networking can be defined as the practice of combining bandwidth and distance, and economics the study of the optimal use of scarce resources, then DCI – which places immense demands on optical fiber’s finite spectrum – surely sits at their nexus.

Few things are as scarce as the usable spectrum in an optical fiber. (OK, I suppose the wireless guys have us on this one.) The ultimate challenge of optical networking and DCI is to use that finite spectrum efficiently. Claude Shannon’s famous capacity limit, expressed as C = B*log2(1+SNR), can be rewritten (by moving the B under the C) as a simple relationship between spectral efficiency and signal-to-noise ratio (SNR). But noise in optical systems is caused by the amplifiers that make long-distance transmission possible. So the challenge is to maximize the use of the SNR the line system provides. We can do this using sophisticated mathematical algorithms implemented in coherent digital signal processors (DSPs).