There’s been a lot of buzz around 4-Level Pulse Amplitude Modulation (PAM-4) in optical interconnect circles lately, and not without good reason.
With the enterprise and metro carriers moving steadily toward 100Gbit/s connectivity, and already setting their sights on 400Gbit/s and beyond, the need to improve the ability of both the transmitter and receiver to handle large amounts of data is growing. Over the past decade or so, providers have been deploying QPSK and other phase and polarization modulation techniques to up the bandwidth, but these solutions require a fair bit of hardware that will only become more complex as the data rates go up.
PAM-4 utilizes four distinct pulses of amplitude to convey data, making it twice as efficient as binary without the extensive hardware requirements of phase modulation. In fact, companies like Inphi have come out with silicon solutions that allow short-haul connectivity at the switch and router level, a development that Microsoft has now embraced as it seeks to integrate data center infrastructure on a highly granular level.
The company is looking to establish switch-level connectivity over as much as 80km using Inphi’s ColorZ platform that integrates modulators and photodetectors on a silicon photonics chip architecture. In this way, the company hopes to establish QSFP28 connectivity over the short-haul, as was demonstrated at the OFC conference in Anaheim last March.
At first blush, it would seem that this would be bad news for the DCI industry. But while it’s true that the design bypasses traditional WDM in the data center, the output from the PAM-4 plug is low power and exhibits poor sensitivity to noise, so the signal still requires a fair bit of amplification and management of chromatic dispersion after it leaves the switch in order to hit its 80km (so far) maximum reach. The OFC demo, in fact, showed a short-haul connection between switches from Arista and Cisco using an open optical line system from ADVA Optical Networking.
Microsoft is looking to deploy the ColorZ platform as a direct detection solution, which minimizes system complexity, power and cost, however, coherent solutions will still play a part in the wider DCI connectivity arena. Where system capacity and simplicity of network designs are required, coherent works best but, in a well-managed short-haul environment, the PAM-4 solution can be a success.
A truly forward-leaning interconnect solution will have the means to incorporate multiple architectures and topologies to support the dynamic data environments on which the world is becoming increasingly dependent.
In this diverse networking universe, there continues to be a need for DCI appliances, both using direct detect and PAM-4 and coherent technologies.