Facebook looks to deploy 100G optical Ethernet

Facebook has helped define a low-cost 100Gb/s optical Ethernet transceiver it plans to start deploying in 2016. The move is another example of ways big data centres are driving design in computers and networking.

Facebook specified a 100G transceiver using single-mode fibre it believes it can drive to a cost of $1/GB. To hit the lower costs, the tech giant relaxed distance requirements to 500m down from 2km and eased specs on operating temperature and product lifetime.

The transceivers are based on QSFP28, a pluggable form factor that uses four 25Gb/s lanes. They also leverage the CWDM4 multi-source agreement, a definition for 4x25G modules using coarse wavelength division multiplexing (CWDM4), supported by vendors such as Avago, Finisar, JDSU, Oclaro and Sumitomo Electric.

Facebook's decision was the result of a major optical interconnects study. The study canvassed options in an effort to deliver greater bandwidth at the lowest possible costs for its rapidly expanding data centres.

The study found the cost of traditional optics are expected to balloon as the industry approaches 400G data rates. However, much of the costs are due to stringent demands from traditional telecom carriers.

Facebook hopes to use its buying power and relaxed specs to drive down by an order of magnitude projected 2016 costs of 100G optical Ethernet transceivers.

"If you spec the boundaries correctly you can get 10x cost decreases, this is the kind of industry-bending work we do," said Katherine Schmidtke, head of optical strategy for Facebook.

The main 100G spec for telcos supports 10km distances, however one variant goes down to as little as 2km. The signaling differences between 2km and 500m versions are not great but, "if you are at the edge of a technology relaxing just a little brings up your yield," said Schmidtke.

Facebooks planned move from multi-mode to single-mode fibre goes hand-in-hand with the reduction 500m distances. Multi-mode fibres have been reducing their reach from 500m to 300m and 100m as speeds go up, opening a widening gap with single-mode fibre. Besides being cheaper, single-mode fibre is easier to install, she said.

Facebook uses 40G multi-mode optics as spine networks linking racks of servers in its latest data centres such as a 500,000 square foot facility in Altoona, Iowa. The centre uses 50,000 optical links that, when placed end to end, would reach 20,000 miles. It was commissioned in November 2014 using a new 40G fabric and 25G copper interconnects linking servers to top-of-rack switches.

The 100G links will act as spines in future data centres. Facebook is working on a second building in Altoona, has plans for a third and operates as many as 19 data centres in various regions around the world.

The Web giant is studying whether its next leap will be to 200G or 400G Ethernet networks.

- Rick Merritt
??EE Times