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Silicon Photonics

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Rockley demos a silicon photonics switch prototype  

Part 1: Rockley Photonics

Rockley Photonics has made a prototype switch to help grow the number of servers that can be linked in a data centre. The issue with interconnection networks inside a data centre is that they do not scale linearly as more servers are added.  


Dr. Andrew Rickman

“If you double the number of servers connected in a mega data centre, you don’t just double the complexity of the network, it goes up exponentially,” explains Andrew Rickman, co-founder, chairman and CEO at Rockley Photonics. “That is the problem we are addressing.”

By 2017 and 2018, it will still be possible to build the networks that large-scale data centre network operators require, says Rickman, but at an ever increasing cost and with a growing power consumption. “The basic principles of what they are doing needs to be rethought,” he says.


Network scale 

Modern data centre networks must handle significant traffic flow between servers, referred to as east-west traffic. A common switching arrangement in the data centre is the leaf-spine architecture, used to interconnect thousands of servers.

A ‘leaf’ may be a top-of-rack switch that is linked to multiple server chassis on one side and larger-capacity, ‘spine’ switches on the other. The result is a switch network where each leaf is connected to all the spine switches, while each spine switch is linked to all the leaves. In the example shown, four spine switches connect to 32 leaf switches. 


A leaf-spine architecture

The leaf and spine switches are built using ASICs, with the largest ICs typically having 32, 100 gigabit ports. One switch ASIC may be used in a platform but as Rickman points out, larger switches may implement multiple stages such as a three-stage Clos architecture. As a result, traffic between servers on different leaves, travelling up and down the leaf-spine, may pass through five stages or hops but possibly as many as nine. 


There is no replacement performance in this area


It is the switch IC’s capacity and port count that dictates the overall size of the leaf-spine network and therefore the number of servers that can be connected. Rockley’s goal is to develop a bigger switch building block making use of silicon photonics.  

“The fundamental thing to address is making bigger switching elements,” says Rickman. “That way you can keep the number of stages in the network the same but still make bigger and bigger networks.” Rockley expects its larger building-block switch will reduce the switch stages needed.

The UK start-up is not yet detailing its switch beyond saying it uses optical switching and that the company is developing a photonic integrated circuit (PIC) and a controlling ASIC. 

“In the field of silicon photonics, for the same area of silicon, you can produce a larger switch; you have more capacity than you do in electronics,” says Rickman. Moreover, Rockley says that its silicon photonics-based PIC will scale with Moore’s law, with its switch's data capacity approximately doubling every two years. “Previously, the network did not scale with Moore’s law,” says Rickman.  


Customers can see something is real and that it works. We are optimising all the elements of the system before taping out the fully integrated devices 



The company has developed a switch prototype that includes ‘silicon photonics elements’ and FPGAs. “Customers can see something is real and that it works,” says Rickman. “We are optimising all the elements of the system before taping out the fully integrated devices.” Rockley expects to have its switch in volume production in 2017.

Last year the company raised its first round of funding and said that it would undergo a further round in 2015. Rockley has not said how much it has raised or the status of the latest round. “We are well-funded and we have a very supportive group of investors,” says Rickman.  

Rickman has long been involved in silicon photonics, starting out as a researcher at the University of Surrey developing silicon photonics waveguides in the early 1990s, before founding Bookham Technologies (now Oclaro). He has also been chairman of silicon photonics start-up Kotura that was acquired by Mellanox Technologies in 2013. Rickman co-founded Rockley in 2013.    

“What I’ve learned about silicon photonics, and about all those electronics technologies, is how to design stuff from a process point of view to make something highly manufacturable and at the same time having the performance,” says Rickman.

There is no replacement performance in the area of data centre switching, he stresses: “The benefit of our technology is to deliver the performance, not the fact that it is cheap or [offers] average performance.”  


For Part 2, Interconnection networks - an introduction, click here

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