PAM-8

Gazettabyte spoke with Jörg-Peter Elbers, vice president, advanced technology at ADVA Optical Networking about the state of the optical industry following the recent OFC/NFOEC exhibition. 

"There were many people in the OFC workshops talking about getting rid of pluggability and the cages and getting the stuff mounted on the printed circuit board instead, as a cheaper, more scalable approach"

Jörg-Peter Elbers, ADVA Optical Networking

Q: What was noteworthy at the show?

A: There were three big themes and a couple of additional ones that were evolutionary. The headlines I heard most were software-defined networking (SDN), Network Functions Virtualisation (NFV) and silicon photonics.

Other themes include what needs to be done for next-generation data centres to drive greater capacity interconnect and switching, and how do we go beyond 100 Gig and whether flexible grid is required or not? 

The consensus is that flex grid is needed if we want to go to 400 Gig and one Terabit. Flex grid gives us the capability to form bigger pipes and get those chunks of signals through the network. But equally it allows not only one interface to transport 400 Gig or 1 Terabit as one chunk of spectrum, but also the possibility to slice and dice the signal so that it can use holes in the network, similar to what radio does.

With the radio spectrum, you allocate slices to establish a communication link. In optics, you have the optical fibre spectrum and you want to get the capacity between Point A and Point B. You look at the spectrum, where the holes [spectrum gaps] are, and then shape the signal - think of it as software-defined optics - to fit into those holes.

There is a lot of SDN activity. People are thinking about what it means, and there were lots of announcements, experiments and demonstrations.  

At the same time as OFC/NFOEC, the Open Networking Foundation agreed to found an optical transport work group to come up with OpenFlow extensions for optical transport connectivity. At the show, people were looking into use cases, the respective technology and what is required to make this happen.

SDN starts at the packet layer but there is value in providing big pipes for bandwidth-on-demand. Clearly with cloud computing and cloud data centres, people are moving from a localised model to a cloud one, and this adds merit to the bandwidth-on-demand scenario.

This is probably the biggest use case for extending SDN into the optical domain through an interface that can be virtualised and shared by multiple tenants.

"This is not the end of III-V photonics. There are many III-V players, vertically integrated, that have shown that they can integrate and get compact, high-quality circuits"

Network Functions Virtualisation: Why was that discussed at OFC?

At first glance, it was not obvious. But looking at it in more detail, much of the infrastructure over which those network functions run is optical.

Just take one Network Functions Virtualisation example: the mobile backhaul space. If you look at LTE/ LTE Advanced, there is clearly a push to put in more fibre and more optical infrastructure.

At the same time, you still have a bandwidth crunch. It is very difficult to have enough bandwidth to the antenna to support all the users and give them the quality of experience they expect.

Putting networking functions such as cacheing at a cell site, deeper within the network, and managing a virtualised session there, is an interesting trend that operators are looking at, and which we, with our partnership with Saguna Networks, have shown a solution for.

Virtualising network functions such as cacheing, firewalling and wide area network (WAN) optimisation are higher layer functions. But as you do that, the network infrastructure needs to adapt dynamically.

You need orchestration that combines the control and the co-ordination of the networking functions. This is more IT infrastructure - server-based blades and open-source software.

Then you have SDN underneath, supporting changes in the traffic flow with reconfiguration of the network infrastructure.

There was much discussion about the CFP2 and Cisco's own silicon photonics-based CPAK. Was this the main silicon photonics story at the show?

There is much interest in silicon photonics not only for short reach optical interconnects but more generally, as an alternative to III-V photonics for integrated optical functions.

For light sources and amplification, you still need indium phosphide and you need to think about how to combine the two. But people have shown that even in the core network you can get decent performance at 100 Gig coherent using silicon photonics.

This is an interesting development because such a solution could potentially lower cost, simplify thermal management, and from a fab access and manufacturing perspective, it could be simpler going to a global foundry. 

But a word of caution: there is big hype here too. This is not the end of III-V photonics. There are many III-V players, vertically integrated, that have shown that they can integrate and get compact, high-quality circuits.

You mentioned interconnect in the data centre as one evolving theme. What did you mean?

The capacities inside the data centre are growing much faster than the WAN interconnects.  That is not surprising because people are trying to do as much as possible in the data centre because WAN interconnect is expensive.

People are looking increasingly at how to integrate the optics and the server hardware more closely. This is moving beyond the concept of pluggables all the way to mounted optics on the board or even on-chip to achieve more density, less power and less cost.      

There were many people in the OFC workshops talking about getting rid of pluggability and the cages and getting the stuff mounted on the printed circuit board instead, as a cheaper, more scalable approach.

"Right now we are running 28 Gig on a single wavelength. Clearly with speeds increasing and with these kind of developments [PAM-8, discrete multi-tone], you see that this is not the end"   

What did you learn at the show?

There wasn't anything that was radically new. But there were some significant silicon photonics demonstrations. That was the most exciting part for me although I'm not sure I can discuss the demos [due to confidentiality].

Another area we are interested in revolves around the ongoing IEEE work on short reach 100 Gigabit serial interfaces. The original objective was 2km but they have now honed in on 500m.

PAM-8 - pulse amplitude modulation with eight levels - is one of the proposed solutions; another is discrete multi-tone (DMT). [With DMT] using a set of electrical sub-carriers and doing adaptive bit loading means that even with bandwidth-limited components, you can transmit over the required distances. There was a demo at the exhibition from Fujitsu Labs showing DMT over 2km using a 10 Gig transmitter and receiver.  

This is of interest to us as we have a 100 Gigabit direct detection dense WDM solution today and are working on the product evolution.

We use the existing [component/ module] ecosystem for our current direct detect solution. These developments bring up some interesting new thoughts for our next generation.

So you can go beyond 100 Gigabit direct detection?

Right now we are running 28 Gig on a single wavelength. Clearly with speeds increasing and with these kind of developments [PAM-8, DMT], you see that this is not the end.   

Part 1: Software-defined networking: A network game-changer, click here

Part 2: OFC/NFOEC 2013 industry reflections, click here

Part 3: OFC/NFOEC 2013 industry reflections, click here

Part 4: OFC/NFOEC industry reflections, click here