Future Proof Your Network: Flexible Grid Architecture

The driver for any development in optical communications technology is, almost without exception, a reduction in cost-per-bit/km travelled (although there’s one interesting exception to this which I’ll talk about at some future point). This intensely capitalistic and utilitarian approach has enabled the dramatic growth in the internet (YouTube needs very low cost/bit to be viable), but are we approaching the point where the rate of cost reduction may start to slow down? In the US, for example, there’s recently been a move away from ‘all you can eat’ data plans to something approaching (very slowly) the capped plans found in many other parts of the world – partially in response to the recognition that there is a finite amount of bandwidth available at any point in the network.

In the optical space, we have, over the past 15 years, moved to higher and higher per-channel bit rates running on more-closely-spaced WDM channels. However, we are (as I mentioned previously) reaching the limit of what can be achieved on the ITU Grid-based systems introduced in the 1990s. Consequently, fibre bandwidth, which only a few years ago was being proselytised by Gilder as being effectively ‘infinite’, is increasingly being seen as a finite resource which needs to be managed as effectively as possible using all the techniques at our disposal.

One solution, which is gaining a great deal of traction at the moment is to move away from the constraints of the ITU Grid to what we term a flexible grid architecture where the channel optical bandwidth can be dynamically adjusted to meet the requirements of the signal being sent through it and hence maximise the data carrying capacity of the fibre.

This ability to control the channel bandwidth and position with GHz resolution has been utilized in many research papers as I discussed in my previous blog. In practice, however, the complexity of managing a network with such fine granularity may outweigh its advantages. Market requirements indicate that a channel bandwidth granularity of 12.5 GHz will meet future channel bandwidth requirements and that even 25 GHz channel bandwidth increments may be sufficient.

First generation WSS (typically based on MEMS and/or Liquid Crystal technologies) allocate a single switching element (pixel) to each channel which means that the channel bandwidth and centre frequency are fixed at the time of manufacture and cannot be changed in service. However, second generation WSS, based on Liquid Crystal on Silicon (LCoS) or 2D MEMS mega-pixel matrix switching arrays, permit dynamic control of channel centre frequency and bandwidth through ‘on the fly’ modification of internal pixel arrays via embedded software.

Furthermore, not only must the core switching elements in a ROADM be capable of supporting flexible grid architectures, but the multiplexer/demultiplexers and filter arrays must support the same degree of flexibility. The flexibility provided by LCoS technology can also be applied to these high-port-count (e.g. 1×23) multiplexer/demultiplexers and programmable filter arrays.

This discussion has focused on the requirements for the wavelength-selective elements in a flexible-grid ROADM as these will be the first part of the flexible grid network that has to be deployed to ensure the network is future-proofed. However flexible grid networks will also require additional component developments including scanning optical channel monitors capable of handling polarization multiplexed signals with varying bandwidths and signal formats and signal (and local oscillator, for coherent systems) lasers capable of operating at the finer frequency increments implied by flexible grid architectures (6.25 GHz for 12.5 GHz channel increments and 12.5 GHz for 25 GHz channel increments). This provides a continuing challenge to those of us in the optical space as we continue to improve the price/performance of our components and modules which underpin the networks of the future.

Flexible grid technology (not grid-less) will allow the optimum usage of the finite operating window in a fibre and allow operators to continue down the path of reducing the cost per bit/km of data travelling through the network.

Any comments are welcome.

Video: WaveShaper M-Series Demonstration at OFC 2010

OFC – One More Time

Well, OFC now seems a distant memory and the pleasures and fleshpots of the San Diego nightlife are unlikely to be reprised until 2015 at the earliest. However, a couple of key take-aways stand out for me now that the dust has settled.

The first is how quickly silicon is becoming a key part of optical communications. I’m not talking here about silicon photonics, which has been around for a long time now without making any real impact on real systems, but rather about the developments in digital signal processing technology of the type pioneered by Nortel. This is not the place to go into all the developments in this technology, but a particularly interesting data point for me was that at the rump session put on to discuss this issue, the ‘do everything in silicon’ guys nearly had it over the ‘optics is still the way of the future’ guys – and this despite it being primarily an optics-focused (pardon the pun) conference.

The second take-away from the haze of workshops, technical presentations and customer meetings was that the next generation of DWDM network architectures will move away from the strict adherence to the current ITU grid and be based on some form of variable channel bandwidth to maximize the data carrying capacity of the fibre. An example of this was the “SLICE” network architecture from NTT which demonstrated a network capable of adjusting the transmission channel bandwidth in 12.5 GHz steps to accommodate varying channel bit rates and to overcome different levels of impairments in the system.

To put this into perspective, it is worth remembering that when the ITU grid was first proposed in the mid ‘90s, the concept of a high-bit-rate dense WDM channel was something carrying 2.5 Gbit/sec in a 200 GHz channel. Since then the channel spacing has dropped, initially to 100 GHz and now to 50 GHz. Simultaneously, the channel data rates have ramped up to the currently-deployed 40G and future extensions to 100G and beyond. The introduction of coherent techniques such as DP-QPSK means that we can, reasonably comfortably, stuff 100 Gbit/sec down a 50 GHz channel but beyond this point will need to start ‘opening-up’ the channel bandwidth again to accommodate the 400 Gbit/sec or 1 Tbit/sec signals of the future.

To achieve this level of flexibility requires re-thinking a number of the elements in an optical comms system, but most critical to this will be the design of the ROADMS and Wavelength Selective Switches which now sit at the core of most networks. Finisar’s LCoS switching technology provides the flexibility needed for these variable-bandwidth architectures and in future columns I’ll discuss more about how this works and how the unique capabilities of LCoS will support future non-ITU-grid architectures.

At this point, I’d like to put in an unashamed plug for our WaveShaper technology. The plethora of activity in new modulation and transmission formats using non-standard channel spacing and non-ITU-Grid wavelengths means that there’s a need for an easily programmable filter which can simulate the (currently) non-standard components required for tomorrows networks. The WaveShaper family of Programmable Optical Processors are based on Finisar’s LCoS optical engine are an ideal tool for such studies as they allow extremely precise control of an optical filter shape, bandwidth and phase. Indeed, WaveShapers were used in many of the OFC post-deadline ‘hero’ experiments - in particular papers pdpb8, pdpc1, pdpc2, pdpc4 and pdpd2. More information on Finisar’s WaveShaper range can be found here.

Finally – Finisar Australia is hiring! As Infonetics recently announced Finisar is now number one in the WSS space. To support this growth and to help us develop the next generations of WSS products which will cement our position as the number one WSS supplier, we are looking for engineers and scientists in a range of disciplines to join our team in Sydney. With year-round great weather and a superb mix of beach and big-city lifestyle, combined with the opportunity to work on cutting-edge technology this is an opportunity not to be missed! Check out our current job postings.

Old Poole’s Almanack: Predictions for 2010 and Beyond

As this is the start of another decade, I thought I’d begin by penning some thoughts on what might occur in the optical communications space in the coming year, a sort of Old Moore’s Almanack, complete with the usual predictions of a rain of frogs and the appearance of little green men in some remote area. However, it became apparent that it would be much more interesting to look a bit further ahead and think about what might happen in the next decade, as this gives time for some of the current research ideas to play out in the market. Before I begin, I must provide this important legal disclaimer: note that I am not obligating myself nor the Company to these crystal ball predictions as neither of us is licensed fortune tellers.

So, for what it’s worth, here’s Old Poole’s Almanack for what may be in store in optical communications in the twenty-teens.

1 Gb/sec to the Home
For those of us with long-term memories, it has been nearly 20 years since the introduction of the Hayes Smartmodem- the original 300 baud modem that kick-started the era of computer–to–computer communications. Today, 100 Mb/sec is becoming commercially available on an increasing scale. As communication bandwidth to the home has typically doubled every couple of years, then home bandwidth connectivity in excess of 1 Gb/sec is a shoo-in for 10 years from now.

Coherent to the Rescue
The rising demand for bandwidth in the home will, in turn, drive the need for more capacity in the backbone. As mentioned in a previous column, coherent transmission is coming soon to a system near you. Whilst significant (as opposed to bleeding edge) 100G coherent deployments are probably still a couple of years away, by the end of the decade, it looks very likely that coherent transmission in one of its various forms and flavors, will play a significant role in high speed communications.

The End of the ITU Grid?
Whilst I’m pretty comfortable about the first two predictions, a slightly more speculative prediction is that over the next 10 years we may see the beginning of the end of the rigid adherence to the ITU DWDM grid. The ITU grid was introduced in the mid 1990s when a high-speed signal was 2.5 Gb/sec and 100 GHz channel spacing seemed pretty dense. We are now reaching the point where 100 Gb/sec on 50 GHz channel spacing looks more like the norm in long-haul networks. Moving forward, however, it is clear that we will need to use the available spectrum within a fiber in an efficient as possible manner. This in turn will require the network to transmit low bit-rate signals in narrow channels and high-rate signals in broader channels. Whilst this approach is common in radio engineering where the issues of channel congestion have been around for many years, we are likely to adopt many of the techniques used there to maximize the capacity of a fiber network (and coherent modulation is just the first of these). Whilst this will require significant advances in Network Operating Systems to manage the complexity inherent in such an arrangement, many of the components required (i.e. transceivers with programmable data rates, Wavelength Selective Switches with variable bandwidth, etc.) are indeed already either developed or under investigation.

Next Stop: 400 Gb/sec/wavelength
Whilst we are still a couple of years away from 100G deployments, the debate has already begun over the next jump in transmission data rates – do we go for 400 Gb/sec or jump straight to 1 Tb/sec? Since the driver of telecom bit-rate is now the need to match the pipe-size on the big-iron core routers, we need to look at what the datacom guys are thinking. Here, it looks like the next step will be 400G, based on an extension of the 4×25 Gbaud multi-wavelength technology that is currently being developed for the CFP MSA. So, will we see 400 Gb/sec/wavelength telecom systems by 2020? Here, I’ll stick my neck out and say yes – we probably will as there are also strong technical reasons for increasing the per-wavelength capacity. However, the technical challenges to be overcome before we can get to this point are massive and will almost certainly limit any deployment to very high capacity backbone links, such as found along the US east coast and between the major cities in Europe.

Green is Good
Finishing up with another no-brainer – despite the atrocious failure of political will in Copenhagen, the need to use the finite resources of the earth more efficiently will become an even greater driver of innovation going forward. Optical communications, as has been shown many times, is by far the ‘greenest’ form of high-speed communications and will continue to be favored as we work to reduce our energy consumption per unit of bandwidth consumed. This will, in turn, drive new network architectures and routing algorithms that favor energy efficiency (and hence more optics) over existing ‘shortest path,’ hence router-intensive designs. Green is good for the planet and for those of us in the optical communications business too!

Well, that’s my two penn’orth – any additional predictions, or comments on the above, are welcome and greatly appreciated.

Out of His Depth Overview

Hi folks. In my new blog I plan to provide a personal take on what’s new, interesting, unusual, and/or under-reported in the area of optical communications. Stay tuned for next week’s official post! First up, a quick introduction to the esoteric joys of the Australian Conference on Optical Fibre Technology (ACOFT) which has just celebrated its 32nd consecutive year, making it nearly as long running as the much better-known OFC series in the US. However, I don’t think any of the recent OFC conferences can offer anything like this video clip, in which Associate Prof Peter Farrell from Melbourne University shows how to entertain the audience between sessions…enjoy.

As always, comments are strongly encouraged.

Simon