Quadrature leap: The science behind record data transmission speeds

14 Feb 2017

Ethernet cables. Image: Bacho/Shutterstock

Do you know your QPSK from your 16QAM? Or how about your WDM from your DWDM? Welcome to the acronym-filled world of photonics and data transmission.

It seems that at least once each year, we hear news that another research group in some part of the world has broken a previously held data transmission speed record.

Last year for example, Dublin made its way into the history books with the recording of a blistering speed of 2.1Tbps, from BT’s research centre in Adastral Park to its exchange in the Irish capital.

Video content driving a need for speed

Of course, what determines the fastest speed depends on who you ask. Various groups have achieved even higher speeds but across much smaller distances, as demonstrated by researchers from the Technical University of Denmark, who achieved 43Tbps on a single piece of fibre in 2014.

Aside from gaining a moment of glory with a new record, what are the real benefits of these kinds of speed?

According to BT’s head of transmissons futures and innovation, Kevin Smith, it is our insatiable demand for online content that is pushing researchers and broadband providers to have the infrastructure in place to prevent an internet slowdown on a national, or even global, level.

Before getting into the details of what they’re doing, it might be worth knowing how exactly the data on your device gets there.

Adastral Park

Adastral Park, in Martlesham, Ipswich. Image: VisMedia/BT

Surpassing old world benchmarks

Quite simply, each fibre optic cable contains different wavelengths of light – as many as 80 – and each carries a large quantity of data; a technology referred to in the industry as wavelength division multiplexing, or WDM for short.

With demand on broadband networks growing on average between 30pc and 40pc every single year thanks to the rapid adoption of services like Netflix, Amazon and YouTube, WDM has quickly become obsolete.

“If you look to WDM networks as they were 10 years ago, then each one of those individual wavelengths might be at a 10Gbps speed,” Smith said in conversation with Siliconrepublic.com.

To add some context, 80 of these wavelengths at 10Gbps per wavelength equals a maximum fibre optic cable speed of 800Gbps.

“That’s the kind of benchmark of the old world,” Smith added. “The stuff that we’re working on, and is now going into our network, has been so for the past two-plus years.”

That “stuff” BT is working on is the next evolutionary step: DWDM, with the extra ‘D’ standing for dense.

DWDM with QPSK or 16QAM?

At its most basic level, DWDM is the ability to squeeze more data into wavelengths of light to the point of reaching a tenfold increase in capacity for each wavelength, from 10Gbps to 100Gbps.

If we increase the number of acronyms again, this DWDM speed of up to 100Gbps is based on quadrature phase shift keying (QPSK) – capable of selecting one of four possible carrier phase shifts of bits.

What Smith and other BT researchers are doing to amp this up again is to increase the speed using something called quadrature amplification modulation (QAM), which doubles the rate of data transfer, from two bits of data per symbol to four.

The rate that BT and many other broadband providers are using at the moment is 16QAM.


A 16QAM constellation diagram. Each quarter on this diagram represents one symbol, with four bits of information in each. Image: Frank Bättermann/Flickr (CC BY-SA 2.0)

The challenge of distance

It is now a matter of squeezing more and more data onto each of these wavelengths to stay two steps ahead of global internet demand.

“We are looking in our labs now at tech that gives 200Gbps per wavelength, [of] which may see early deployment in the next year or two. We are also studying – which is longer-term research – to see how we can get 400Gbps in a single wavelength,” Smith said.

“You can see this progression from 100Gbps to 8Tbps, and then going from 200GB to 16TB, and then in the near future, 400GB to 32TB.”

So what is stopping research groups like the one in Adastral Park from simply increasing the capacity tomorrow if it appears to be – at least on the surface – a relatively straightforward exercise?

Migrating to a flexible grid

With existing DWDM resting on what is referred to as a ‘fixed grid’, there is only so much speed that can be achieved before it hits an optical brick wall – that being 200Gbps.

Designed 30 years ago when 10Gbps was considered an astronomical amount of data, researchers thought that it would be at least half a century before that amount would ever be surpassed.

But using a new technology simply called ‘flexible grid’, the 50GHz barrier that existed in a fixed grid is removed from an individual wavelength, thereby offering far, far superior broadband speeds.

So while 80 wavelengths might have been the limit using a fixed grid, the BT team was able to demonstrate – a then world record – 120 wavelengths on a single fibre optic cable.

By running some basic math, this would increase a cable’s capacity to increase the amount of data by an extra 50pc.

Fibre optic cable laying

Optical fibre cable drum. Image: Matjoe/Shutterstock

It all boils down to efficiency

But in the real world, the largest amount of data does not necessarily mean the most feasible, as Smith admitted this would likely be scaled back to 30pc extra capacity in the next generation.

In the end, it all comes down to efficiency. One day, it is hoped that BT and other researchers will be able to push fibre optic cables even further, to somewhere in the region of 100Tbps.

“What we don’t want to do is really put loads and loads of fibres in, as it’s very wasteful and more expensive. You want to take each fibre and, as realistically as possible, put as much capacity on that specific of piece of fibre,” Smith said. “That’s what’s driving us.”

With news that Nokia recently achieved a record 1Tbps direct-to-home fibre speed – significantly more than the 1Gbps speed of Google Fiber – researchers will need to work at ‘light speed’ ahead of a world of billions upon billions of connected devices.

Colm Gorey was a senior journalist with Silicon Republic