IBM researchers
have made cheap low-powered analogue to digital converters (ADCs) which could
allow 100Gbps networks over long-distance fibre, with a cheap device to send and
receive data each end.
The breakthrough will allow cheaper and simpler devices that can convert the
signals on fibres into digital information. This will cut energy use and could
revolutionise mobile phones and radio astronomy, while making high speeds easier
and cheaper to deliver to a wider range of devices. The technology, produced by
IBM researchers, working with Ecole Polytechnique Fédérale de Lausanne (EPFL) in
Switzerland, is based around tiny ADCs integrated onto the same CMOS chips which
hold other functions.
Smaller ADCs than anyone else
Although light signals are sent along fibres in digital form, by the time
they reach the far end of the cable they have to be processed to clean up and
receive the data which was sent, explained Martin Schmatz, who manages the
systems division of IBM’s Zurich lab: “The problem is fibre has dispersion,” he
explained, so the signal is blurred when it reaches its destination. “During
transit, some energy is dispersed to other frequencies. We need to shift that
bit of energy back to the right place.”
Currently, long distance links use “dispersion compensating” fibres, in which
the main fibre is combined with another which has the opposite dispersion
characteristics. This is complex and expensive says Schmatz – and the IBM
breakthrough allows the signal to be cleaned up with dispersion cancelled out
electronically.
“You can actually correct for dispersion by a mathematical approach,” Schmatz
told TechWeekEurope. “If you digitise the signal, you can then apply
mathematical functions to correct for the dispersion.” Telephone lines regularly
use maths to correct for signal dispersion but – as Schmatz pointed out – they
only have to work at around 56kbps on digitised voice traffic. Digital
correction on a fast fibre network would have to work far faster.
Fibre networks carry long-haul Ethernet data at 100Gbps, but to view them as
analogue signals and clean them up, needs an eight bit resolution, said Schmatz:
“If you have four channels, all of a sudden you have 2.5 Terabits persecond
(Tbps) coming out of the ADC.”
ADCs have normally been implemented as separate components made with
different technology, because traditional CMOS circuits aren’t optimised for
analogue signals. However, it is expensive and inefficient to take data at that
rate off one chip and onto another, Schmatz told us.
The IBM team managed to show it is possible to make a very tiny ADC out of
standard 32nm CMOS, opening the way to integrating many of these components onto
the same chip as the rest of the network hardware.
“It is power inefficient to ship the data from an ADC to a CMOS chip,” he
said. “We were able to show that it is possible to use a plain vanilla digital
process on a digital CMOS chip, not optimised for analogue, to build such an ADC
which has an extremely high content of analogue circuits.”
These ADCs are very power efficient, and consume a tiny amount of the CMOS
chip’s area: “The majority of the power – and area – needs to be assigned to the
digital circuit. The ADCs need to be tiny.”
100Gbps Ethernet signals are sent as four 25Gbps streams, separated by phase
and polarisation. Because the Nyquist-Shannon theorem says they need
“oversampling”, a 100Gbps Ethernet channel would actually need four 64Gbps ADCs.
That’s faster than can be easily done, so the group proposes using more ADCs,
each of which handles a time slice of the total signal (so it might use 64 ADCs,
each at 1Gbps).
“We will end up with 256 ADCs on one side of the chip and a lot of processing
on the other,” predicted Schmatz. “Is this possible? Absolutely!”
Radio telescopes and phones
The technique could be available in 2014, he said, and will have applications
beyond fast long-haul Ethernet.”There are many applications where the signal you
are using is in the analogue domain,” he said.
For instance, a set-top box could take in the whole frequency spectrum from a
cable modem and sort out the individual channels in a single chip.
The approach could also revolutionise radio astronomy, which is all about
finding signals in a big analogue spectrum. Schmatz hopes to provide signal
processing equipment for the Square Kilometer Array (SKA), an international
project to build the world’s largest and most sensitive radio telescope.
Another possibility would be to have one chip to handle all the radio signals
coming into a mobile phone. “You could have one wideband antenna, sample
everything from 400MHz to 6GHz – and then sort it out by number crunching so
you have Wi-Fi and Bluetooth at 2.4 GHS, and 3G and 4G at frequencies like
900MHz – and you do all of that in the digital domain.”
“Most of the ADCs on the market today weren’t designed to handle the massive
Big Data applications we are dealing with today – it’s the equivalent
of funnelling water through a straw from a fire hose.”
By:
Peter Judge
Source: http://www.techweekeurope.co.uk/news/ibm-adc-breakthrough-100gbps-big-data-108364
