Today, the fastest throughput on most of the global telecommunications network is 10 billion bits (gigabits) per second so sending the contents of a full DVD would keep a link tied up for around 4 seconds. It has been that way since 1996,an era when users stepped onto the information superhighway via dial-up modems and the original Netscape Navigator browser.Masses of optical fibre cables were added to the backbone during the dotcom boom a decade ago, initially producing a huge glut in capacity. Now new users and new services – social media, video downloads, streaming audio and video, file sharing and cloud computing are filling up those fibre pipes. More capacity will soon be needed, but providing it poses considerable challenges.
In today’s fibre-optic backbone, digital 1s and 0s are represented by switching a laser beam on and off. Lasers send dozens of separate signals down each optical fibre at slightly different wavelengths, which together can convey 10 gigabits of data per second. But this techniques has its limitations: trying to raise the data rate for each wavelength won’t work, as the signals start to blur together. The problems of signal integrity are ”100 times worse at 100 gigabits than they are at 10”.
The starting point for the new 100-gigabit technology was to ditch the off-and-on switching, and instead modulate the phase of the light waves moving them ahead or behind by a fixed increment. The simplest approach is to shift the phase by 90 degrees – one-quarter of a wavelength – to distinguish a 0 from a 1.Higher data rates require a more elaborate process, called quadrature phase-shift keying, which has four possible shifts, +135, +45, -45 and -135 degrees, each representing a different pair of bits, 00, 01, 10 or 11.
About Gigabit:
The 100-gigabit system abandons on-off switching in favour of changing the phase of the light waves That alone isn’t enough to reach 100 gigabits. To achieve that goal requires signals with two different polarisations, which can be separated at the receiver, each carrying 50 gigabits.Even then, after passing through hundreds of kilometres of fibre, the input signal must be processed with light from an internal laser to extract a clear signal. The receivers are equipped with powerful electronic circuits, which analyse the signal and minimise noise added along the cable, says Amin. “The end points got a lot smarter and can deal with everything in between.”
The Canadian telecoms equipment company Nortel, which built the Verizon system, has shown it can transmit signals more than 2000 kilometres in a test on an Australian network owned by Telestra. “The 2000 kilometres was a bit of heroism. For most applications we’re saying it’s more like 1000 kilometres,” says John Sitch, senior adviser on optical R&D at Nortel.There are still some problems facing the ultra-fast backbone. Noise can be a killer if 10 and 100-gigabit channels are transmitted through the same fibre at closely spaced wavelengths. And the first generation of 100-gigabit systems can only stretch half as far as today’s 10-gigabit systems before signals are lost.
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