[Note: This item comes from friend Steve Goldstein. DLH]
From: Steve Goldstein <steve.goldstein@cox.net>
Date: February 3, 2010 6:47:12 AM PST
To: Hendricks Dewayne <dewayne@warpspeed.com>
Subject: Internet backbone breaks the 100-gigabit barrier (phase shift mod. + polarization mux.)
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The first inklings of what the upgrade might look like can be seen in an ultra-fast 900-kilometre fibre-optic link between Paris in France and Frankfurt in Germany installed by telecoms firm Verizon. It is a foretaste of a high-speed internet backbone with enough capacity to satisfy bandwidth-hungry applications well into the future.
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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″, says Dimple Amin of network equipment maker Ciena of Linthicum, Maryland.
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.
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.”
Without this, “we could never have gotten into the ultra long haul” of 1000 to 1500 kilometres, says Glenn Wellbrock, Verizon’s director of network backbone architectures.
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, Wellbrock says.
“But you don’t need to try 4000 kilometres,” Wellbrock points out. “The majority of traffic in the US is 1500 kilometres or less, and it’s less in Europe.” As first steps go, a near 900-kilometre link isn’t a bad effort.