http://www.economist.com/printedition/PrinterFriendly.cfm?Story_ID=987690&CFID=301055&CFTOKEN=47471685&
Optical communications Cutting the cable Feb 14th 2002 >From The Economist print edition "Free-space" optics requires no fibre. That may be an advantage FIBRE optics revolutionised communication by abolishing the law that light can travel only in a straight line. From that point on, light signals could be treated in the same way as electrical ones, and bent round corners. Some people, however, are never satisfied. And these dissatisfied engineers are trying to turn the clock back by developing systems that use "free-space" optics-in other words sending information from place to place by shining laser beams through the air. Free-space optics has three advantages. It is easy to install. It can handle a technology known as wavelength division multiplexing (WDM) without, as it were, blinking. And it seems suited to a new-and allegedly uncrackable-encryption technique called quantum key distribution. Speed of installation comes from not having to dig up the road to lay conduits. Free-space optics may thus be an answer to the difficulty of providing broadband connections to customers' homes and offices-the so-called "last mile". Free-space links that operate at speeds of up to 20 gigabits a second-as good as fibre-have now been demonstrated. They can be installed in hours rather than the weeks or months normally needed for broadband access. And if they can be put into place quickly, they can be upgraded quickly, too. That matters in the context of WDM, a technique that allows a single optical path to carry thousands of parallel channels, as long as each is encoded in a slightly different colour. Upgrading a fibre network for WDM is hard. First, individual fibres are each compatible with only a few WDM schemes. The exact chemical composition of a fibre's glass determines how transparent it is to different frequencies, and also its tendency to disperse those frequencies even when it is transparent. Both restrictions reduce the number of channels that can be carried. Moreover, even if a particular fibre can be used with a particular scheme, the light sources, amplifiers, switches and associated paraphernalia usually cannot. Amplifiers, for instance, will not boost all colours equally, so special devices are necessary to compensate. Free-space optics suffers from none of these problems. Air is transparent to a wide range of frequencies and has few dispersive tendencies (at least, when the weather is good). And with the associated kit clustered together in base stations, upgrades are easy to carry out. The third advantage-for quantum key distribution-is more speculative. The technique exploits the arcana of quantum mechanics to let two computers swap a cryptographic key (and thus the means to decode a message) with perfect security. Quantum key distribution has been demonstrated successfully in fibres, but it suffers from one major drawback: it requires a dedicated link, and so cannot be implemented in a network. However, two experiments carried out in the past few weeks have shown that it works with free-space optics. First, researchers at QinetiQ, a British-government-owned company, and Ludwig Maximilian University, in Munich, Germany, exchanged keys between two alpine mountain-tops more than 23km apart, though they did so at night, when sunlight could not confuse the signal. Then, another group of researchers, from Los Alamos National Laboratory in New Mexico, announced that they had performed a 10km key exchange in broad daylight. These two groups are working towards military applications in which the key is exchanged from the ground to a satellite. But both recognise that the technology might be exploited commercially, and are part of a European Union collaboration called QuComm that is encouraging this. Free-space optics would have the odd drawback, such as flocks of birds, showers of snowflakes or banks of fog interrupting the beams. But message-encoding systems are already set up to cope with lost data. Many customers might be willing to put up with a 99.999% available service that could be installed straight away, rather than waiting indefinitely for the 100% availability of fibre. Copyright © 2002 The Economist Newspaper and The Economist Group. All rights reserved. -- ----------------- R. A. Hettinga <mailto: [EMAIL PROTECTED]> The Internet Bearer Underwriting Corporation <http://www.ibuc.com/> 44 Farquhar Street, Boston, MA 02131 USA "... however it may deserve respect for its usefulness and antiquity, [predicting the end of the world] has not been found agreeable to experience." -- Edward Gibbon, 'Decline and Fall of the Roman Empire' --------------------------------------------------------------------- The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to [EMAIL PROTECTED]