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Wednesday, Mar 12, 2003 8:30 PM UTC
The myth of interference
Internet architect David Reed explains how bad science created the broadcast industry.
Topics: Entertainment News
There’s a reason our television sets so outgun us, spraying us with trillions of bits while we respond only with the laughable trickles from our remotes. To enable signals to get through intact, the government has to divide the spectrum of frequencies into bands, which it then licenses to particular broadcasters. NBC has a license and you don’t.
Thus, NBC gets to bathe you in “Friends,” followed by a very special “Scrubs,” and you get to sit passively on your couch. It’s an asymmetric bargain that dominates our cultural, economic and political lives — only the rich and famous can deliver their messages — and it’s all based on the fact that radio waves in their untamed habitat interfere with one another.
Except they don’t.
“Interference is a metaphor that paints an old limitation of technology as a fact of nature.” So says David P. Reed, electrical engineer, computer scientist, and one of the architects of the Internet. If he’s right, then spectrum isn’t a resource to be divvied up like gold or parceled out like land. It’s not even a set of pipes with their capacity limited by how wide they are or an aerial highway with white lines to maintain order.
Spectrum is more like the colors of the rainbow, including the ones our eyes can’t discern. Says Reed: “There’s no scarcity of spectrum any more than there’s a scarcity of the color green. We could instantly hook up to the Internet everyone who can pick up a radio signal, and they could pump through as many bits as they could ever want. We’d go from an economy of digital scarcity to an economy of digital abundance.”
So throw out the rulebook on what should be regulated and what shouldn’t. Rethink completely the role of the Federal Communications Commission in deciding who gets allocated what. If Reed is right, nearly a century of government policy on how to best administer the airwaves needs to be reconfigured, from the bottom up.
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Spectrum as color seems like an ungainly metaphor on which to hang a sweeping policy change with such important social and economic implications. But Reed will tell you it’s not a metaphor at all. Spectrum is color. It’s the literal, honest-to-Feynman truth.
David Reed is many things, but crackpot is not one of them. He was a professor of computer science at MIT, then chief scientist at Software Arts during its VisiCalc days, and then the chief scientist at Lotus during its 1-2-3 days. But he is probably best known as a coauthor of the paper that got the Internet’s architecture right: “End-to-End Arguments in System Design.”
Or you may recognize him as the author of what’s come to be known as Reed’s Law — which says the true value of a network isn’t determined by the number of individual nodes it connects (Metcalfe’s Law) but by the far higher number of groups it enables. But I have to confess that I’m biased when it comes to David Reed. I first encountered him in person three years ago at a tiny conference when he deftly pulled me out of a hole I was digging for myself in front of an audience of my betters. Since then, I’ve watched him be bottomlessly knowledgeable on a technical mailing list and patiently helpful as a source for various articles I’ve worked on.
It doesn’t take much to get Reed to hold forth on his strong, well-articulated political and social beliefs. But when it comes to spectrum, he speaks most passionately as a scientist. “Photons, whether they are light photons, radio photons, or gamma-ray photons, simply do not interfere with one another,” he explains. “They pass through one another.”
Reed uses the example of a pinhole camera, or camera obscura: If a room is sealed against light except for one pinhole, an image of the outside will be projected against the opposite wall. “If photons interfered with one another as they squeezed through that tiny hole, we wouldn’t get a clear image on that back wall,” Reed says.
If you whine that it’s completely counterintuitive that a wave could squeeze through a pinhole and “reorganize” itself on the other side, Reed nods happily and then piles on: “If photons can pass through one another, then they aren’t actually occupying space at all, since the definition of ‘occupying’ is ‘displacing.’ So, yes, it’s counterintuitive. It’s quantum mechanics.”
Surprisingly, the spectrum-as-color metaphor turns out to be not nearly as confounding to what’s left of common sense. “Radio and light are the same thing and follow the same laws,” Reed says. “They’re distinguished by what we call frequency.” Frequency, he explains, is really just the energy level of the photons. The human eye detects different frequencies as different colors. So, in licensing frequencies to broadcasters, we are literally regulating colors. Crayola may own the names of the colors it’s invented, and Pantone may own the standard numbers by which digital designers refer to colors, but only the FCC can give you an exclusive license to a color itself.
Reed prefers to talk about “RF [radio frequency] color,” because the usual alternative is to think of spectrum as some large swatch of property. If it’s property, it is easily imagined as finite and something that can be owned. If spectrum is color, it’s a lot harder to think of in that way. Reed would recast the statement “WABC-AM has an exclusive license to broadcast at 770 kHz in NYC” to “The government has granted WABC-AM an exclusive license to the color Forest Green in NYC.” Only then, according to Reed, does the current licensing policy sound as absurd as it is.
But if photons don’t interfere, why do our radios and cellphones go all crackly? Why do we sometimes pick up two stations at once and not hear either well enough?
The problem isn’t with the radio waves. It’s with the receivers: “Interference cannot be defined as a meaningful concept until a receiver tries to separate the signal. It’s the processing that gets confused, and the confusion is highly specific to the particular detector,” Reed says. Interference isn’t a fact of nature. It’s an artifact of particular technologies. This should be obvious to anyone who has upgraded a radio receiver and discovered that the interference has gone away: The signal hasn’t changed, so it has to be the processing of the signal that’s improved. The interference was in the eye of the beholder all along. Or, as Reed says, “Interference is what we call the information that a particular receiver is unable to separate.”
But, Reed says, “I can’t sign on to ‘It’s the receiver, stupid.'” We have stupid radios not because we haven’t figured out how to make them smart but because there’s been little reason to make them smart. They’re designed to expect signal to be whatever comes in on a particular frequency, and noise to be everything on other frequencies. “The problem is more complex than just making smart radios, because some of the techniques for un-confusing the receiver are best implemented at the transmitter, or in a network of cooperating transmitters and receivers. It’s not simply the radios. It’s the systems architecture, stupid!”
One of the simplest examples of an architecture that works was invented during World War II. We were worried that the Germans might jam the signals our submarines used to control their radio-controlled torpedoes. This inspired the first “frequency-hopping” technology: The transmitter and receiver were made to switch, in sync, very rapidly among a scheduled, random set of frequencies. Even if some of those frequencies were in use by other radios or jammers, error detection and retransmission would ensure a complete, correct message. The U.S. Navy has used a version of frequency-hopping as the basis of its communications since 1958. So we know that systems that enable transmitters and receivers to negotiate do work — and work very well.
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