We live in a world where digital technology is so advanced that even what used to be science fiction looks quaint. (Really, Kirk, can you download Klingon soap operas on that communicator?) Yet underlying it all is a mathematical theory that dates back to 1948.
That year, Claude Shannon published the foundational information theory paper, A Mathematical Theory of Communication (PDF). Shannon’s work underlies communications channels as diverse as proprietary wireless networks and leads on printed circuits.
Yet in Shannon’s time, as we all know, there were no personal computers, and the only guy communicating without a desk telephone was Dick Tracy. In our current era, the number of transistors on chips has gotten so dense that the potential limits of Moore’s Law are routinely discussed. Multi-core processors, once beyond the dreams of even supercomputers, are now standard on consumer laptops. Is it any wonder then, that even Shannon’s foresighted genius may have reached its limits?
From the Internet to a child’s game of telephone, the challenge in communications is to relay a message accurately, without losing any of it to noise. Prior to Shannon, the answer was a cumbersome redundancy. Imagine a communications channel as a conveyer belt, with every message as a pile of loose diamonds: If you send hundreds of piles down the belt, some individual diamonds might fall off, but at least you get most of the diamonds through quickly. If you wrap every single diamond in bubble wrap, you won’t lose any individual diamonds, but now you can’t send as many in the same amount of time.
Shannon’s conceptual breakthrough was to realize you could wrap the entire pile in bubble wrap and save the correction for the very end. “It was a complete revolution,” says Schulman, “You could get better and better reliability without actually slowing down the communications.”
In its simplest form, Shannon’s idea of global error correction is what we know as a “parity check” or “check sum,” in which a section of end digits should be an accurate summation of all the preceding digits. (In a more technical scheme: The message is a string n-bits long. It is mapped to another, longer string of code words. If you know what the Hamming distance—the difference in their relative positions—should be, you can determine reliability.)
Starting back as far as the late 1970s, researchers in the emerging field of communication complexity were beginning to ask themselves: Could you do what Shannon did, if your communication were more of a dialogue than a monologue?
What was a theoretical question then is becoming a real-world problem today. With faster chips and multi-core processes, communications channels are becoming two-way, with small messages going back and forth. Yet error correction still assumes big messages going in one direction. “How on earth are you going to bubble wrap all these things together, if what you say depends on what I said to you?” says Schulman.
The answer is to develop an “interactive” form of error correction. In essence, a message sent using classic Shannon error correction is like a list of commands, “Go Left, Go Right, Go Left again, Go Right…” Like a suffering dinner date, the receiver only gets a chance to reply at the end of the string.
By contrast, an interactive code sounds like a dialog between sender and receiver:
“Going left.”“Going left too.”
“Going right.”
“Going left instead.”“Gzzing zzzztt.”
“Huh?”
“Going left too.”
The underlying mathematics for making this conversation highly redundant and reliable is a “tree code.” To the lay person, tree codes resemble a branching graph. To a computer scientist they are “a set of structured binary strings, in which the metric space looks like a tree,” says Schulman, who wrote the original paper laying out their design in 1993.
“Consider a graph that describes every possible sequence of words you could say in your lifetime. For every word you say, there’s thousands of possibilities for the next word,” explains Amit Sahai, “A tree code describes a way of labeling this graph, where each label is an instruction. The ways to label the graph are infinite, but remarkably Leonard showed there is actually one out there that’s useful.”
Tree codes can branch to infinity, a dendritic structure which allows for error correction at multiple points. It also permits tree codes to remember entire sequences; how the next reply is encoded depends on the entire history of the conversation. As a result, a tree code can perform mid-course corrections. It’s as if you think you’re on your way to London, but increasingly everyone around you is speaking French, and then you realize, “Oops, I got on at the wrong Chunnel station.”
The result is that instructions are not only conveyed faster; they’re acted on faster as well, with real-time error correction. There’s no waiting to discover too late that an entire string requires re-sending. In that sense, tree codes could help support the parallel processing needs of multi-core machines. They could also help in the increasingly noisy environments of densely packed chips. “We already have chip-level error correction, using parity checks,” says Schulman. But once again, there’s the problem of one-way transmission, in which an error is only discovered at the end. By contrast, a tree code, “gives you a check sum at every stage of a long interaction,” says Schulman.
Unfortunately, those very intricacies are why interactive communications aren’t coming to a computer near you anytime soon. Currently, tree codes are in the proof of concept phase. “We can start from that math and show they exist, without being able to say: here is one,” says Schulman. One of the primary research challenges is to make the error correction as robust as possible.
One potential alternative that may work much sooner was just published by Sahai and his colleagues. They modified Schulman’s work to create a code that, while not quite the full theoretical ideal of a tree code, may actually perform nearly up to that ideal in real-world applications. “It’s not perfect, but for most purposes, if the technology got to where you needed to use it, you could go ahead with that,” says Schulman, “It’s a really nice piece of progress.”
Sahai, Schulman, and other researchers are still working to create perfect tree codes. “As a mathematician, there’s something that gnaws at us,” says Sahai, “We do want to find the real truth—what is actually needed for this to work.” They look to Shannon as their inspiration. “Where we are now is exactly analogous to what Shannon did in that first paper,” Schulman says, “He proved that good error correcting codes could exist, but it wasn’t until the late 1960s that people figured out how to use algebraic methods to do them, and then the field took off dramatically.”
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