Issue 4.10 | Oct 1996
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While ATM co-creator Sandy Fraser suggests that RSVP is simply a case of the Netheads "reinventing telephone technologies and giving them a new name," it really is a new twist on what came before. In essence, RSVP applies a software sensibility to a problem that ATM tries to solve with rigid hardware.
Leave it to the diplomats
The third and final core debate in the war between the Bellheads and the Netheads is over something terribly mundane: the number of bytes in an ATM packet. Yet this may be the fault line that ends up mattering most. Packet size is the primary reason Doran hasn't deployed ATM equipment in Sprint's network; it is also the reason MCI's Vint Cerf did.
An ATM packet, technically known as a cell, is 53 bytes long. The first five bytes (called the header) contain information about the cell and its connection; the next 48 bytes (the payload) contain the actual data. Contrast this with an IP packet, which has a 20-byte header and a variable-length payload of anywhere from 12 to 65,536 bytes. It's difficult to convey how insane ATM's cell scheme sounds to anyone in the data community, but it's roughly equivalent to Ford announcing a new car that is shaped like an upright obelisk. Sure, it could be made to work, but it's neither aerodynamic nor practical.
Computers, after all, are binary systems. That means they like to work with powers of two: 64, 128, 256.... At the very least, they like to work with even numbers, which divide easily. What they definitely don't like working with is prime numbers. But if you had to pick a prime number for cell length, you would at least want to make it large. The shorter the length, the greater the percentage of overhead. With an ATM packet, 5 of the 53 bytes are essentially worthless. That means you're throwing away about 10 percent of your bandwidth right from the start - an effect known as the "cell tax." Cerf sums up the situation: "The only good thing about 53 is that's how old I am."
So why did 53 get chosen? Because an extremely corrosive influence was injected into the technical debate over ATM: international politics.
When the telecom community wants to set a standard, it must go to the International Telecommunications Union, a UN treaty organization. Each country is represented by a delegation. Arguments are carried out formally and with great attention to protocol. And final votes are cast by government representatives. (In the case of the US, this means someone from the State Department.) It would be hard to come up with an atmosphere less conducive to setting technical standards.
Imagine, then, the 1988 ITU meeting in Geneva, where delegations from two dozen countries met to decide on the length of an ATM packet. It was immediately obvious that consensus was going to be difficult: The Europeans wanted 32-byte payloads, because that would be best for voice, while the Americans and Japanese wanted 64-byte payloads, since that would be better for data. (Although the American delegation was firmly in the Bellhead camp, it was at least aware of Nethead concerns. The Europeans saw little demand for data and didn't expect that to change anytime soon.)
According to Richard Vickers, who was part of the US delegation, the discussion quickly turned confrontational, with the US and France becoming the main combatants. As the invective became more heated, pressure built to solve the question the diplomatic way: split the difference. And so was born the 48-byte payload, which, combined with a 5-byte header (the smallest that the ITU could agree upon), added up to a 53-byte cell.
No one was terribly pleased with the result. Sandy Fraser, forced to watch his technology haggled over like some kind of border dispute, now says, "In my view, they picked the worst of both worlds." The Netheads were far more vocal. T-shirts and buttons with a slash through a red 53 became popular at subsequent Internet conferences. Doran practically sputters when the topic comes up, claiming that the short cell size results in almost 30 percent overhead when used to carry IP traffic. That's because longer IP packets have to be distributed across multiple cells, often leaving the last cell only partially filled. "Why should I throw 30 percent of my bandwidth away?" he asks incredulously.
Carl Cargill, who has studied high tech standards for 14 years and currently handles standards strategies for Netscape Communications Corp., sees the episode as a study in the opposing ways that the telecom and data industries think about standards: "Interconnection is really important for telecom companies. So first they agree on a standard, then they compete. Computer companies, on the other hand, compete right off the bat and let the market pick the standard."
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