The Planetary Society SETI@home Update, 5/20/2002:

A Question of Bandwidth
by Amir Alexander

Fascinated as we are by the idea of contacting alien civilizations, we
sometimes forget that SETI@home is not just a search for intelligent beings
among the stars. It is also a gigantic computer science experiment of
unprecedented magnitude taking place here on Earth. With over 3.5 million
users worldwide, SETI@home is by far the largest and most powerful computing
network ever assembled.

Keeping this gigantic network operating, it turns out, is no easy matter.
Every day around 400,000 people from around the world connect to the
SETI@home server in Berkeley, California. First-time users sign up,
established users send in processed work units and download new ones. Each
work unit is around 350 kilobytes in size, and the constant movement back
and forth of thousands of units has created something of an information
traffic jam on SETI@home's internet connection.

The communications problem can be summed up in one word: "Bandwidth."
Because of the enormous amount of web traffic it generates, SETI@home needs
a very wide communications band to operate smoothly. And as time goes on,
and more and more users join the SETI@home network, the bandwidth
requirements only get larger and larger.

On Cars and Bytes

To visualize the problem, consider any road or a highway near your home. If
you use it when traffic is light, say, on a weekend morning, all you see is
open road. You can drive more or less as fast as you want, or as fast as
your local highway patrol will allow. But try the same route during Monday
morning rush hour and you will find things very different. Even the widest
roadway will often be packed with vehicles as far as the eye can see, and
traffic will slow to a crawl. The reason is that (given speed limitations)
any roadway can only allow the movement of a certain number of vehicles at
any given time. If more vehicles try to pass through, the road will become
hopelessly clogged, with dire (but not uncommon) results for the morning

Eventually, one would hope, your local government will address the situation
by widening the roadway and adding a lane or two. This will alleviate the
problem, because the wider roadway will have a greater capacity and will be
able to move a larger number of vehicles on their way. But if your area is
at all like mine (I live in Los Angeles.) it will not be long before the
maximum capacity of the wider road will be reached, and the problem will
repeat itself.

What is true of automobile traffic is also true of electronic web traffic.
The fiber-optic cables that carry information to SETI@home headquarters in
Berkeley can only move so many work units along at a given time. And as
SETI@home users have recently noticed, that limit is fast approaching.

A Clogged Highway

The signs were unmistakable early in January. SETI@home users trying to send
or receive work units found that the speed of uploading and downloading
information had slowed to a crawl. A process that usually takes less than a
minute would now drag on for hours, causing many to give up the effort
altogether. And although conditions were considerably better during off-peak
hours (such as night-time in California), even then the information was
moving at a far slower pace than it should.

The problem was particularly severe for users who run several computers with
the SETI@home program simultaneously and are therefore constantly sending
and receiving data. When every online transaction became an adventure of
indeterminate duration, these dedicated users had to shut down their
operations, or at least trim them down drastically.

The source of these troubles, as you've probably surmised, was bandwidth.
Since SETI@home is located at the Space Science Laboratory of the University
of California at Berkeley, all web traffic to and from the SETI@home servers
passes through the university's internet system. Now U.C. Berkeley's total
bandwidth is 70 Megabits per second (or "70 Mbps"), meaning that it allows
for the transfer of 70 million bits of digital information every second.
This capacity must suffice for all of the university's e-mail and web
traffic in all of its academic and administrative departments.

When SETI@home went online in 1999, it initially didn't make much of a dent
in this overall capacity. But over time, as millions around the world joined
the program, it grew into the single largest user of U.C. Berkeley's
bandwidth. By early 2002, SETI@home traffic ranged between 20 and 30
Megabytes per second, or about 40% of the university's total bandwidth.

This proved to be too much for U.C. Berkeley's web system. The SETI@home
traffic, when combined with the natural growth of the university's other
internet requirements, was using up the university's maximum full bandwidth
capacity. Since SETI@home data is a relatively low priority for the
university compared to carrying on its day-to-day operations, users trying
to upload and download information bore the brunt of this traffic jam. The
flow of SETI@home data, like traffic on an overused roadway, ground nearly
to a halt.

What to Do?

The SETI@home team had been aware of the growing problem for quite a while,
and was considering possible solutions. Nevertheless, when the crisis came
in early January, it surprised everyone with its suddenness and impact.
According to Project Director David Anderson, they are still not sure what
initiated the crisis at this particular time. For some reason, outgoing data
from U.C. Berkeley had increased in January, using up the full capacity of
the university's bandwidth.

The first response of the SETI@home team was literally to buy time. For a
monthly payment of $6000.- the university agreed to guarantee at least 20
Mbps in bandwidth for SETI@home's use at any given time. With this
arrangement in place, SETI@home users could again send and receive their
work units, though not quite at the rate they were used to.

This, however, was only a temporary stopgap measure, and the SETI@home crew
immediately started looking for a permanent solution. What was needed was an
independent web connection, completely separate from the general U.C.
Berkeley line and with sufficient capacity. They found exactly that: Cogent
Communications, a company based in Washington D.C., was offering 100 Mbps
worth of bandwidth at an affordable price on their network of fiber optic

While Cogent's proposal is highly attractive, there are still some obstacles
on the way. Cogent's network, it turns out, cannot be accessed in Berkeley,
but only in Palo Alto, on the other side of the San Francisco Bay. The
SETI@home team must find a way to transmit and receive data across the 40
miles separating Berkeley from Palo Alto with sufficient bandwidth so as not
to create a bottleneck. The U.C. Brekeley campus network administrators have
proposed using their existing connection with Palo Alto, via a company named
(some would say appropriately) UFO Communications. According to SETI@home
chief scientist Dan Werthimer, however, it is not yet certain whether this
particular arrangement is going to work. A preliminary test a few days ago
failed, and more tests are scheduled for the coming days.

Once all the problems are resolved, however, the new bandwidth will be an
enormous boon for SETI@home. For one thing, 100 Mbps would provide the
project with four times the capacity it needs today, and would enable users
to contact SETI@home quickly and easily for years to come. For another,
according to David Anderson, such bandwidth would also enhance the project's
scientific value. With that kind of capacity at its disposal SETI@home will
be able to receive many more data packages from Arecibo, and quickly
distribute them to its millions of users. As a result, the project would be
able to analyze radio signals recorded at a wider range of frequencies then
the relatively modest 2.5 MHz it tracks today.

A victim of its own phenomenal success, SETI@home has outgrown the academic
environment in which it was conceived, and is now struggling with the
constraints it places on the project's growth. When this transition is
completed, SETI@home will reemerge as a larger, more efficient, and more
scientifically ambitious project than ever before.

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