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Januar 14, 2012
A Wiele Road Aond Daa TafficJamB ANNE EISENBERG
THE vast data centers that process information for the Facebooks
and Amazons of the Web
work at a brisk clip. But even so, they cant always keep up.
During sudden bursts of activity, bottlenecks occur as traffic
moves among dense clusters of
servers. Typically, the servers are stacked one on top of
another in rack after rack and are
connected by switches, routers and cables.
To better handle the congestion, researchers are testing a
shortcut that doesnt involve costly
rewiring. They are experimenting with wireless links, mounted
atop the server racks, to supply
extra bandwidth for moving data along at crunch times.
Researchers in the field, as well as data center administrators,
initially were skeptical about the
idea of applying wireless technologies inside data centers,
which have stringent requirements
for reliability and security, saysVictor Bahl, director of the
mobile computing research center atMicrosoft Research in Redmond,
Wash. His group began trying the links to supplement wired
systems three y ears ago.
That skepticism is normal, Dr. Bahl says. Wireless links have
always been considered flaky.
Wireless performance, as anyone who has lost a cellphone
connection can attest, can be affected
by many things, from a door opening or a microwave oven starting
to changes in the weather.
But a data center is a highly predictable environment, with
controlled temperatures andinfrequent movement of people and
equipment. That minimizes fluctuations in the quality of
wireless links. You set up these racks of servers and walk away,
Dr. Bahl says. The
environment stays the same.
The Microsoft team forged ahead with the project, building and
testing a system with tiny
directional antennas at the top of each rack to send and receive
data. A central controller
monitors traffic patterns, finds network bottlenecks, configures
the antennas and turns on the
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wireless links when more bandwidth is required, says Daniel
Halperin, a Ph.D. candidate at the
University of Washington, who worked on the project as an intern
at Microsoft. Signals go out
on a horizontal plane and are steered right or left. The design
sped up traffic by at least 45
percent in 95 percent of the cases tested, Mr. Halperin
says.
The wireless signal that the Microsoft group uses to carry
multigigabits of data per second
between racks isnt the familiar Wi-Fi of coffee-shop hot spots.
That ty pe of signal spreads outover an entire room so that many
people can open their laptops and go to work.
The wireless signal for the data centers is a narrow beam of
extremely high-frequency radio
waves in a part of the spectrum known as the 60 gigahertz band.
The band is already used in
home entertainment, replacing bulky cables that connect
high-definition televisions to Blu-ray
players, for example. Hardware for this bandwidth, which is
available worldwide for unlicensed
use, is likely to drop in cost as the band gains more use, Dr.
Bahl says.
At the University of California, Santa Barbara, Heather Zheng,
an associate professor in thecomputer science department, and
colleagues are also designing multigigabit wireless links to
augment wired data center networks. The links in the system can
travel horizontally, but they
can also leave one rack, bounce off a reflective ceiling at an
angle, then move to another rack.
When we bounce signals off the ceiling, we take advantage of
much more bandwidth than we
could if we restricted ourselves to a horizontal plane, says
Amin Vahdat, a professor at the
University of California, San Diego, who specializes in
large-scale data center architecture and
deployment and is working with Dr. Zheng on the project. Dr.
Vahdat, who is on sabbatical leavefrom the university, is also a
principal engineer at Google.
Craig Mathias, a principal at the Farpoint Group, an advisory
firm specializing in wireless
communications and mobile computing, agrees that the 60 GHz band
is suitable for indoor,
high-bandwidth use in information technology, but he maintains
that wired systems are best for
connecting servers.
The wired connections used in a data center are highly reliable,
so why introduce variability in
a mission-critical situation? he asks.
MARK THIELE, executive vice president for data center technology
at Switch Communications,
a Las Vegas builder and operator of data centers, says he thinks
wireless technology offers a
promising solution to congestion. If it deals effectively with
bottlenecks, a wireless connection
could eliminate hundreds of thousands of dollars in required
network connectivity, he says.
Much work is still ahead. Youll have to prove its security, he
says, and youll have to prove
