A WIRELESS ROAD AROUND DATA TRAFFIC JAMS
By ANNE EISENBERG
Published: January 14, 2012, The New York Times
THE vast data centers that process information for the Facebooks and Amazons of the Web work at a brisk clip. But even so, they can’t 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 doesn’t 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, says Victor Bahl, director of the mobile computing research center at Microsoft Research in Redmond, Wash. His group began trying the links to supplement wired systems three years 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 and infrequent 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 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 isn’t the familiar Wi-Fi of coffee-shop hot spots. That type of signal spreads out over 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 the computer 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 leave from 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. “You’ll have to prove its security,” he says, “and you’ll have to prove that 100 times out of 100 times” no information is lost.