IEEE Spectrum May, 2008 - 8

But Christensen and Nordman's concept will take some effort
to implement. Switching between
Ethernet speeds is time-consuming.
"When you change link rate today,
you have to drop the link and
reestablish it, which takes [up to]
2 seconds," says Nordman.
However, rate switching would
have to happen in less than a millisecond to be practical. That means
researchers will need to come up
with a much faster protocol for
the two ends of an Ethernet link-
say, a PC and a switch-to coordinate their link rates.
The industry is weighing
the Adaptive Link Rate scheme
against another one, hatched at
Intel, which promises to be even
more energy efficient. Called lowpower idle, it proposes transferethernet connections waste lots of watts.
ring data on an Ethernet link at
it need not be so
the highest possible rate and then
putting the network controller
thernet link speeds of
to become a standard that if put
chip into a sleeplike state. "You're
100 megabits per second or
into practice would save some of
better off sending data faster and
even 1 gigabit per second
the wasted watts. Their seemingly
getting to sleep quicker, which
are typical right now in local area
simple solution: adapt the Ethernet
allows you to save more power
networks, but it's very unlikely that link's speed to match a device's
over the long haul," says Robert
you need that much bandwidth all
needs. If you were checking e-mail,
Hays, a strategic planner for
networking products at Intel.
the time. Studies show that on aver- for instance, 100 Mb/s would be
The trouble is that turning on a
age, people use their Ethernet links
enough, but the network controller
at full throttle less than 5 percent
would shift to 1 Gb/s when downdormant network card quickly is
loading a large file. The researchers
of the time. But the circuitry on the
a challenge. Still, for link speeds
network-interface controller, the
described the concept, called
up to 1 Gb/s, Hays says, turning
chip that connects your computer
circuits on and off is easier than
Adaptive Link Rate, last month in
to the network, is always running
IEEE Transactions on Computers.
switching between rates. An IEEE
at full speed, wasting power. In
At the low data speeds, the netstandards task force recommended
2005, all the network-interface
work controller chip's circuits
the Intel scheme for 1-Gb/s links.
controllers in the United States-
would work at slower clock rates,
But for faster, 10-Gb/s links,
computers, switches, and routers
and some might be turned off, cut- where there is more potential for
all have them-burned through
ting power use. Christensen and
power savings, it's not yet clear
5.3 terawatt-hours of energy,
Nordman estimate that with netwhich of the schemes would be
enough to keep 6 billion 100-watt
working devices in homes, offices,
easier to implement and would
and data centers running at 1 Gb/s, save more power.
lightbulbs shining all year.
switching to 100 Mb/s whenever
"There's no reason to have
No matter what scheme the
a 1-gigabit link when there's no
possible could save more than
industry chooses, a complete
traffic on it," says Ken Christensen,
US $300 million in energy costs.
redesign of the network-interface
computer science and engineering
The savings would be even
controller system is needed, says
professor at the University of South greater if the links were switching
Hugh Barrass, a technical leader
between 10 Gb/s and 100 Mb/s. Ten- at Cisco. "[We] should expect
Florida, in Tampa. Christensen
and Bruce Nordman, a researcher
gigabit links-expected to be wideto take two to three generations
spread by 2010-use 10 to 20 W
at the Lawrence Berkeley National
before equipment gets the most
more power than 100 Mb/s links,
Laboratory, in California, have
efficient it can be," Barrass says.
devised one of two schemes vying
while 1 Gb/s uses about 4 W more.
-Prachi Patel-Predd

energy-efficient
ethernet

news
brief
I see you
Researchers from
the Fraunhofer
Institute for Photonic
Microsystems plan
to demonstrate an
organic LED built on a
CMOS chip this month
at the Society for Infor-
mation Display. The
technology should
allow the integration
of light-emitting pixels
and light-sensing ele-
ments, yielding a com-
bination of camera
and display in one chip.
photo: FRaunhoFeR
institute FoR photoniC
miCRosystems

olaF KowalziK/Getty imaGes

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Table of Contents for the Digital Edition of IEEE Spectrum May, 2008