Last month, MicroSemi and Akros Silicon
demonstrated the first compatibility
between ICs for IEEE 802.11at Power
over Ethernet Plus/HiPOE power source
equipment (PSE) and powered devices
(PDs). The PSE chip was MicroSemi’s
PD64001, and the PD chip was Akros Silicon’s
AS1135.
Conducted by MicroSemi, the test predates
the actual release of the final 802.11at standard.
But the draft standard was released last
fall, and an approved version is expected late
in 2008. So, the compatibility demonstration
is a genuine milestone.
Compatibility means that it is now possible
to design an Ethernet switch or midspan
power injector or an Ethernet device such as
a video-over-IP telephone that will
together negotiate variable power
requirements up to about 30 W
per channel on the fly. The ability
to dynamically shift power among
multiple loads promises lower costs
for switches and midspans by sizing
their power supplies based on statistical
analyses of peak power demands, rather than
on theoretical maximum demands.
A LONG ROAD
Power-over-Ethernet (PoE) is a scheme for
powering Ethernet-connected devices over
the Ethernet cable itself, rather than via a wall
adapter. It evolved from a proprietary Cisco
approach developed for voice-over-IP phones.
Defined by the IEEE 802.11af standard,
its first inception limited power to
15 W, delivered at the switch or
midpoint. A simple hardware
handshake established
the presence of a PoEpowered
device at one
end of the cable and the
presence of a PoE-enabled
switch at the other. There were
four possible maximum power
levels, and once the handshake was
accomplished, the maximum power
available to the PD could not be changed.
Basic 802.11af PoE proved adequate
for some applications, especially stationary
security cameras and Wi-Fi
hot-spots. But 15 W wasn’t
enough
for applications such
as video
phones, which have
power-hungry backlights, nor for
WiMAX hotspots and surveillance
cameras
with pan/tilt/zoom motors.
That need led
to the formation of the IEEE
802.11at Working Group.
The Working Group soon established that
CAT-5 Ethernet cable and its associated RJ-45
connectors could handle at least double the
maximum 802.11af current. (The nominal
PoE voltage at the PD is 48 V dc.) But the
group also saw the need for dynamic power
rationing. Without it, the manufacturers
of big Ethernet switches
would have to install power
supplies capable of providing
full power to every
port.
While the 802.11at standard has
been pending, several companies, including
MicroSemi and Akros, have
developed
products that allow individual
Ethernet
ports to be powered with up to
60 W, using
the basic 802.11af handshake. That’s really
all that’s required for a security camera or
WiMAX installation, given a dedicated run of
CAT5 cable. That approach wouldn’t work,
however, in a cube farm where workers and
their IP phones would be frequently shuttled
here and there.
TWO-EVENT CLASSIFICATION
The process of dynamically negotiating
maximum power for a PD involves both the
basic 802.11af hardware handshake and an
ongoing dialog between the PD and the PSE
via the Ethernet data link itself
View the chip
