The draft standard for IEEE 802.3at Power over Ethernet Plus (PoE Plus) remains
on track for an August release. The original 802.3af PoE standard offered a
fairly straightforward way to supply loads with 13 W or so of usable power delivered
at 48 V dc. But IEEE 802.3at PoE Plus, which ups usable power to something over
50 W, introduces some wrinkles that designers and even IT managers must understand.
One catch is that designers can still supply power in a limited fashion in
some existing Ethernet installations via a mid-span bridge. But in that case,
designers can’t implement power negotiations between a powered device
(PD) and power source equipment (PSE). This implies dedicated PoE Plus ports
and relatively high duty-cycle power supplies in midspans.
Something else to watch out for are PDs that dynamically negotiate power requirements
with the PSE via their Ethernet connection. This requires more code in the PD
micro-controller and a greater understanding of dynamic power requirements on
the part of the engineer writing that code.
A potential pitfall for end users is that PDs can meet the standard by operating
in a fall-back mode if there’s not enough power for full functionality.
(For example, a video phone could fall back to operating voice-only, with- out
a video display.) Alternatively, a PD application could meet the standard simply
by signaling “insufficient power.” IT managers who bought a lot
of “compliant” video phones could find themselves embarrassed
by a system that didn’t work as expected if a “compliant”
switch didn’t possess a sufficiently robust power supply.
HISTORY LESSON
To get comfortable with PoE Plus, it helps to understand its genesis and subsequent
evolution. In the beginning, Cisco had a proprietary approach for powering Voice
over Internet Protocol (VoIP) business phones that involved powering some pairs
in the router with 48 V.
The rest of the industry saw that this was good and wished for an open standard,
which became IEEE 802.3af. To be conservative, the IEEE subcommittee limited
power to 15 W at the PSE, which was enough for the non-video VoIP phones that
then dominated the market. They also expanded Cisco’s idea by allowing
the “spare pairs” in an Ethernet cable to be powered by a midspan,
making it possible to retrofit PoE to legacy Ethernet plant.
When PoE hit the streets, many potential vendors saw its advantages and jumped
on the bandwagon. VoIP phones would no longer need power plugs, making them
more like old-fashioned public-branch-exchange (PBX) phones. Wireless hotspots
could be located anywhere someone could pull a CAT5 cable. Supermarket shelves
would twinkle with up-to-date price tags that would always match the prices
in the cash register. And, PoE musical instruments, mixers, and recording equipment
would displace the MIDI bus and revolutionize the music business.
Obviously, some of these goals were more realistic than others. In the three
years since basic PoE was released, three killer applications have taken hold:
VoIP phones, Wi-Fi hotspots, and security cameras. Within those applications,
though, there immediately appeared a need for power beyond 13 W.
For example, there’s an anticipated demand for video conferencing using
VoIP phones, and backlighting a video screen takes power. Simple short-range
Wi-Fi is happy with 13 W, but WiMAX takes more power. And while fixed security
cameras don’t require much power, once motors are added for panning,
tilting, and zooming, power does become an issue.
But the makers of Ethernet switches, concerned about over-specifying power
supplies, pointed out that video phones and pan/zoom/tilt cameras don’t
need full power all of the time. Most of the time, the phone is just sitting
there. Even when there’s a call, video isn’t always necessary.
Unless it’s a formal conference, most people would prefer to remain invisible
to the other party. Similarly, those high-end security cameras only move when
a guard touches a joystick. In other words, the requirement for higher power
changes continuously.
The dynamic-power issue transformed the questions facing the IEEE 802.3at task
force from simply “How much current can a bundle of CAT5 cables and their
associated RJ45 connectors safely handle?” to “How can we create
a protocol that allows PDs to dynamically negotiate for power with a PSE?”