The IEEE 802.3at Power over Ethernet Plus (PoE-Plus)
task force anticipates the release of a "usable" draft
standard in August, with a firm specification to follow a year later. PoE Plus will boost the maximum power delivered
to applications to something around 60 W from the basic
802.3at PoE standard's 13 W at the application.
Increasing the power was relatively simple, once the
task force agreed that CAT5 cable and RJ45 connectors
could handle more current (the approach some parties
wanted) and/or that it would be acceptable to use all
four twisted pairs in the CAT5 cable—the data pairs and the so-called "spare pairs"—to carry current (the
approach some other parties wanted).
One challenge was updating PoE "classification." There are
two kinds of power devices in a PoE system: power source
equipment (PSE) and powered devices (PDs). Classification
allows the PoE controller in a router or bridge to intelligently allocate its power to PDs.
In plain-vanilla 802.3af PoE, when a PSE chip (one for each
powered CAT5 port) senses a PD on the port, it asserts a short
pulse on the pairs it is using for power. The PD responds
to the pulse by briefly drawing a certain amount
of current. The amount of current represents the
class of equipment the PD is supporting, which
essentially boils down to how much power that
equipment will be drawing. Basic PoE included
classes 0, 1, 2, 3, and 4, and 4 was "reserved" The 802.3at task force determined that the best
way to extend the classification system was a two-level method. All PSE chips will now send the first classification pulse, but PoE-Plus PSEs will also send a second pulse.
Standard PoE PDs will respond in the usual fashion to the
first pulse. But when PoE-Plus PDs see the second pulse that
tells them the PSE is PoE Plus-capable, they will signal that
they are class 4 devices by drawing the class 4 current. The
PSE and the PD then will enter into a data exchange with the
PSE that will provide information about peak power needs,
average power, duty-cycle, and other factors. The task force is
now working on the data format for that exchange.
There is one catch. Two-layer classification will only work with
new routers. It won't be possible to just stick a PoE-Plus midspan
bridge into a legacy system and have all the benefits of PoE-Plus
classification. On the other hand, that doesn't rule out higher
PoE power on selected ports without classification. Several
sources have announced single-port chips that will power a single higher-power PoE application such as a WiMAX access point
or a pan/tilt/zoom security camera ().
CURRENT CONCERNS
The working number for current in the data pair of the spare pair is 720 mA. That might go
up to 750 mA. The wires could carry more, but the task force
has to deal with the potential heat buildup in bundles of 100
CAT5 cables 100 m long, with all the cables carrying current on
all four pairs.
The RJ45 connectors are sufficiently robust. There's a possibility of arcing on disconnect, but the portions of the connector
that might arc and develop pitting aren't the sections that are
in contact when the connectors are fully mated. Meanwhile,
data transformers are being beefed up because there was
some possibility that the cores could saturate.
For more on the state of PoE applications and for more
details of the IEEE 802.3at draft standard, see our Engineering
Essentials article in our upcoming July 5 issue.