PoE
was, and is, supposed to make the powering of devices easy. You take your
camera or other device that accepts power via the Ethernet port, you plug in
the RJ45 jack to the port, and you walk away. Inside the head end, you plug the
other end of the same Ethernet cable into a PoE switch or PoE injector and
voila, power is magically delivered to the device along with the data
connection. In theory, all of the normal worries are gone. AC power or DC power
is irrelevant, and you don't even have to worry about over-powering a camera
that, were you to fry it, could potentially set you back a few thousand dollars
in equipment costs and man hours!
Why has it become so
complex? Who knows! Unfortunately it has, and confusion has also shared a taxi
with a lack of education on the road to PoE's widespread acceptance. People see
a label on a device that says “802.af” or “IEEE Compliant” and then
automatically assume that they can plug it into a PoE switch or midpsan and
have it work with no problem. What makes the education problem worse is that
often it does work with no problem, and this leads people to the assumption
that PoE is really nothing more than Windows “plug and play” for power. Unlike
Windows, however, there is no “blue screen of death” when using PoE. Instead
there is a device that does not power on, or (in rare cases) a device that does
power on followed by smoke, the smell of singed chip boards and fried
capacitors, and then what was a very expensive security device becomes an
equally expensive paperweight.
PoE
was supposed to be this way, but practical reality has diverged from the perfect
world concept in such a way that the actual installation is almost never that
easy. So set aside the “perfect world” notions you have, and let’s start with
the basics, so you can understand how PoE works.
There
are four classes of PoE: Class 1, 2, 3 and 0. Each PoE classification denotes a
range of power that is available to the end device as well as the power that
must be available on the port of the power sourcing equipment (PSE):
PoE
Classifications
- Class 1 -- 4.5 watts at PoE port; 3.84 watts at device
- Class 2 -- 7.5 watts at PoE port; 6.49 watts at device
- Class 3 -- 15.4 watts at PoE port; 12.95 watts at device
- Class 0 -- 15.4 watts at PoE port; .44 to 12.95 watts at device
In
the world of PoE there are two kinds of switches that can provide PoE; the kind
that operates with a “guarantee per port” and the kind that operates with a
“total power budget”. Both kinds of switching are useful but there is a
significant difference between them. If you happen to have a switch nearby,
look at it and see if you can tell into which one of the above two categories
your switch falls.
A
switch that guarantees a certain wattage per port -- 15.4 watts per port, for
example -- means that you can be sure that no matter how many Class 3 or Class
0 devices are plugged in, the switch will be able to power them. Of course,
these switches tend to be bigger, more expensive and ill-suited for use outside
of a nice climate controlled room, but they do prevent errors in power
planning.
The
second type of switch mentioned above -- the kind with a total power budget --
can only power as many PoE devices as it has power to spare. Imagine that you
are working with a 4-port switch that carries a total power budget of 30 watts.
This kind of switch could power four Class 2 cameras (4 devices x 7.5 watts =
30 watts needed). It could also easily power four Class 1 devices (4 devices x
4.5 watts = 18 watts needed). Continuing with that math, it would be able to
power Class 3 or Class 0 devices, but it could only power two of those types of
devices.
Power
planning is where the rubber meets the road, and it brings up a challenge in
our industry.
What happens if a chosen device (i.e., a PoE powered camera) does not
clearly specify the PoE class and instead simply gives an operating wattage?
You might think that this is OK since a camera which says “6.01 watts” is
within the Class 2 specifications and therefore must be Class 2. But that’s
where reality often diverges from common sense. In theory, what is supposed to
happen is that a device is clearly labeled with a PoE classification so that
when said device is plugged into a PSE device, the power budget has been worked
out such that each device will receive its required PoE.
What I believe the security industry needs – right now, since PoE is
happening today -- is clear labeling of the correct classification of PoE on
each and every device that uses PoE. It is all well and good to place the
operating or maximum wattage on the device, but industry manufacturers need to
take the next step!
Manufacturers should label the device, print it on in large type and with
bold colors, CLASS 1, CLASS 2, CLASS 3, CLASS 0, or whatever PoE Plus will hold
as a classification. It's OK if your device actually only draws 3 watts during
normal operation but for some reason is Class 0. Just tell your integrator
channel partners and end users by labeling the device in the manner in which it
was intended to be used. This lets system designers know the classification so
that they might properly create a power plan and buy the correct devices. No
one wants to be in the field trying to get a project done on time and only then
realize that their switches don’t have enough power for the devices they’ve
purchased.
While I am solidly standing on my PoE soapbox, let me also make a plea for
PoE classification to be a priority on data sheets and marketing slicks. Some
camera manufacturers make wonderful versions of these spec sheets. You’ll find
photos, technical illustrations, cross reference charts, and more -- and often not
a hint of PoE classification to be found anywhere. As someone who works with
PoE, it sometimes seems as though PoE has become the crazy uncle that everyone
has and who no one wants to invite to the party. Unfortunately for all of us,
the crazy uncle could actually be the life of the party -- he makes it easy to
entertain the guests and always has enough cash to pay for pizza -- but we
haven't managed to take advantage of him yet!
PoE is supposed to make things easy, and between the standards bodies, the
independent PoE offerings, the lack of classification usage, the errors in PoE
chip usage within devices, and the propensity of some manufacturers to create
Class 0 signatures in devices that draw minimal wattage, PoE's original purpose
has been obfuscated in a way only rivaled by the current explanation of the
financial bailout.