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Hack 68 Home-Brew Power over Ethernet

Power your access point without running a
separate power cable by using free pairs on the CAT5.

A number of access point manufacturers (Lucent,
Symbol, and D-Link, to name three) are now offering Power over
Ethernet (PoE) add-ons for their access points. A PoE module inserts
DC voltage into the unused wires in a standard Ethernet cable (pairs
7-8 and 4-5). The idea is to supply the AP's power
and UTP Ethernet connectivity requirements via a single Ethernet
cable. This works great in areas where you may not have power easily
accessible, such as a roof. This also allows you to more easily
place the AP closer to the antenna, thus reducing signal loss over
antenna cabling. Ethernet signal travels well over
CAT5 cable; a 2.4 GHz signal
doesn't do as well over antenna cabling. Also,
Ethernet cabling is much cheaper than antenna cable such as LMR400.
The following hack demonstrates how to build a simple PoE module
pair.

Step by Step

Solder wires to the DC Male Power Plug. Solder one pair (two wires
twisted together) to the inner-contact connection. These will be the
positive power wires. Solder another pair to the outer-contact
connection. Notice that on this DC Male Power Plug, there are three
connectors. One is for the center pin, one is for the outer surface,
and one goes to the plug-housing. You do not need to solder anything
to the plug-housing connector. Figure 4-30 shows
what it should look like when finished.

Drill a hole in your two-port mount housing. Mount the Male DC plug
in the housing, as shown in Figure 4-31.

Connect the wires in your two-port jack as follows (note that this is
the Intel Symbol, Orinoco Standard, not the
Cisco standard for wiring):

Wire the one port wall mount jack as follows:

Plug in and test. The completed modules are shown in Figure 4-32.

The DC
resistance of Cat5 is about 3 ohms per 100 feet per conductor, so a
250-foot cable has at least 7 ohms resistance. Most of the time, the
APs draw much less than 0.8A, so you would still be above 6V at the
AP. In fact, the access points typically use linear regulators to
drop the voltage down to 5V on their insides, so as long as
you're giving them something better than 6V at the
terminals, they're likely to work.

There is a good calculator online at http://www.gweep.net/~sfoskett/tech/poecalcl
that calculates the voltage drop for a given length of
CAT5. Use it to estimate how much power you need to provide at one
end of your cable run in order to power your access point.

Terry Schmidt