Power Over Ethernet (POE) is a fancy name for simply using the 4 unused conductors on most CAT5 ethernet cable to pass the low-voltage DC to your Access Point (AP). Sounds easy, right?
But, there are significant traps for the unwary, and you can have unintended problems if the POE cable is not engineered correctly.
The Main Problem
The problem with POE is voltage loss in the cable. CAT5 conductors were never designed to carry significant current, and are very thin. Consequently, the resistance of (say) a 20m POE cable is significant; 1.8 ohms per conductor to be precise. As we pass the current necessary to power the AP though such a cable, there is an associated voltage loss. And if the voltage at the AP end of the cable falls too low, the AP does not work at all, or performs at low efficency.
One common symptom of a poorly engineered POE cable is:
- AP at the end of a POE cable appears to work fine when idle (i.e. little or no wireless traffic)
- AP continually reboots as soon as the AP starts a high-speed file transfer, where the file transfer data is being sent from the AP to a 'client'.
The reason: As the AP starts transmitting, it draws more current over the POE cable. That means a bigger voltage drop, and the voltage at the AP is no longer sufficient. So, it crashes, reboots, and starts over.
An Example
It all comes back to that basic equation known as Ohm's Law, which states:
(Voltage Drop in cable) = (Cable resistance) x (Current)
or
V = I x R
Lets say we have the following:
- 20m POE cable. The round-trip resistance totals 1.8 ohms.
- An AP that:
- draws 200 mA (idle)
- draws 500 mA (transmitting full speed)
- requires 5.0V (+/- 0.5V) for proper operation
- comes with a 5.0V regulated power supply
So, we can calculate the voltage loss in the cable as follows:
V = I x R
Idle: V = 0.200 x 1.8 = 0.36
Transmitting: V = 0.500 x 1.8 = 0.90
That means the voltage at the AP is:
Idle: 5.0 - 0.36 = 4.63 V (OK - as it above 4.5V)
Transmitting: 5.0 - 0.90 = 4.10 V (Not OK - as it is below 4.5V)
Engineering Your Own POE Cable
You need to do the following:
- Operate your AP as normal (i.e. no POE cable) in such a way that it is transmitting full speed. Typically you will setup a simple wireless network (AP and Client), connecting two PCs with high speed ethernet interfaces (NICs), and start a long file transfer using File Transfer Protocol (ftp) or similar.
- Measure these values:
- DC Voltage at the AP (Va)
- DC Current into the AP (Ia)
- Note the required AP voltage (Vr). This is often written on the AP, or in the manual.
- Calculate the maximum length of CAT5 POE cable using this equation:
Max Length = (Va - Vr) / (Ia x 0.094). [Note: 0.094 is the resistance per meter of CAT5e cable]
An example:
Vr = 12 Volts (written on AP)
Va = 13 Volts (measured with AP transmitting)
Ia = 400 mA (measured with AP transmitting)
Max Cable Length = (13 - 12)/(0.400 x 0.094) = 26 meters.
Where does the 0.094 ohms/m come from?
CAT5e has a maximum DC resistance specification of 0.0938/m. (See this link.) We wire the 4 unused conductors in a POE cable like this:
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where each pair of wires is shorted at each end (ie each pair is wired in parallel). So considering a 1m length of cable:
- Resistance of one conductor = 0.094 ohms
- Resistance of two conductors wired in parallel = (0.094/2) = 0.047 ohms
- Round Trip Resistance for both conductor pairs = (2 x 0.047) = 0.094 ohms
This sounds all too hard! Have you already engineered any POE cables for some APs?
Yes.
Freenet Antennas has already done the above measurements/calculations on the APs listed below, and confirmed they work perfectly with up to 20m or CAT5e POE cable.
You can purchase pre-made POE cables from our Online Store. 
