Monday, November 07, 2011

Power over Ethernet and the Throwing Star LAN Tap

Since handing out hundreds of Throwing Star LAN Tap printed circuit boards as business cards at DEF CON, I've received a number of interesting questions about the device in my email. A couple people were hoping to use Throwing Stars to monitor connections that use Power over Ethernet (PoE). When I designed the current version of the Throwing Star LAN Tap, I decided to pass all eight conductors through from J1 to J2 (the target ports) even though this necessitated the addition of two filtering capacitors. I did this primarily to support RS-232 monitoring (a feature that I imagine is very rarely used), but I also thought that having eight conductors might be handy for other things such as PoE. It wasn't until recently that I actually verified PoE capability, however.

PoE allows a device to be powered by direct current (DC) running over an Ethernet cable that may also be used for communication. It is popular for VoIP telephones, IP security cameras, wireless access points and other network-connected devices that are commonly deployed at multiple locations within a building. There are several different ways that PoE has been implemented over the years, but most devices these days follow the IEEE 802.3at-2009 standard or its similar predecessor, IEEE 802.3af-2003. I looked at these standards and also at the most common non-standard implementations and found that they are all compatible with the Throwing Star LAN Tap.

Twisted pair Ethernet cables consist of eight wires arranged into four pairs. In some cases two of those pairs are unused. Each pair carries a differential signal with one wire carrying the inverse of the signal on the other wire; when one goes high, the other goes low. This is an alternating current (AC) signal.

What all of the PoE schemes have in common is that they introduce a DC bias between one pair and another. In the figure to the left, the purple and green lines represent the voltage on one pair and the blue and orange lines represent the voltage on a second pair. In each pair, the AC component (the rapidly changing difference in voltage between the two wires) is relatively small. This is the signal that carries network data. From one pair to the second pair there is a larger voltage difference, the PoE DC bias.

The Throwing Star LAN Tap provides a DC path for all eight conductors between the target ports, J1 and J2, but it only extends a subset of those conductors to the monitoring ports, J3 and J4. This is done in such a way that Power over Ethernet on the target network can pass through the tap but does not extend to the monitoring ports. It's almost like I meant to do that. :-)


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