How 802.3af PoE Works
Before 802.3af, some LAN equipment vendors used proprietary power injectors to insert power into an Ethernet cable. Although this approach worked, some injectors posed a real hazard to wired equipment. For example, one product we tested in our NETWORK COMPUTING Green Bay, Wis., Real-World Labs came with large yellow warning stickers for the powered Ethernet cable. If you plugged that "hot" Ethernet cable into a PC or laptop, the network interface could suffer electrical damage, or even worse, the hot cable could fry the entire machine or start a fire.
Luckily, 802.3af remedied this safety issue. 802.3af detects PoE-capable devices and puts power on the cable only if there's a compatible device on the other end of the line. If the device on the opposite end is not PoE-capable, the port behaves just like a normal LAN switch port. No sparks or smoke.
Power can be delivered to an 802.3af-compliant PD over active data lines or, in the case of 100-Mbps Ethernet, over the two unused pairs in the cable. This method requires new 802.3af-compliant Ethernet switching hardware that can put power on the line. Alternatively, power may be supplied to the PD using midspan power injection, whereby powered patch panels inject power onto the unused pairs in a 10/100 cable. "Power It Up," left, illustrates these three methods.
Before 802.3af, many vendors recommended midspan devices because they provide PoE without requiring changes to the LAN switching equipment. The downside is that midspan devices can add power only to unused cable pairs. In a network with all four pairs in use, you can't use a midspan injector.
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