PoE basically "injects" power into an ethernet cable at the source, and "picks" off that power at the destination. The power "injector" and "picker" can be external adaptors or can be "embedded" into the devices themselves. Either way, the destination device is completely powered by the ethernet cable, no extension cord or outlets required.
Many applications benefit from electrical power sent over ethernet cables. This avoids a separate power cord for things like internet phones and wireless access points. Fortunately and unfortunately, there are several ways to do this. Fortunately you have several choices, unfortunately they are not all compatible. Fortunately you are at the right place to sort out the confusion between standards.
There are basically three methods of sending PoE on the ethernet cable:
First type of PoE, using "data" wires. The IEEE (Institute for Electrical and Electronics Engineers), same folks who standardized Ethernet itself, standardized PoE in June 2003. The IEEE 802.3af standard uses the same "data" pairs as ethernet, leaving the "spare" pairs free. This PoE adds DC power to the data pairs using signal transformers, and pickes off power at the far end the same way. A comprehensive set of technical standards for Power Sourcing Equipment (PSE) and Powered Devices (PD) create an "idiot proof" system protected from shorted wiring, polarity reversal, or accidentally plugging in non-PoE equipment. IEEE 802.3af is technically complex and best implemented with power management chips specially designed for the purpose, supplied by Dallas, Maxim, Linear Technology, Texas Instruments, and others, that are intended to be embedded into the PoE devices themselves. Expect PoE device makers using their own standard to migrate to IEEE 802.3af in the future.
Second type of PoE is a combination of the two. The new IEEE 802.3af standard alternatively allows the "spare" wire pairs to be energized, to be compatible with both types of wiring. You may mix 802.3af with older or homebrew PoE devices, but the result may not be "idiot proof". If your mixed network has only PoE sources that are IEEE 802.3af compliant, your mixed network is pretty safe from damage, but older devices may or may not operate correctly. This mixed PoE allows brands to migrate to the common standard.
Third type of PoE, using "spare" wire pairs. The Unshielded Twisted Pair wiring (UTP) for ethernet has 8 wires, twisted in 4 pairs. Ethernet connects to only two pairs for data, leaving the other two free. Simply using the "spare" pairs is the hands-down most economical and efficient way to do PoE. But, beware of incompatible standards for polarity and voltage, which vary from brand to brand. More info below. Beware also, some other devices, such as ordinary analog phones, may already be using the "spare" pairs in your network. Also, if you plug your laptop or some kind of non-PoE ethernet device into a port energized with PoE, it may or may not harm your devices. All homebrew and many commercial PoE devices use the "spare" pairs in UTP wiring, and it is the absolute fastest and easiest way to adapt a non-PoE device to use PoE.
If you are merely a consumer, any IEEE 802.3af compatible device will work with any other. If your PoE devices are not IEEE 802.3af compliant, best stick with one brand, or at least with PoE devices known to be compatible with your favorite brand. More info below. If you are comfortable working with low voltage, you have come to the right place to sort out the various standards and roll-your-own PoE network.
Many applications benefit from electrical power sent over ethernet cables. This avoids a separate power cord for things like internet phones and wireless access points. Fortunately and unfortunately, there are several ways to do this. Fortunately you have several choices, unfortunately they are not all compatible. Fortunately you are at the right place to sort out the confusion between standards.
There are basically three methods of sending PoE on the ethernet cable:
First type of PoE, using "data" wires. The IEEE (Institute for Electrical and Electronics Engineers), same folks who standardized Ethernet itself, standardized PoE in June 2003. The IEEE 802.3af standard uses the same "data" pairs as ethernet, leaving the "spare" pairs free. This PoE adds DC power to the data pairs using signal transformers, and pickes off power at the far end the same way. A comprehensive set of technical standards for Power Sourcing Equipment (PSE) and Powered Devices (PD) create an "idiot proof" system protected from shorted wiring, polarity reversal, or accidentally plugging in non-PoE equipment. IEEE 802.3af is technically complex and best implemented with power management chips specially designed for the purpose, supplied by Dallas, Maxim, Linear Technology, Texas Instruments, and others, that are intended to be embedded into the PoE devices themselves. Expect PoE device makers using their own standard to migrate to IEEE 802.3af in the future.
Second type of PoE is a combination of the two. The new IEEE 802.3af standard alternatively allows the "spare" wire pairs to be energized, to be compatible with both types of wiring. You may mix 802.3af with older or homebrew PoE devices, but the result may not be "idiot proof". If your mixed network has only PoE sources that are IEEE 802.3af compliant, your mixed network is pretty safe from damage, but older devices may or may not operate correctly. This mixed PoE allows brands to migrate to the common standard.
Third type of PoE, using "spare" wire pairs. The Unshielded Twisted Pair wiring (UTP) for ethernet has 8 wires, twisted in 4 pairs. Ethernet connects to only two pairs for data, leaving the other two free. Simply using the "spare" pairs is the hands-down most economical and efficient way to do PoE. But, beware of incompatible standards for polarity and voltage, which vary from brand to brand. More info below. Beware also, some other devices, such as ordinary analog phones, may already be using the "spare" pairs in your network. Also, if you plug your laptop or some kind of non-PoE ethernet device into a port energized with PoE, it may or may not harm your devices. All homebrew and many commercial PoE devices use the "spare" pairs in UTP wiring, and it is the absolute fastest and easiest way to adapt a non-PoE device to use PoE.
If you are merely a consumer, any IEEE 802.3af compatible device will work with any other. If your PoE devices are not IEEE 802.3af compliant, best stick with one brand, or at least with PoE devices known to be compatible with your favorite brand. More info below. If you are comfortable working with low voltage, you have come to the right place to sort out the various standards and roll-your-own PoE network.
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