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POE - Power Over Ethernet

(source = http://www.altair.org/labnotes_POE.html)

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. 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. Voila, Power-over-Ethernet !

There are basically three methods of sending PoE on the ethernet cable: First 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. Second 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. Third 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. 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.

Various Standards for PoE, both Commercial and HomeGrown

All PoE uses standard UTP ethernet cabling, patch panels etc, your old familiar Cat 3, 5, 5e or 6 cabling is fine. But which wire pairs, what voltage and what polarity were only recently standardized by IEEE. This means a lot of brands have not yet converted to the new standard, and a lot of older equipment is still out there. Also, non-PoE devices are frequently run via PoE by simply running their "wall cube" power brick output thru the ethernet "spare" pairs to the device. These PoE adapter kits for non-PoE devices are often available for less than $100, or you can wire it yourself for less than $10.

The IEEE standard for PoE is mainly used for "Embedded" power, that is, factory builtin PoE.

Table 1. Summary Comparison of Various PoE Standards SOURCE LOAD Ethernet RJ-45 connector pin number * Load DC Load 1 2 3 4 5 6 7 8 Voltage Connector RX, RX, TX, TX, spare spare spare spare (embedded) DC+ DC+ DCDCRX RX TX DC+ DC+ TX DCDC(embedded) REMARKS

STANDARD IEEE 802.3af using data pairs IEEE 802.3af using spare pairs Intel, Symbol, Orinoco Cisco (OLD old standard) Cisco (NEW old standard) D-Link (Adapter) Apple MacIntosh AirPort PoE, Extreme

Source Voltage 48 V DC, protected 48 V DC, protected Usually 12 or 24 V DC 48 V DC

Industry Standard for embedded PoE Industry Standard for embedded PoE Most Brands of PoE Older Cisco polarity is REVERSED New Cisco is IEEE compliant D-Link PoE Adapter for non-PoE products.

RX

RX

TX

DC+

DC+

TX

DC-

DC-

(embedded)

RX

RX

TX

DC-

DC-

TX

DC+

DC+

(embedded)

48 V DC

RX

RX

TX

DC+

DC+

TX

DC-

DC-

(embedded) DC coaxial 5.5/2.5m m DC coaxial (???)

48 V DC

RX

RX

TX DC?? DC??

TX DC?? DC??

5VDC @ 2.5A

48 V DC

RX

RX

TX DC?? DC??

TX DC?? DC??

Converted to ???

Mac Polarity Unknown

HyperLink NYC Wireless "Roll Your Own" NOTES:

Many DC Voltages Available 12 or 24 or 48 V DC

RX

RX

TX

DC+

DC+

TX

DC-

DC-

same as input same as input

RX

RX

TX

DC+

DC+

TX

DC-

DC-

DC Variety of Options coaxial Available to Fit Most and others Brands of PoE available DC New York City coaxial Wireless PoE or as reqd

* Wire Color Codes shown for EIA 568B. Swap Green and Orange for EIA 586A color code.

PoE Industry Standard IEEE 802.3af

802.3af, also known as Power over Ethernet, defines a way to build Ethernet powersourcing equipment and powered terminals. The specification involves delivering 48 volts DC power over unshielded twisted-pair wiring. It works with existing cable plant, including Category 3, 5, 5e or 6; horizontal and patch cables; patch-panels; outlets; and connecting hardware, without requiring modification. IEEE 802.3af divides the PoE world into Power Sourcing Equipment (PSE) like hubs and routers, and Powered Devices (PD) like IP phones and wireless access points. PDs are classified by the amount of power they consume. Ethernet ports on PSE may supply a nominal 48 V DC power on the data wire pairs or on the "spare" wire pairs, but not both. A

PSE must never sent power to a device that does not expect it. PoE is managed by a multi-stage handshake to protect equipment from damage and to manage power budgets. Here's how IEEE 802.3af works. SIGNATURE. First the PSE probes the device to see if it is IEEE 802.3af compliant. Probing with two current limited voltages between 2.7 V and 10 V, the PSE looks for the "signature impedance" of 25k ohms. The PD is allowed two diode voltage drops in series with the signature impedance, so two V-I points above the diode drops must be used. Non-PoE devices will usually be below 1 k ohm or many megohms. If the signature impedance of an IEEE 802.3af device is not seen, the process stops here. CLASSIFICATION. The PSE now forces a classification voltage between 15 V and 20 V and the PD responds by drawing a specific current to identify itself in a power class according to the following table. Table 2. Summary of IEEE 802.3af Power Classifications CLASS USEAGE 0 1 2 3 4 Default Optional Optional Optional Optional PD Power (W) 0.44 to 12.95 0.44 to 3.84 3.84 to 6.49 6.49 to 12.95 Reserved* Classification Current (mA) <5.0 10.5 18.5 28 40

*Class 4 is currently reserved and should not be used. DISCONNECT. A PSE must never sent power to a device that does not expect it, even when connections are changed. Therefore a PSE is required to remove PoE power when a cable is unplugged, and to reapply power only after the signature and classification phases are correctly repeated. The PSE detects the disconnect by either of two methods and removes power. The DC disconnect method detects when PD current falls below a given threshold (5 to 10 mA) for a given time (300 mSec to 400 mSec). The AC Disconnect superimposed a small AC voltage on the power and measures the resulting AC current, similar to power supply ripple voltage and load ripple current. If the impedance is above 26.25 k ohms, power is shut off intil a valid signature and calssification is repeated. As you can see, full compliance with IEEE 802.3af is best done with special chip sets, however its not hard to design your own simple devices to draw PoE power from an IEEE 802.3af compliant PSE. The easiest way is to implement Class 0 on the "spare" pairs, with polarity protection, a signature impedance, a voltage threshold and a voltage regulator.

To Learn More: IEEE 802.3af Standards Group PoE industry website IEEE 802.3af Consumer PoE Adaptor Products: Add-on POE Injector converts any hub port to an IEEE standard PSE port HyperLink Technology PoE Adaptors and Kits for non-PoE devices PoE Adaptors and Kits for various devices PoE Adaptors and Surge supressors for IEEE "compatible" systems (NOT fully protected) Australian "RPOE" Standard Adaptor uses voltage regulator at load. MacIntosh MacWireless Classic PoE in a 5 1/4 Drive Bay home project (Uses OLD Cisco polarity) PoE Circuit Components: Chip from Maxim Semiconductor combines IEEE standard PD front end and voltage regulator. Chip from Lineaar Technology for IEEE standard PD front end Chip from Texas Instruments for IEEE standard PD front end Chip from SuperTex for IEEE "Class 0" PD front end Eval board from SuperTex for IEEE "Class 0" PD front end Web-based Voltage Regulator designer at National Semiconductor, ideal for converting 48 VDC to your desired voltage. Amateur Standard: New York City Wireless http://www.nycwireless.net/poe/ D-Link Standard http://www.dlink.com/products/?pid=48 Voltage . Input: 48VDC, 400mA . Output: 5VDC, 2.5A www.altair.org labnotes_POE.html, © Altair EMail [email protected]

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