PoE= Power Over Ethernet, specifically ordinary Cat5Cable
IEEE adopted a standard for Power over Ethernet in June 2003: 802.3af. IEEE 802 series standards are usually made available at a public download area 6 months after publication; until then you need to buy the standard from IEEE.
However, many non-standard, simpler, PoE implementations have been homebrewed or manufactured into products.
Homemade PoE modules Howto http://www.nycwireless.net/poe/
I (JayPrimePositive) followed the NycWireless HowTo and learned a thing or two. Read about JppsHomeBrewPoe to learn a little about reasonable lengths of PoE cable you can make. (Super brief answer, 25 feet is probably safe, longer under some circumstances.)
I've been running PoE over two pairs to an Apple Airport base station over 250 feet for almost a year now, no problems. --TerrySchmidt
I (John Adams) have another way to do PoE using easily available Dual Cat5 Jacks from Radio Shack. It's here: JnaPoe
PowerDsine has a technical application note on the circuitry in powered devices (e.g. AP's).
See also the PoEWiringProposal
One nice thing of the 802.3af standard is that 802.3af DTE equipment can receive power from a non standard 802.3af midspan POE device.
Subject: RE: Power-over-Ethernet question./Fri, 15 Mar 2002/ From: "Brian MacLeod" <bmacleod@cet.com>/To: <dev@seattlewireless.net>
PoE means different things to different people. The general idea is to supply enough low-voltage d.c. to run small electronic devices. Sometimes you want to put a small piece of electronics (e.g. an AP) in a place where the cost of running power (professionally and to code) will be several times the cost of the equipment to be powered. [The IEEE has adopted a standard in 6/2003, although the industry in its usual enthusiasm produced many products prior to its completion which may or may not comply with the final document. -ch]
The standard is concerned with the general world where RJ45 connectors are used for everything from ISDN and Ethernet to proprietary PBX and various CSU/DSU equipment. Since you can get up to 200 V on phone lines and some phone systems have "phantom" power, this is a serious consideration. The danger of plugging a cable into the wrong jack and blowing something (or someone) up are real. The IEEE is creating something that will sense the twisted pair being used and try to avoid injecting a voltage if there is something there already. The draft standard also allows for long runs/voltage drops and makes sure that everything is SELV (International standard for 'Seperated Extra Low Voltage' which keeps everything below where it will cause harm to humans).
**note orignal text was 'Safe Electrical Low Voltage' - see note below for explanation**
In the current climate, there are simpler and cheaper "hacks" in use every day. If you are setting up in your own home and control the installation, there is probably little risk in using them. However, do be aware that there will be a flood of compatible products on the market from dozens of vendors once the standard is complete (high volumes and compatibility means cheap). There is a good chance that some of the products available today and many of the hacks will not work with the standard products.
Charles Deitlin wrote to dev@seattlewireless.net on 1 Aug 2003:
For what it's worth, "real" PoE (802.3af) runs at 48VDC, with a max current of 350mA. That's 16.8W theoretical, but 12.95W is the stated maximum given losses.
"End span" 802.3af provides power on either the data pairs (1,2;3,6) or spare pairs (4,5;7,8), while "mid span" 802.3af provides power on the spare pairs (4,5;7,8).
802.3af works with 10BASE-T, 100BASE-TX, or 1000BASE-T.
I have a PowerDsine PD-6001 that is a mid-span device. It works great with my Soekris net4521, which is 802.3af-compatible. It works 10Mbps or 100Mbps.
True 802.3af hubs/supplies are able to detect if a device (router/AP/whatever) is 802.3af compliant, so you can just plug-and-play and not worry about frying hardware that does not like excess power coming down the cable.
note on 'Seperated Extra Low Voltage' this is the correct description for SELV. The precise technical specification varies depending on the electrical standards of the country in question, but are broadly the same across the globe. (e.g. BS 7671 in the UK (a.k.a. IEE Wiring Regulations Sixteenth Edition))
The key bits are :-
- 'Seperated' - meaning 'electrically seperated from earth and from other systems in such a way that a single fault cannot give rise to the risk of electric shock.'
- 'extra low voltage' - meaning less than 50V a.c. or 120v d.c. whether between conductors or to earth.
Ok so it's nice to know the correct definition but so what - well it means if you are creating your own home cooked POE system you should :-
never connect it direct to the mains
never use an auto-transformer - these don't isolate the system from the mains earth.
Personally I would never use more that about 25v a.c. / 50v d.c. in a POE system, any more is asking for trouble, and make sure you stay within the voltage & current ratings for the ethernet cable you are using.
