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802.3af

802.3af, also known as Power over Ethernet, defines a way to build Ethernet power-sourcing equipment and powered terminals. The specification involves delivering 48 volts of AC 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.

Traditionally, network devices, such as IP phones, wireless LAN access points, laptop computers and Web cameras, also have required both types of connections. Given the increasing number of LAN devices in corporations, wiring AC connections for each of them is a costly task.

The 802.3af specification eliminates the need for additional outlets and the labor cost incurred from contracting electricians to install them.

What's more, the technology supports a point-to-multipoint power distribution architecture, parallel to the data network. This lets managers use a single UPS at the network core to back up multiple scattered devices on the LAN. 802.3af also provides remote access and management via SNMP Web-based control.

The current delivered to each node is limited to 350 milliamps. The total amount of continuous power that can be delivered to each node, taking into account some power loss over the cable run, is 12.95 watts. IP phones and wireless LAN access points typically consume 3.5 to 10 watts.

Power is carried on two wire pairs, to comply with safety standards and existing cable limitations. 802.3af power sourcing equipment contains a detection mechanism to prevent sending power to noncompliant devices. Only terminals that present an authenticated Power over Ethernet signature will receive power, preventing damage to other equipment.

802.3af defines two types of power source equipment: end-span and mid-span.

End-span refers to an Ethernet switch with embedded Power over Ethernet technology. These new switches deliver data and power over the same wiring pairs - transmission pairs 1/2 and 3/6.

Mid-span devices resemble patch panels and typically have between six and 24 channels. They are placed between legacy switches and the powered devices. Each of the mid-span ports has an RJ-45 data input and data/power RJ-45 output connector. Mid-span devices tap the unused wire pairs 4/5 and 7/8 to carry power, while data runs on the other wire pairs.

For new deployments, you'd typically buy an end-span Ethernet switch. Mid-spans make sense for upgrading a network without replacing switches and for low port density.

However, it's probably wise to consider deploying a new end-span switch because it will be attached to IP phones, wireless LAN access points and other popular powered terminals during its expected life span.

With either type of power-sourcing equipment, you can safely mix legacy Ethernet devices and new LAN-powered terminals.

From 802.3af powers up LAN, lowers cost, Network World Tech Update, 03/10/03.

Additional resources

IEEE 802.3af Task Force

There are 7 comments:

802.3af uses DC!
By Kurt Albershardt

802.3af specifies 48 Volts DC at 300-400mA

See http://www.ieee802.org/3/af/requirements.pdf
and http://en.wikipedia.org/wiki/Power_over_Ethernet


Power over Ethernet is more complex
By Lance Rasmussen

The article directs the reader to deploy end span switches without going over any of the drawbacks for this choice. PoE switches notably are limited to 200 watts divided by 24 ports or 8.3 watts per port if all ports are in use. Many access points and IP cameras take more wattage than that.While the article discusses the merits of the IEEE standard, the fact is half the market place is composed of end devices that are non standard (12 volt, 24 volt, proprietary Cisco CDP protocol.An end span switch can only forward one PoE style, not all the others. a modular Midspan PoE Hub is the only product that is capable of addressing the need for different PoE protocols and higher wattage applications.


needed information for a short deffinition
By Rowan Hawkins

The 48vac would be for powering the device from a telco equipment rack. I believe that the PoE standard is 5v out. At least all devices that I've seen take PoE also have 5v input, cameras, wireless, etc.

It would be nice to know the pinout for the standard since we are talking about it. Going into detail about wattage per port is included, so why not also the pinout for the standard, which is actually the information I was looking for. I dont think it is to much to ask, considering they mention the standard 1-2,3-6 device information.

I don't know if there is a seperate entry for HighPowerPOE or if that is part of the spec (39w/port).


Power over Ethernet
By Bassem

Brief but to the point,
No details about the protocol but any more details would be too much for the artical,
In one sentance:
"the artical is short enough to read in a few minutes with just enough information to have an idea about how PoE works"


802.3af
By Christopher Sundene

90 to 264 VAC(47-63Hz). DC voltage on pins 7/8 and 4/5. These data are from the PowerDsine 6006.


802.3af
By Mike Warnecke

The article still leaves some important questions unanswered:
Is the 48 VAC at 50 or 60 Hz? Or something else?

For mid-span devices, are 4/5 carrying a neutral and 7/8 the hot (binding a twisted pair together into 1 connector), or 4/7 neutral and 5/8 hot (like what 1/2 and 3/6 do)?


802.3af
By David McCormick

This was very helpful in defining what 802.3af, end-span, and mid-span are and accomplish.



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