Power over Ethernet: Why do it?

July 1, 2004
Almost all networked devices require data connectivity and a power supply.

Almost all networked devices require data connectivity and a power supply. Basic analog telephones are powered from the central office through the same twisted pair that carries the voice.

By using Power over Ethernet (PoE), the same thing is possible for Ethernet services, such as:

  • VoIP (Voice over Internet Protocol) telephones;
  • IEEE 802.3af Wireless LAN access points;
  • Bluetooth access points;
  • Web cameras.

In response to a growing need to supply power over the Ethernet cables, IEEE began work on 802.3af in 1999.

On June 12, 2003, the Power Over Ethernet Standard was approved by the IEEE Standards Board to be published: IEEE802.3af-2003, Data Terminal Equipment (DTE) Power via Media Dependent Interface (MDI).

More control, fewer truck rolls

The IEEE 802.3af standard was developed to provide a 10Base-T, 100Base-TX, or 1000Base-T device with a single interface to both the data it requires and the power to process the data. Besides the obvious, "You do not have to plug the device into a local power receptacle," there are also other advantages.

PoE allows use of a centralized uninterruptible power supply (UPS), guaranteeing power to the devices during power failures. This is becoming very important as more security, access control, and building automation services are moving to Ethernet. And my personal favorite: devices can be shut down or reset remotely—no need for a site visit to press the rest button or toggle the power switch. More control with fewer truck rolls.

Cabling system requirements

IEEE 802.3af uses ISO/IEC IS 11801:2002 Generic Cabling for Customer Premises Standard cabling. The minimum cabling channel requirement in ISO/IEC IS 11801:2002 is Category 5e; however, use of lesser-category cable is also allowed by IEEE 802.3af. IEEE always wants to use the installed base of cabling for their new applications. (Let me fill you in on a little secret: that's because it leaves more money in our IT budgets for their new equipment.)

How do they get the power onto cabling?

Power is injected into the cabling by power-sourcing equipment (PSE), which is located at the data terminal equipment (DTE) at either end of the channel or at a cross-connection point along the way (midspan). The power is used by a powered device (PD) located at the end of the channel.

A PSE can provide power to the PD in one of two wiring configurations. Alternative A, sometimes called "phantom mode powering," supplies the power over pairs 2 and 3. Alternative B supplies power over pairs 1 and 4.

Endpoint PSEs can be compatible with 10Base-T, 100Base-TX, and/or 1000Base-T systems, and can use Alternative A, Alternative B, or both.

Midspan PSEs can be used only with 10Base-T and 100Base-TX, and can only use alternative B. Why? Because using a midspan PSE contributes an additional connection in the signal path (and puts three connectors close together in the telecommunications room), which can affect the channel's near-end crosstalk, power-sum near-end crosstalk, equal-level far-end crosstalk, power-sum equal-level far-end crosstalk, and return-loss performance. Trading line powering of the device for a channel that cannot support the application would not be a good idea.

How much power are we talking about?

IEEE 802.3af allows delivery of a maximum of 12.95 watts of power over two pairs to a PD at one end of the cabling. While there are several power classifications in the standard, Class 0—which covers the entire range—is the default. Class 0 allows a maximum power level of 15.4 watts at the output of PSE and power levels ranging from 0.44 to 12.95 watts at the input of PD.

The maximum continuous output current from the PSE under normal operation is 350 milliamps direct current (DC) per pair, or 175 milliamps DC per conductor. This was based on the calculation of the maximum capacity of 26 AWG stranded conductor cables at their maximum environment operating temperature with the maximum allowable temperature rise. But now consider that at start-up, peak current can be as high as 5 amps for the first 1 millisecond, and 450 milliamps DC for up to 50 milliseconds.

Power is only applied when a PSE detects a device that requires power and is capable of accepting it. This ensures that there is no electrical arcing when inserting the plug into the jack, and protects ordinary DTE that would likely be destroyed if power was injected onto the signal conductors.

Do we really need TIA-568B.2-8?

A year after the IEEE published IEEE 802.3af, TIA TR-42.7 is still working on SP-3-3727-AD8 (TIA-568B.2-8), "Commercial Building Telecommunications Cabling Standard—Part 2: Balanced Twisted-Pair Cabling Components—Addendum 8: Additional Component Requirements for DTE Power."


Think of structured cabling as an interstate highway system. All sizes and shapes of vehicles travel on the system. But no vehicle is allowed on the highway that cannot meet the minimum speed requirements or would damage the highway so that other vehicles could not use the same highway in the future. Size, speed, and weight were all considered by the civil and structural engineers before they designed an interstate highway system. The same is true of a structured cabling system, only the engineers tend to be electrical and mechanical.

The idea of structured cabling is that you can plug in a telephone one day, an Ethernet link the next, and a month from now. . .who knows what?

TIA-568B.2-8 will address components that will support a wide variety of low-voltage power-limited applications that will benefit from using remote power supplied over balanced twisted-pair cabling. This is not just IEEE 802.3af-compliant applications. Yes, believe it or not, there are systems available today that are not running over IP—like 1394b and proprietary control systems for everything from lighting to lawn sprinklers.

(For further information on Power over Ethernet, see www.PowerOverEthernet.com. To download a free copy of IEEE 802.3af, see standards.ieee.org/getieee802. For further information on work in TIA TR-42, see www.tiaonline.org)

Next month: A look at progress on the Augmented Category 6 standard.

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Donna Ballast is BICSI's standards representative, and a BICSI registered communications distribution designer (RCDD). Send your questions to Donna via e-mail: [email protected]

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