Connections +

Power over Ethernet Plus

It certainly has merit, but there are a number of cabling issues that need to be investigated to assure satisfactory performance.

July 1, 2005  

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There is a new effort underway in IEEE 802.3 to augment the capabilities of the IEEE 802.3af-2003 standard with higher power levels and improved power management information.

At press time, a project authorization request for Power over Ethernet (PoE) Plus was due to be submitted for approval in late July. The assigned project number is 802.3at and the expected completion date is November 2007.

The existing 802.3af standard limits the power supplied to a powered device to 12.95 watts with a maximum current limitation of 350 mA over 2 pairs of conductors.

One option called end-span powering is to apply DC power to the remote device over the data pairs (pairs 3-6 and 1-2). Another called mid-span powering is to apply DC power over the unused pairs (pairs 4-5 and 7-8) for 100 Mb/s Ethernet.

The PoE Plus study group has identified significant market demand for applications such as multi-band wireless access points and controllable security cameras that require power beyond the level currently defined in the standard.

A maximum power level in the range of 30 to 40 watts is needed to meet market demand, which is anticipated to be approximately 80 million units by 2008.

The table above provides some examples of power levels required for different devices that are or are coming on the market.

The benefits of PoE Plus are that it does not require a local power supply, eliminates the electrical installation cost to provide supplementary outlets (e.g. wireless access points), and provides automatic backup via an Uninterrupted Power Supply (UPS) connected to the Power Sourcing Equipment (PSE).

Power Management enables the same 400-watt power supply to address 24 ports at 15.4 watts or to manage the power between low power and high power devices (e.g., 16 ports at 7.7 Watts + 8 ports at 30Watts)

The maximum amount of power that can be applied is limited by the cabling infrastructure. An important consideration that limits how much power can be applied is the heating of the cable.

Derating factor

FCC Part 68.215 specifies that the current on individual wiring conductors shall be limited to values that do not cause an excessive temperature rise.

For 24 AWG multi-conductor cables the current limit is 420 mA per conductor at 25 degrees C. At higher ambient temperatures, the current needs to be de-rated. The derating factor for cable is not precisely known.

Further work needs to be done to determine cable derating factors for closely grouped cables where power is applied for 2-pair and 4-pair operation at different temperatures.

Another important consideration is the change in connector performance after subjecting the connector (plug-jack connection) to multiple cycles of mating and unmating under load.

There is evidence that removing the plug under load can cause arcing at the contact interface for higher current levels.

This effect needs to be further investigated to assure long term contact reliability, in particular for 2-pair operation where the current levels per conductor are twice as high for the same power level.

As you can see there are a number of cabling issues that need to be investigated to assure satisfactory performance at higher levels of power. Implementing 4-pair power at 802.3af (350mA per pair or 175 mA per conductor) current levels would be the fastest route for the committee to take in order to get a higher power standard.

However, the tradeoff is that 4-Pair PoE incurs a significant increase in cost and complexity.

That is why the PoE Plus study group is also considering a 2-pair powering option at power levels in the range of 20 Watts to 30 Watts. What can be achieved will depend on the results of tests that measure the effect of cable heating and connector performance in a worse case environment.

Paul Kish is Director, Systems & Standards at Belden CDT. He is also vice chair of the TR-42 engineering committee.

Disclaimer: The information presented is the author’s view and is not official TIA correspondence.

Power Level Application
13 Watts to 20 Watts 802.11n / Biometric Access Control / Thin Clients
20 Watts to 30 Watts RFID Readers / Video IP Phones / PTZ IP Cameras / 802.16 Base Stations / Thin Clients / Industrial Sensors
30 Watts to 40 Watts Thin Clients / Video IP Phones / PTZ IP Cameras / Workgroup switches / Point of Sales / Information Kiosks