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Remotely Powerful

A pending standard is poised to enable a new generation of low-power Ethernet devices into the market, opening up all kinds of possibilities.


January 1, 2003  


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The year 2002 was an eventful one that was marked with the completion of a number of forward-looking standards. These include the Category 6, the laser-optimized multimode optical fiber, the Building Automation Standard and the revised Administration Standard.

I have written about these in previous columns of this magazine. Looking forward into this year, I thought it would be a good opportunity to describe an important standard in the making.

It is the IEEE 802.3af standard for remote powering of Ethernet devices, such as IP phones, wireless access points, point-of-sale terminals, security devices, etc. The remote powering standard is poised to enable a new generation of low-power Ethernet devices into the market, opening up all kinds of possibilities.

Two types of power sourcing equipment (PSE) are specified in the draft 802.3af standard: Data terminal equipment (DTE) powering and midspan powering.

In the first case, power is sourced directly from the network equipment.

These types of devices are called DTE power sourcing equipment (DTE PSE). In the second case, midspan power is provided by inserting power sourcing equipment between the DTE and the device to be powered.

For midspan PSE, power is supplied on the pairs not used for 10/100Base-T equipment — that is, pin-pair assignments (4,5) and (7,8). Midspan powering will permit the support of legacy equipment that lacks powering capability.

The current IEEE 802.3af specification requires that the transmission performance of a channel and the channel length be maintained after the insertion of the midspan PSE. This means that the transmission performance of a midspan PSE is equivalent to either a mated connector or a work area cord and can replace one of these components without affecting the overall channel performance.

For power sourced from 10/100/ 1000Base-T equipment (DTE PSE), the power is provided on the same pin-pair assignments as the data, that is, TX (1,2) and RX (3,6). Power is superimposed on the data pairs through a “phantom” DC circuit utilizing the secondary winding center taps of the transmitter and receiver transformers at each end of the link segment.

One of the important functions of the DTE PSE or midspan PSE is that it can recognize the powered devices (PDs) that are enabled to receive power and that it can provide the required power levels and remove power in the event that the PD is disconnected from the link.

The detection mechanism is an extremely important function of the PSE to prevent unintended application of power to the wide range of devices that can be plugged into an 8-pin modular jack.

The PSE output voltage range is 44 Volts to 57 volts with a maximum output current of 350 mA.

CABLING ISSUES

TIA-TR42, the TIA/EIA Engineering Committee responsible for User Premises Telecommunications Cabling Infrastructure, is in the process of evaluating the IEEE 802.3af specification.

A task group has been assigned to lead the investigation. The scope of the task group work includes identifying the cabling channel performance required, and evaluating the use of the midspan PSE, both the implementation and transmission performance. It is anticipated that the outcome of the work will result in an addendum to TIA/EIA-568-B.1 and TIA/EIA-568-B.2.

The addendum will address all of the requirements for DTE powering to ensure full support of IEEE 802.3af and other applications that can use the cable plant for powering equipment.

One of the most important cabling parameters is the pair balance characteristic, such as the resistance unbalance of the powered circuit, which could affect the operation of the PSEs.

The main driving force for powered Ethernet is IP telephony to ensure that the phone remains operational in the event of a power failure and in an emergency.

Traditional telephone sets are sourced from batteries in the central office or in the main equipment room where the PBX is located. Centralized remote power with battery backup has enhanced the telephones reliability and indispensability.

This remains an expectation of end-users who want to migrate to IP telephony. Other powering applications include wireless access points and building automation system devices, where it is not always feasible or convenient to supply local power.

It supports the need for copper data cabling as a vital link in the horizontal distribution network.

Paul Kish is Director, IBDN Systems & Standards at NORDX/CDT. He is also vice chair of the TR-42 engineering committee. The author would like to thank Chris DiMinico of CDT for the information provided for this article.


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