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A powerful new Ethernet

The benefits of converging data and operating power into one LAN cable are being realized.


January 1, 2004  


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There is power running over cabling networks, enough to power IP telephones, IP cameras, wireless access points, or even recharge the battery on my PC.

Power over Ethernet (PoE) is an exciting new technology for the networking industry. It creates expanded opportunities for new powered devices that can be used in security and building automation systems.

It also offers a more reliable and cost effective implementation of IP telephony for enterprise networks.

Work on Power over Ethernet started when the Institute of Electrical and Electronics Engineers (IEEE) formed a taskforce in 1999, called IEEE 802.3af.

It developed a standardized approach to deliver direct current (DC) power using two pairs in a 4-pair cable. The PoE standard takes into account the regulations for Safety Extra Low Voltage or SELV systems per IEC 950, which ensures that the application of power is controlled and does not pose a hazard to personnel and equipment.

It was approved by the IEEE Standards Board last June and published as “802.3af-2003.”

Two methods

The 802.3af standard defines two methods to implement PoE. One is to source both power and data directly through the port of an Ethernet switch. The other is called end-span power.

For end-span power, the DC power is applied on the data pairs (pin assignment: 1-2 / 3-6) through the center tap winding of a transformer. The high frequency data signals have always been transformer-coupled at each end of an Ethernet link to provide common mode interference rejection.

The DC voltage is placed on the center tap at one end and is recovered from the center tap at the other end of the line. Opposing DC current flow in each half of the transformer winding prevents dc saturation of the ferrite core allowing the data signal to pass unhindered.

An alternative method, called mid-span power, inserts the power sourcing equipment (PSE) between the Ethernet switch and the powered device (PD).

Mid-span power equipment resembles a patch-panel with a “data in” port and a “data + power out” port.

For mid-span power, the DC power is applied on the unused pairs (pin assignment: 4-5 / 7-8) for 10/100 BASE-T networks. Mid-span power is typically used for low port density applications or for adding power to a network without replacing the switches.

According to the 802.3af standard, a PSE must deliver a nominal 48Vdc at up to 350mA to a PD when it’s been detected. The maximum power applied to the device is 12.95 Watts. This is sufficient to power many devices such as IP phones, LAN access points and cameras that draw between five to 10 Watts of power.

An important function of the PSE is to identify the PDs that are enabled to receive power to provide the required power levels and to remove the power when the PD is disconnected from the link.

To perform this function, the PSE applies test voltages to determine the load characteristic of the PD, which is called the PD detection signature. The detection signature enables the PSE to provide the right level of power, providing a form of power regulation.

In support of the work done by IEEE 802.3af task force, the TIA TR42.1 subcommittee is in the process of balloting on SP-4425-AD6 Draft 2.0, “Additional Cabling Requirements for DC Power”, to be published as an Addendum to TIA/EIA-568-B.1.

This document contains requirements for testing different mid-span power implementations. From a cabling standpoint, the intent of a mid-span power device is to replace a connector or a patch cord, without compromising the performance of a channel.

In summary, the benefits that come with converging data and operating power into one LAN cable are being realized.

PoE technology saves the time and cost of installing separate power cabling, AC outlets, and local power supplies (wall warts). It further eliminates the need for a dedicated Uninterrupted Power Supply (UPS) for individual devices connected to the Ethernet.

By connecting a UPS in the telecommunications or equipment room, the entire IP telephony network becomes more reliable and ensures continuous operation in the event of a power failure.

PoE technology will also create new markets for network powered equipment for control, management and maintenance of security, building automation and industrial control systems.

Paul Kish is Director, IBDN Systems & Standards at NORDX/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.


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