The PoE standard, or IEEE 802.3af-2003 Data Terminal Equipment (DTE) Power via Media Dependent Interface (MDI), helped increase the value of an Ethernet port by connecting and powering devices using a common network infrastructure. An upcoming improvement to the PoE standard known as Power over Ethernet Plus (PoEPlus), or IEEE P802.3at, promises to deliver more power, enabling a new breed of Ethernet devices while continuing to support IEEE 802.3af.

PoE, which began shipping in 2003, is enjoying adoption in an increasing number of applications. PoE equipment is divided into two categories: powered devices and power sourcing equipment. While powered devices are end devices that require power, such as IP phones or Wi-Fi access points, power sourcing equipment refers to devices that deliver power, such as PoE switches or midspans (power injectors).

Getting more power

Many of today's powered devices require power greater than the allowable 12.95 W specified in IEEE 802.3af, including IEEE 802.11n Wi-Fi access points, pan-tilt-zoom security cameras, and IP phones that come with video conferencing and other advanced features. These sophisticated powered devices will benefit from IEEE P802.3at, which the IEEE P802.3at Task Force expects to ratify by 2009.

IEEE P802.3at helps combine services such as video used for perimeter security in a single infrastructure. For example, a closed caption TV camera requires multicabling and an analog-based receiver/recorder such as a VCR. A pan-tilt-zoom security camera that supports IEEE P802.3at only requires one RJ-45 cable to capture the scene, transmit the video, and power the camera. This provides customers a simplified infrastructure, enormous cost savings, and greater security coverage.

In September 2005, the IEEE P802.3at Working Group defined the following goals as binding objectives for the IEEE P802.3at PoE Enhancements Task Force:

• Ensure that IEEE P802.3at:
• Uses Category 5 (Cat 5) only. IEEE P802.3af supports Cat 3 and Cat 5.
• Abides by the power safety rules and limitations pertinent to IEEE 802.3af-2003.
• Is compatible with IEEE P802.3af.
• Provides the maximum power to powered devices as allowed within practical limits.
• Powered devices connected to IEEE P802.3af power sourcing equipment indicate to users that IEEE P802.3at power sourcing equipment is needed.
• Research the operation of midspans for 1000BASE-T.
• Research the operation of midspans and endspans for 10GBASE-T.
• Create a powered device management information base.

Cabling, current, voltage, and wattage

PoEPlus requires Cat 5 (8-wire) instead of Cat 3 (4-wire) because more power can be transmitted over more conductors and because Cat 5 cables have lower impedance to reduce power dissipation. The Cat 5 cable is required to operate in ambient temperatures of no more than +50 °C.

Based on IEEE P802.3at/D3.0, March 2008, the IEEE P802.3at Task Force set a maximum current of 600 mA, significantly higher than the 350 mA maximum current in IEEE P802.3af. All parameters used to determine overload conditions and maximum consumption levels are adjusted for IEEE 802.3at.

The IEEE P802.3at draft standard also increased the minimum voltage from 44 V to 50 V to support battery-based powered devices, an important factor for embedded applications. This modification in voltage increases the available power by 16 percent.

Currently, the maximum wattage specified in IEEE P802.3at for a powered device is 25.5 W, which is almost twice the maximum wattage specified in IEEE P802.3af. The IEEE P802.3at Task Force considered modifying the maximum wattage to 60 W but decided against it because doing so would have increased the environmental requirements for power sourcing equipment and powered devices.

Powered device taxonomy

Table 1 shows the powered device classification defined in IEEE P802.3at, which matches the powered device classification defined in IEEE P802.3af to meet the 12.95 W backwards compatibility maximum wattage requirement. Power sourcing equipment powers up a Type 1 powered device using a 1-Event Physical Layer Classification.

Type 2 powered devices, which require power from 12.95 W up to 25.5 W, must support the following classifications:

• 2-Event Physical Layer Classification: In this method, the power sourcing equipment emits two classification pulses to detect the powered device. Powered devices that support IEEE P802.3at and require more than 12.95 W send a Class 4 signature to the power sourcing equipment.
• Data Link Layer Classification: In this method, the power sourcing equipment emits a single classification pulse and provides power to the powered device. The powered device can then begin communicating to the power sourcing equipment using a data link layer protocol such as Link Layer Discovery Protocol (LLDP). Subsequently, the powered device and power sourcing equipment can begin negotiating for power using LLDP.

While IEEE P802.3at requires that powered devices support both classification methods, power sourcing equipment is only required to support one method.

Design considerations

When designing a PoEPlus system, engineers should observe the following rules of thumb:

Power sourcing equipment design

• Use a power supply with the highest possible voltage because for any given powered device load, the current will be lower and thus the power dissipated inside the power sourcing equipment will be lower. The overall system conversion with the best-case voltage can be as much as 5 percent more efficient.
• Implement the DC maintain power signature disconnection method, which does not require a diode in series with each port. This greatly reduces the power dissipated in the power sourcing equipment.
• Make certain that wire races have at least the minimum width recommended by the power sourcing equipment IC manufacturer.
• Ensure that ventilation is appropriate for the power dissipated by the circuit.
• If possible, use more than one power supply to feed the system. Most users will not need 30 W in every port, especially when the switch or midspan has 24 or more ports. Employing more than one power supply allows most users to pay only for the power they use and advanced users to add power via an external power supply. Make sure to use a system that can rapidly disconnect ports in case a power supply fails.
• Use nonsaturating pulse transformers that can withstand 600 mA ongoing current with at least 120 microH inductance.

Powered device design

• Follow the same requirements regarding thermal, traces, and pulse transformers.

Moving forward

Today, PoE is commonly used in VoIP deployments. PoE port shipments grew to 45 million in 2007 and are expected to jump to 145 million in 2011. Many companies are adopting this technology to benefit from the promise of consolidating voice with data using PoE.

Embedded platforms such as video cameras will be the next wave of devices powered by PoE. Because PoEPlus offers higher wattage support, new applications with higher-powered devices will continue to appear. Now and in the future, the growth of PoE will help increase the value of Ethernet in both enterprise and embedded applications.

Daniel Feldman is a senior product line manager at Microsemi's Analog and Mixed Signal Group, based in Irvine, California, where he manages products in the PoE, xDSL remote power feeding, and telephony ring generation markets. He is an active member of the IEEE 802.3at Task Force and chairs the Ethernet Alliance PoE/PoEPlus Technical Committee. Daniel holds a BSc (cum laude) in Computer Engineering from the Technion – Israel Institute of Technology and is an MBA candidate at UC Berkeley's Haas School of Business.

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