Traditional Power over Ethernet

When the IEEE created the IEEE802.3af task force, which pursued the addition of power transmission to the venerable IEEE802.3 standard, the IT world was a different place. In 1999, CAT3 was still considered to be an abundant connectivity option to the desktop, Enterprise IP telephony did not really exist and WLAN AP's were nowhere to be seen.

Since then (and since the approval of IEEE802.3af in 2003) this has changed. The cabling infrastructure was upgraded to CAT5E and above while IP telephony has become a part or in the plans of every enterprise. WLAN is found everywhere. These, together with Network Cameras, were the main applications enabled by the original Power over Ethernet standard. And for the most part, these applications can do pretty well with the power limit imposed by IEEE802.3af, 12.95W.

However, the popularization of these applications triggered the inventiveness of developers and marketers into creating more power hungry derivative devices, such as Video IP Phones, Multi-channel WLAN Access Points (like the ones in the IEEE802.11n upcoming MIMO standard) and Pan-Tilt-Zoom IP Cameras. All these applications require typically more than 13W, normally between 13W and 30W. And of course, going further laptops and even desktop PC’s could be powered too…

Enhancing Power over Ethernet

With these new high power applications in mind, the IEEE802 created in December 2004 a study group called PoEPlus, which studied the market needs and possible technical solutions to address these, still maintaining backwards compatibility with IEEE802.3af.

As a result, the group laid out a set of objectives, approved in September 2005 as binding for the IEEE802.3at Power over Ethernet Enhancements Task Force:

  • 802.3at should operate on CAT5E and higher infrastructure, unlike 802.3af, that had take into account the CAT3 limitations
  • 802.3at should follow the power safety rules and limitations pertinent to 802.3af
  • A High Power 802.3at PSE must be backwards compatible with 802.3af, being able to power both 802.3af and high power 802.3at PD's
  • 802.3at should provide the maximum power to PD's as allowed within practical limits, at least 30W
  • 802.3at PD's, when connected to a legacy 802.3af PSE, will provide the user an indication that an 802.3at PSE is required.
  • Research the operation of Midspans for 1000BASE-T
  • Research the operation of Midspans and Endspans for 10GBASE-T
  • Creation of a Powered Device MIB

Cabling and High Power

The assumption of using CAT5E and not CAT3 cabling is a crucial one, as CAT5E cables have by default 8-wires, twice the amount of the wires present at CAT3 cables, which means that without any modification in the current or voltage parameters of the IEEE802.3af , it should be straightforward to get twice the power.

But the advantages don't stop here: CAT5E cables have only 62.5% of the resistance found in CAT3 cables and therefore dissipate proportionally less power for the any given current.

At the time of writing, the IEEE802.3at task force has decided to follow recommendations by the TIA TR42.7 to set maximum current at 720mA. This assumes that the CAT5E cables are in ambient temperatures of no more than 45oC. The Icut and Ilim parameters in the standard where updated accordingly, so that PD’s can take up to 820mA for short periods of time. This is more than twice the maximum current in IEEE802.3af, 350mA.

Voltage levels

Raising the current is not the only way of getting more power through the existing infrastructure. Increasing the minimum voltage is also possible. When the IEEE802.3af standard was written, several companies believed that it was important to have support for battery-based powering. This is the source of the minimum 44V PSE output. Based on the experience acquired with IEEE802.3af installations on the field, this seems no to be a common limitation, and one that can be overcome by boosting the battery voltage.

Therefore, the IEEE802.3at task force intends to raise the minimum PSE output voltage from 44V to 50V, increasing the power available to the PD by 16% with virtually no cost involved.

High Power vs. Medium Power

With CAT5E cabling, an 720mA of current and a higher voltage level, one question remains: is IEEE802.3at a synonym of using always all the 8-wires available to feed PD's, or is there a place for rising current and voltage but use only 4-wires?

While clearly using the 8-wires gives more power (59W), depending on the final maximum current determined by the IEE802.3at task force, 4-wires solutions could be enough to power many applications, as it provides 29.5W to the powered device.

Backwards Compatibility

Interoperability with existing hardware is key for the success of any standard, and this not different with IEEE802.3at. Backwards compatibility with IEEE802.3af PSE's and PD's is a mandatory requirement. If from the IEEE802.3at PSE perspective this seems a relatively simple task, as the new IEEE802.3at PSE would be detecting an existing IEEE802.3af PD, the same cannot be said about IEEE802.3at PD's. There are two possible interoperability scenarios envisioned by the IEEE802.3at Task Force:

a) IEEE802.3at PD can work in a low power mode: the IEEE802.3at PD should be able to identify the PSE to which it is connected as an IEEE802.3af one and automatically operate in the low power consumption mode.

b) IEEE802.3at PD requires more than 12.95W to operate: the IEEE802.3at PD must actively indicate to the end-user that it is connected to a legacy IEEE802.3af PSE that cannot power it.

Future-proofing the standard

Another aspect of the IEEE802.3at specification is related to data: can it operate with IEEE802.3an 10Gbit/s Ethernet? How about Gigabit Ethernet and Midspans?

The group has committed to study how raising the power influences the new IEEE802.3an PHY.

Gigabit Midspans, which were not precluded by IEEE802.3af but out of scope of the specification, but have become very popular in the marketplace, are going to be formally addressed by the new IEEE802.3at standard.

Classification

The last point of interest, but not necessarily the least important, is the method of classification. IEEE802.3af provides 5 classes (0 to 4), which translate into three PSE power allocation values: 4.0 Watts, 7.0 Watts or 15.4 Watts.

This scheme, which is not efficient even for IEEE802.3af applications (every WLAN access point that requires 7.1 Watts receives a 15.4 Watts allocation, more than 50% waste), is clearly not fit for higher power applications.

One of the goals of the IEEE802.3at task force is therefore to increase the granularity of the classification method, leaving also place for future enhancements. This must be done in a backwards compatible way, so IEEE802.3af PD's can still be classified by IEEE802.3at PSE's, and IEEE802.3af PSE's don't wrongly classify IEEE802.3at PD's.

Two methods may be used to perform classification in IEEE802.3at:

2-Event Classification, in which the PSE emits 2 classification pulses to detect the PD. IEEE802.3at PD’s that require more than 12.95W to operate present a class 4 signature, telling the PSE that they require maximum power. The IEEE802.3at PD senses the 2 classification pulses to discover that the PSE is capable of providing high power. In this method, the goal of classification is to provide mutual identification. PSE’s that use this method either provide full power at every port or use an intelligent statistical method to manage power between different ports.

With Layer 2 Classification, the alternate method, the PSE issues a single classification pulse and turns the PD on. The PD, not recognizing that this is an IEEE802.3at PSE, communicates with the PSE using the LLDP protocol, to negotiate the reception of more power, in case it requires over 12.95W.

While PD’s must support both classification methods, PSE’s have to support only one method.

Summary

Power over Ethernet is about to change in ways that most of us cannot imagine. The increase in the power available to a PD from 12.95 Watts to 59 Watts will transform the RJ45 connector in the first universal power plug. With the IEEE802.3at activities expected to be finished in Q2 2008, and pre-standard equipment such as Microsemi's PD8012 and PD7001G already available, High Power over Ethernet is closer to your desk than you think.

Biography

Daniel Feldman serves as a Product Line Manager at Microsemi’s Analog Mixed Signal Group, in charge of the P&L of all products in the PoE, xDSL remote power feeding and Ring Generation markets, including ICs and modules. He is a member of the IEEE 802.3at Task Force and Chairs the Ethernet Alliance PoE/PoEPlus Technical Committee.

Previously, Mr. Feldman worked for PowerDsine as a Senior Product Manager, at IC4IC as the System Architecture Group Manager, as a VHDL Engineer at NICE Systems and as VLSI Engineer at RAFAEL. Mr. Feldman holds a B.Sc. (Cum Laude) in Computer Engineering from the Technion in Israel.