Question of POE, POE+ and UPOE

“Cisco now has a 60 watt POE option. What was the limitation that caused POE to have to be developed in stages of 15, 30, and then 60 watts? Why could they not just do 60 watts from the inception of POE?”

802.3af was largely implemented to power VoIP phones and early access points. The list of devices that started using it grew quickly, and as it grew and the technology changed, it was found that 802.3af didn't deliver enough power for everything people wanted. For instance, it could power a video camera, but maybe not a PTZ video camera. Or it could power that single radio access point, but not an 802.11n 

dual radio access point.

Basic VoIP phones were fine, but high definition screen with video capability VoIP phones didn't have the power needed.

It was also found to be somewhat inefficient in its power delivery. Short of vendor proprietary extensions, a PoE port always delivered 15.4W of power to the device, even if the device didn't need that much.

So, another standard was developed to meet these needs, 802.3at. This provides up to 30W of power and allows devices to negotiate their power needs. If you only need 3W of power, it can do so and doesn't need to deliver 30W. Interoperation with 802.3af devices is accomplished by delivering 15.4W to any device that doesn't negotiate for more or less.

Cisco came up with the 60W for exactly the reason I gave to start out (they were also one of the first to deliver inline power, and higher than 15.4W of power through proprietary protocols). If the "ability" is there, then people will come up with ways to use it. Their thought process is "why limit what we can do within the power budget? Let's just provide more power."

This is both good and bad. Good because we will see new abilities by PoE devices or entirely new PoE devices that were not previously realized. Bad because there are other concerns to keep in mind.

For instance, most people don't consider heat in their cable plant when thinking about PoE.

The more power you run through a cable, the more heat that it generates and needs to be dissipated. This may reduce how far you can run cables depending on the category of your cables. Others have raised concerns because data cabling is often "bundled" with up to hundreds of data cables being tightly bound together and this can result in higher temperatures in the center of the bundles.

Another concern is that the more power you need to deliver to end devices through PoE, the more power the switch needs to draw. How big would the power supplies on a Cisco 4500 have to be to provide up to 60W on its potential 384 UPoE ports (in addition to the power needs of the switch itself)? UPSes to provide reliable power to these pieces of network equipment would then have to be upsized as well.

If it shapes up that the industry find use for 60W, then the IEEE will draft another standard/ amendment.

More about PoE & PoE+

PoE (IEEE 802.3af)

PoE is the ability of the LAN switching infrastructure to provide power over the Ethernet copper wire to an endpoint. This capability, sometimes also referred to as "inline power," was originally developed in 2000 by Cisco to support the emerging IP telephony solution. IP phones, like standard desktop private branch exchange (PBX)-supported phones, require 48 volts of power, which can be provided in one of two ways: by plugging the phone into a power outlet or by powering the phone over the network cable. The latter option was chosen because there was less chance of phone power failure. Cisco supports both its original proprietary technology for inline power and the IEEE 802.3af PoE on all Cisco Catalyst LAN switches.

The specification for PoE calls for two devices: the power source equipment (PSE) and the powered device. The Cisco Catalyst switch, when populated with PoE-capable line cards, functions as the PSE and provides power to the end device, which is the powered device. The powered device can be one of many different devices, including the IP phone or wireless access point. Other powered devices are introduced and covered later in this document.

The standard also supports another mode of operation. Because today many existing switches still in service do not support 802.3af or any type of inline power, the powered devices must support midspan PSE. This device sits between the LAN switch and the powered device, inserting power on the Ethernet cable to the powered device. A technical difference between the two mechanisms should be noted: If the Cisco Catalyst switch is the PSE device, the power is transmitted over the same pairs (pins 1, 2 and 3, 6) of the Ethernet cable used to transmit data (this capability is sometimes referred to as "phantom power"). If a midspan PSE, such as the Cisco Catalyst Inline Power Patch Panel, is used, then the power is delivered on the unused pairs (pins 4, 5 and 7, 8).

The 802.3af standard also provides for five power classes to which a device might belong. The PSE vendor does not have to implement all these classes and can choose to support the maximum of 15.4W. This might require the facilities manager to invest significantly more resources in providing PoE if a vendor chooses not to use power management. It should also be remembered that even though a powered device might support IEEE 802.3af-2003 power classification, the PSE might not, and 15.4W delivery is the common denominator.

Cisco supports the capabilities described in the 802.3af standards as well as its own pre-standard implementation of PoE. Cisco's prestandard PoE implementation includes support for Cisco Intelligent Power Management (IPM). Cisco IPM, which is described later in this paper, provides better efficiency in the use of power in the Cisco Catalyst switch relative to other potential PSEs.

Cisco IPM is just one early example of Cisco's continuing innovation in PoE.

PoE+ (IEEE 802.3at)

The introduction of network devices that require more than 15.4W has compelled the IEEE to develop a new PoE standard that can deliver even more power than defined in the IEEE 802.3af standard. This new standard, IEEE 802.3at, is designed to deliver at least 30W per port of inline power.

Transmitting more than 15.4W of power per port poses some significant challenges. One such challenge lies in the physical characteristics of copper cabling, which can overheat or get damaged when transmitting power above certain thresholds. The IEEE is exploring different means of transmitting higher levels of power subject to these limitations.

Another challenge is backward compatibility with the IEEE 802.3af standard. This interoperability could be crucial to the successful adoption of 802.3at. Therefore, the IEEE is working to make sure that 802.3at-compliant PSEs are able to interoperate with 802.3af powered devices and the reverse. As a result of these and other implementation challenges, the IEEE 802.3at standard is not expected to be

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