The original 802.3af PoE standard offered a fairly straightforward way to supply loads with 13 W or so of usable power delivered at 48 V dc. But IEEE 802.3at PoE Plus, which ups usable power to something over 50 W, introduces some wrinkles that designers and even IT managers must understand.

One catch is that designers can still supply power in a limited fashion in some existing Ethernet installations via a midspan bridge. But in that case, designers can’t implement power negotiations between a powered device (PD) and power source equipment (PSE). This implies dedicated PoE Plus ports and relatively high duty-cycle power supplies in midspans.

Something else to watch out for are PDs that dynamically negotiate power requirements with the PSE via their Ethernet connection. This requires more code in the PD microcontroller and a greater understanding of dynamic power requirements on the part of the engineer writing that code.

A potential pitfall for end users is that PDs can meet the standard by operating in a fall-back mode if there’s not enough power for full functionality. (For example, a video phone could fall back to operating voice-only, without a video display.) Alternatively, a PD application could meet the standard simply by signaling “insufficient power.” IT managers who bought a lot of “compliant” video phones could find themselves embarrassed by a system that didn’t work as expected if a “compliant” switch didn’t possess a sufficiently robust power supply.

HISTORY LESSON
To get comfortable with PoE Plus, it helps to understand its genesis and subsequent evolution. In the beginning, Cisco had a proprietary approach for powering Voice over Internet Protocol (VoIP) business phones that involved powering some pairs in the router with 48 V.

The rest of the industry saw that this was good and wished for an open standard, which became IEEE 802.3af. To be conservative, the IEEE subcommittee limited power to 15 W at the PSE, which was enough for the non-video VoIP phones that then dominated the market. They also expanded Cisco’s idea by allowing the “spare pairs” in an Ethernet cable to be powered by a midspan, making it possible to retrofit PoE to legacy Ethernet plant.

When PoE hit the streets, many potential vendors saw its advantages and jumped on the bandwagon. VoIP phones would no longer need power plugs, making them more like old-fashioned public-branch-exchange (PBX) phones. Wireless hotspots could be located anywhere someone could pull a CAT5 cable.

Supermarket shelves would twinkle with up-to-date price tags that would always match the prices in the cash register. And, PoE musical instruments, mixers, and recording equipment would displace the MIDI bus and revolutionize the music business.

Obviously, some of these goals were more realistic than others. In the three years since basic PoE was released, three killer applications have taken hold: VoIP phones, Wi-Fi hotspots, and security cameras. Within those applications, though, there immediately appeared a need for power beyond 13 W.

For example, there’s an anticipated demand for video conferencing using VoIP phones, and backlighting a video screen takes power. Simple short-range Wi-Fi is happy with 13 W, but WiMAX takes more power. And while fixed security cameras don’t require much power, once motors are added for panning, tilting, and zooming, power does become an issue.

But the manufacturers of Ethernet switches, concerned about over-specifying power supplies, pointed out that video phones and pan/zoom/tilt cameras don’t need full power all of the time. Most of the time, the phone is just sitting there. Even when there’s a call, video isn’t always necessary. Unless it’s a formal conference, most people would prefer to remain invisible to the other party. Similarly, those high-end security cameras only move when a guard touches a joystick. In other words, the requirement for higher power changes continuously.

The dynamic-power issue transformed the questions facing the IEEE 802.3at task force from simply “How much current can a bundle of CAT5 cables and their associated RJ45 connectors safely handle?” to “How can we create a protocol that allows PDs to dynamically negotiate for power with a PSE?”

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