Electronic Design

Sneak Preview: IEEE 802.3at Power Over Ethernet Plus Implementation Details

The IEEE 802.3at Power over Ethernet Plus (PoE-Plus) task force anticipates the release of a "usable" draft standard in August, with a firm specification to follow a year later. PoE Plus will boost the maximum power delivered to applications to something around 60 W from the basic 802.3at PoE standard's 13 W at the application.

Increasing the power was relatively simple, once the task force agreed that CAT5 cable and RJ45 connectors could handle more current (the approach some parties wanted) and/or that it would be acceptable to use all four twisted pairs in the CAT5 cable—the data pairs and the so-called "spare pairs"—to carry current (the approach some other parties wanted).

One challenge was updating PoE "classification." There are two kinds of power devices in a PoE system: power source equipment (PSE) and powered devices (PDs). Classification allows the PoE controller in a router or bridge to intelligently allocate its power to PDs.

In plain-vanilla 802.3af PoE, when a PSE chip (one for each powered CAT5 port) senses a PD on the port, it asserts a short pulse on the pairs it is using for power. The PD responds to the pulse by briefly drawing a certain amount of current. The amount of current represents the class of equipment the PD is supporting, which essentially boils down to how much power that equipment will be drawing. Basic PoE included classes 0, 1, 2, 3, and 4, and 4 was "reserved" The 802.3at task force determined that the best way to extend the classification system was a two-level method. All PSE chips will now send the first classification pulse, but PoE-Plus PSEs will also send a second pulse.

Standard PoE PDs will respond in the usual fashion to the first pulse. But when PoE-Plus PDs see the second pulse that tells them the PSE is PoE Plus-capable, they will signal that they are class 4 devices by drawing the class 4 current. The PSE and the PD then will enter into a data exchange with the PSE that will provide information about peak power needs, average power, duty-cycle, and other factors. The task force is now working on the data format for that exchange.

There is one catch. Two-layer classification will only work with new routers. It won't be possible to just stick a PoE-Plus midspan bridge into a legacy system and have all the benefits of PoE-Plus classification. On the other hand, that doesn't rule out higher PoE power on selected ports without classification. Several sources have announced single-port chips that will power a single higher-power PoE application such as a WiMAX access point or a pan/tilt/zoom security camera (see the figure).

The working number for current in the data pair of the spare pair is 720 mA. That might go up to 750 mA. The wires could carry more, but the task force has to deal with the potential heat buildup in bundles of 100 CAT5 cables 100 m long, with all the cables carrying current on all four pairs.

The RJ45 connectors are sufficiently robust. There's a possibility of arcing on disconnect, but the portions of the connector that might arc and develop pitting aren't the sections that are in contact when the connectors are fully mated. Meanwhile, data transformers are being beefed up because there was some possibility that the cores could saturate.

For more on the state of PoE applications and for more details of the IEEE 802.3at draft standard, see our Engineering Essentials article in our upcoming July 5 issue.

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