Electronic Design

POE+: Which Road Leads To High Power Over Ethernet?

Almost as soon as the IEEE 802.3af Power-over-Ethernet standard was released, users started clamoring for more power. Thirteen watts is adequate for basic IP phones, but motorized cameras, multiradio access points, and devices with large color screens are seriously constrained. The fact that (under ideal conditions) typical CAT-5 cabling tolerates much higher current levels than the standard allows has led adventurous manufacturers to develop ad-hoc techniques to deliver extra power down the wire. In response, the IEEE has formed a new study group dubbed PoE+ that will develop a new standard to enable higher-power devices to coexist with 802.3af devices available today.

Although typical CAT-5 cabling includes four twisted pairs, the 802.3af standard permits only two pairs to carry current at once. One option is to allow additional current down the third and fourth pairs, doubling the available power. A second option is to raise the current limit, permitting more power down the same two pairs. A third obvious solution, raising the line voltage, is prevented by safety agency regulations. Each of these techniques has appeared in proprietary PoE systems. Yet each has drawbacks, complicating the choice between them.

Using all four pairs has the potential to deliver the most power to the powered device (PD) because this technique uses all of the conductors in the cable, minimizing the end-to-end resistance and the resulting power loss. Four-pair power also can be deployed in an interim arrangement by connecting an 802.3af midspan in series with an 802.3af power sourcing equipment (PSE) device. However, powering all four pairs roughly doubles the cost of the port controller circuitry, because a four-pair PSE must provide detection and fault control for each set of pairs. A four-pair PD, in turn, must limit the current draw from each pair set, and current balance must be maintained even when the incoming pair sets have significantly different voltages.

The two-pair option prevents the cost and current balance problems, requiring only minor modifications to the circuitry already in place in today's 802.3af designs. The cost for two-pair designs is the power lost when the cable gets long, because only half as many wires carry current compared to the four-pair case. This lost power heats the cable (especially when many cables are bundled together), and it adds to the stress on connector fingers and patch panel traces. Above 30 W, these factors can become significant. Higher current levels also aggravate ringing due to parasitic inductance during plug and unplug events, increasing the risk of damage to PD or PSE circuitry.

Any high-power scheme must remain backward-compatible with 802.3af systems. PoE+ PDs must fail gracefully when plugged into 802.3af PSEs, either reducing their capabilities to work within the limited available power or providing some indication to the user that they are plugged into the wrong type of PSE.

Market acceptance of this or any new standard requires stuff that just works or fails in an easily understandable way. If the new PoE+ devices are seen as twitchy or unreliable, they won't sell and the standard will not be widely adopted. Into this void will pour proprietary, non-interoperable solutions--exactly the scenario that PoE+ is meant to prevent.

To keep costs in check, most PoE+ PSEs will ship with undersized supplies. Left out of 802.3af, power management will need to be part of PoE+. In particular, the behavior of a port when the power supply is maxed out will need to be defined. Again, compliant systems must work like the end user expects. If a PD doesn't come on immediately, many users will assume it is broken and call tech support before they consider that the PSE might be oversubscribed.

Maximizing the power available from a powered Ethernet port will enable new PoE applications, and a standard that takes full advantage of the infrastructure's capabilities will last longer than one that leaves room for ad-hoc solutions. Adding power while preserving the ease of use and consistent behavior that made 802.3af a success is the challenge that the PoE+ study group must meet.

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TAGS: Components
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