Planning for PoE Plus

Richard Chirgwin explains the new revision to power over Ethernet and offers advice on how to prepare your business to take advantage of its new features.

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Let me say from the start that power can be dull. However, with a significant revision to the Power-over-Ethernet standard now making its way into silicon – and therefore well on the way to reaching users – it's worth taking a look at what PoE-plus will mean to network administrators.

Power-over-Ethernet isn't that old a technology, but its usefulness gave it quick popularity. Wiring a new office is always costly, but no matter how many power points you decide your workstations will need, it seems there are never enough. With a laptop (many users want external screens when they're at the office), a powered telephone, and a phone charger, the minimum for any workstation is probably four outlets. Since re-cabling an existing office is disruptive as well as expensive, power-board and extension cord sales proliferate along with the devices that users demand.

So it's no surprise that PoE took off, if only because powering the LAN phone over the Ethernet cable helped eliminate at least one outlet.

And, of course, there's also the burgeoning number of devices that work best in inconvenient places, specifically wireless access points and LAN-connected security cameras. Both need power, and while their typical ceiling-level installation means cabling can be run through false ceilings, they're also a natural for PoE.

And here's the problem: in particular, two classes of devices lift the power requirements significantly. As 802.11n takes off, it will drive up power requirements beyond the original 15W of PoE; and similarly, higher-quality security cameras need more juice than static, low-to-medium resolution cameras.

So there are drivers for a higher-power PoE, but there are also downsides. Higher power also adds to the enterprise's electricity bill – and that doesn't play well in a world increasingly preoccupied with “green IT”.

You Want Watt?

Originally, PoE Plus had proposed a 30 Watt capability for the technology. However, as the IEEE subcommittees worked through their testing and modeling, it became clear that this target was too ambitious. The higher currents implicit in the original proposal caused too much heating in large cable bundles, and it was decided to limit the new specification to 25 Watts.

Unfortunately, for those keen on even higher power, because the limit is imposed by the cables rather than what's in the switch or the power supply, more “grunt” in any future PoE would depend not only on what's in the standard, but whether a new cabling standard can be designed to increase the current the Ethernet cables can carry.

The original PoE – now known as Type 1 PoE to distinguish it from PoE Plus – offered four device power classes: Class 0, which supports devices from 0.44 W to 12.95 W; Class 1, which has a maximum of 3.84 W; Class 2, which supports devices from 3.84 W to 6.49 W; and Class 3, for devices from 6.49 W to 12.95 W. PoE Plus adds Class 4, for devices between 12.95 W and 25.5 W. The device class only affects the maximum current the power supply can deliver; the voltage remains at a nominal 48 V.

The standard also has to take into account cable losses, so the PoE Plus nominal output voltage at the power supply is 53 V rather than PoE's 48 V. This is well within the scope of the connected devices.

The PoE power supply (which may be built into the switch or may be a separate device) determines at connection which class of device is connected (a process known, prosaically enough, as Classification), and periodically re-checks the cable to ensure that the device is still connected. If the device is disconnected, the power supply removes power from the port.

So as not to damage unpowered devices connected to a powered port, the Classification process uses a lower voltage (between around 14 V and 20 V), only lifting the port to full voltage once it has determined that a PoE-capable device is connected.

The challenge is in the cabling itself, because it's the cabling that sets PoE Plus's deployment limits. The biggest deployment change in PoE Type 2 (PoE Plus) is that it adds a maximum power dissipation of 5 kW within a cable bundle. PoE Type 1 had no deployment limit.

This won't change anybody's life in the SME sector, but companies with large installations will have to audit their cabling before embarking on an upgrade. If any of their bundles (for example, cable runs between floors, in false ceilings, or long runs servicing many workstations) would supply more than 5 kW worth of devices, the cabling layout will have to be re-designed to reduce the bundle size. If all the connected devices are running at the maximum 30 W, an enterprise would need more than 150 powered devices to hit the limit, so this is only a concern for the largest enterprise installations.

Cable plant designers and installers will also have to keep an eye on temperature, since both the maximum power and, more importantly, the maximum current per device are affected by the operating temperature. In an office environment, this may not be a problem: offices typically experience their warmest ambient temperature if air-conditioning is switched off (for example, over weekends), and at such times, many of the powered devices might also be shut down. However, if the cable plant has to cope with a warm environment, it will limit the amount of power the specification allows.

New deployments should focus on Cat 6 or Cat 7 cables, since these improve the heat-current trade-off.

Remember the UPS

With the ability to connect higher-power devices to the PoE infrastructure, and with more devices to choose from, PoE Plus will probably expand the total number of powered devices, and this raises yet another consideration: what impact will it have on your UPS infrastructure?

In the first instance, this will depend on your usage profile. Sensible UPS design and selection already takes into account the needs of different devices; in particular, you'll want to consider what kinds of devices are using the PoE infrastructure. In today's environment, PoE is generally limited to a few device classes – wireless access points, VoIP phones and security cameras. In the event of a power outage, most users want to maintain their telephones and security cameras as long as possible, but will sacrifice the computer network since the desktops will probably go dark anyway.

However, if both the phones and the security cameras are upgraded to take advantage of PoE Plus, the implementation will also demand an upgrade to the UPS and associated batteries.

PoE Management

As PoE Plus reaches the market, we can expect it to be supported by a raft of new devices, and to meet energy consumption concerns, vendors are already making their management of powered devices more sophisticated.

Even if you're not concerned with environmental issues, it makes sense to take advantage of power-down capabilities, since the electricity you save is money you won't spend.

For implementations of any scale, Intelligent power management is set to become the vendors' battleground in the PoE Plus world. So what sorts of capabilities will bring down your spend on electricity?

If you plan to use PoE Plus infrastructure to better manage your power usage, the following features are worth looking for:

  • More granular detection – Switches that offer highly-granular detection of devices, and can tailor the power supply accordingly, will use less power than less flexible power supplies;
  • Time-based power management – Look for management software that allows you the ability to set policies to shut down equipment out-of-hours, but is still flexible enough to allow users to power-up if they're working late.
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