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Tracking and Managing Power with 802.3af

When power is being supplied through 802.3af, it's also possible to monitor—and control—power consumption, according to 3Com's Doggart, which helps prevent connecting and powering unauthorized devices.

"We can track and meter power on a per-device or per-watt basis," says Doggart. "For example, in a net deploying VoIP phones, we can limit the power going to the phones to exactly what the phone needs to power up, so someone can't plug in a higher-power device like a camera or an Access Point."

As the use of PoE grows, predicts Doggart, it will also be increasingly important to monitor power consumption using SNMP. "If anything's failing, we can provide power budgeting; provide reports to the managers, like 'You need to provide more power, extra switches in this location.'"

3Com is working on integrating network management for Ethernet traffic and power, so managers can see a topological map what what's being powered, utilization levels, temperatures, and other data, according to Doggart.

As a company's Ethernet-powered devices climbs above the two dozen mark (often into the hundreds or more), management of the power budget becomes increasingly important.

"Being able to cycle power on and off is good, but pretty straightforward. As you build the PoE, you have to know how you can manage all these devices, and the available power resources and reserves available," states Steve Shalita, Senior Manager, Worldwide Product Marketing for LAN switching. "What happens when the LAN switch runs out of power or a device needs more power? The SysAdmin needs to think about the power budget overall, how to manage and support all these connected devices."

Cisco's solution to this, implemented in new versions of its Catalyst intelligent switching family include the optional 802.3af Power Classification feature, which lets the device tell the power sourcing equipment how much power it will need. So if a device needs only 9 watts of power, the power sourcing equipment won't assume and assign it the full default of 15.4 watts of power. Otherwise, either the switch will run out of available power sooner.

"Having the ability to identify the amount of power a device required is critical for any deployment of over 20 ports. Or else you'll be building often huge overcapacity," notes Shalita. "For large-scale deployments of hundreds or thousands of ports, you're talking about kilowatts and kilowatts of power."

Other key features, Shalita says, include being able to override power requests—turn power off on a per-port basis so devices won't consume power unexpectedly. This helps prevents rogue devices from being attached or legitimate devices from consuming power that wasn't planned for.

Similarly, the ability to regulate the total amount of power that can be delivered to a port for when power classification isn't available but you know the amount of power a device will need (for example, 5 watts) "because you want to get as many devices connected without overextending," explains Shalita.

"The biggest [PoE] issue that a sysadmin will have to think about is power capacity. Like with network bandwidth, the problem of oversubscription, under the assumption that it won't all be used at the same time," states Shalita. So you need this intelligent ability to manage your power deployment. Or else you'll build capacity you'll never need, which cascades with needing bigger power supplies, greater wall power, amperage, and so on. And over some threshold—about 1,500–2,000 watts, the next level power supply needs a 220 Amp connection, which costs more."

Shalita discusses one other critical element to know in deploying PoE: How does the LAN switch behave when power goes out? Or, what happens when I've oversubscribed my power, don't have the ability to restrict power, or otherwise the draw is greater than the available power? Will the switch go down? Reboot? Randomly close ports down? Switches that have intelligence and granular control are preferred, because the admin can define these things.

For their Catalyst Intelligent Switching Family, Cisco recently announced 802.3af support, including power prioritization, Shalita notes. "Our switches let you define prioritized groups of ports, so in a reduced power mode, the low-priority ports go first."

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