POE: Data Center Friend or Foe?

Power over Ethernet can reduce total cost of ownership, but there's plenty of hurdles to cross

October 13, 2004

3 Min Read
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Power over Ethernet (POE) is a seemingly innocuous innovation that will have a huge impact on the way we design, purchase, install, and manage our networking infrastructure. In a nutshell, POE allows us to power small network-ready devices without a separate power supply. Instead, an injector” (which is either external or built into a switch) provides power over unused wires in the Ethernet twisted-pair cable.

POE has a lot going for it, including accessibility where there are no nearby AC outlets. It can also eliminate many of those unsightly cords and transformers as well as the need for having an uninterruptible power supply (UPS) at the injector source (one UPS on a POE-ready switch can cover dozens of devices). With the advent of the 802.3af standard, POE is taking off like a rocket.

There are a huge number of components available for POE, from chips to injectors to RJ-45 jacks with integrated Web servers. And we’ve only seen the tip of the iceberg for plug-and-play products. A sampling of POE-ready devices include VOIP phones, 802.11 access points and bridges, Web cams, and card access systems. There is even a clock that synchronizes to the National Institute of Standards and Technology (NIST) standard and provides local-time services via Telnet and a low-power, XP-based, flat-panel computer.

In our quest to lower total cost of ownership (TCO) and reduce the cost of our physical infrastructure, POE seems ideal. But there are some potential pitfalls to be aware of.

For one, a proliferation of Ethernet "micro-nodes" will require more switch ports to support them and perhaps upgrades to support POE. Historically, we’ve gone from shared cable (the original Ethernet), to hubs, to switches and hubs, to a switch port for every user. Now imagine a ten-to-one ratio of switch ports-to-users in the not-so-distant future.Working out the power and cooling requirements for our wiring closets and data centers will become even more critical as we expand into POE. This comes at a time when we are trying to reduce investments in data-center (and wiring-closet) power, not to mention heating, ventilation, and air conditioning. Even though POE is low voltage and relatively low power, having a large number of POE ports requires careful consideration of the number of POE devices drawing power, how much each device draws, and, ultimately, how much power our switches can supply.

We will also need to manage a much larger number of IP addresses and devices. These micro-nodes are intelligent IP devices just like the big stuff. Soon, the combined number of access points, Web/security cameras, loud speakers/PA systems, clocks, and other devices will eclipse the number of users. Do such nodes have their own virtual local-area network (VLAN)? And what about IP space? Who’s responsible for security and hacking? Or identifying rogue nodes? How do we identify problems at the POE electrical layer? Or overdrawn switches?

Finally, watch out if you are attempting to wire your own POE injector. A likely scenario is inadvertently wiring a jack or patch cable incorrectly. And remember -- providing the wrong power at the wrong time could seriously damage a device.

— Scott Haugdahl, CTO, WildPackets Inc.

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