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Capacity Planning Goes Electric

If you still think of network- capacity planning as speeds and feeds, take a deep breath. Electrical power may become the limiting factor in equipment purchases in the next couple of years.

Low-wattage desktop devices like VoIP phones are being powered using Power over Ethernet (PoE), as defined by IEEE 802.3af. PoE runs over regular CAT 3 and 5 cable without interfering with other network devices. Distributing power and data through the same cable makes sense; fewer cables means lower costs. But as more devices draw power over PoE, the load on the switch infrastructure will increase. Power meets networking.

Cisco Systems recently announced a new line of switches, the Catalyst 3750-E and 3560-E, that can supply 15.4 watts per 48 switch ports. That works out to 740 W for the entire switch if all the PoE ports are outputting power. Start stacking those in a rack, and heat dissipation becomes an issue. Of course, if you're running VoIP phones and your PoE switch power supply gives up the ghost, you lose data and voice.

And 15.4 W isn't that much power. The IEEE 802.3at task force is defining a new specification to support 30 W over CAT 3 and 5 cable. If and when products roll out, expect power requirements to expand in the wiring closet. That means increased draw and increased heat.

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