If you???re still thinking of network capacity planning as speeds and feeds, then take a deep breath. Electrical power may well become the limiting factor in equipment purchases. Low wattage desktop devices like VoIP phones are on the market and are being deployed and powered using Power of Ethernet as defined by IEEE 802.3af. PoE is designed to run over regular CAT 3&5 cable and not interfere with other network devices. As more devices are powered over PoE, the load on the switch infrastructure will increase. Power meets networking.
Cisco recently announced a new line of switches, the Catalyst 3750-E and 3560-E switches, that can supply 15.4 watts per 48 switch ports. That works out to 740 watts for the entire switch of all the PoE ports are outputting power. Start stacking those in a rack, and heat dissipation could certainly become an issue. Of course, if your running VoIP phones, they need to be just as reliable as your current handsets. Power supplies do fail and if your PoE switch power supply gives up the ghost, you not only loose data, but voice as well. Cisco???s Redundant Power Supply 2300, is an external redundant power source that uses the same hot swappable power supplies as the Cat 3750-E???s. You can connect upto 6 fixed configuration Catalyst switches or ISR???s to the RPS, but the unit can only power 2 devices at a time, and that includes full PoE power.
Distributing power and data through the same cable makes sense. Fewer cables means lower costs, but as representatives from Cisco pointed out, the facilities people look at PoE as a power network that distributes data too. It???s a different world with power having to maintain cooling, power dissipation, and power management. 15.4 watts isn???t much power, but it is more than the 2.5 watts supplied over a USB cable. More power hungry devices like web cams, displays, and other stuff may increase demand. The IEEE 802.3at task force is in the process of defining a new specification to support 30 watts over CAT3&3 cable. If they achieve that goal and products roll, you can expect power requirements to expand in the wiring closet, of all places. That means increased draw and increased heat.
Not to panic, it???s still a few years out.