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VoIP power calculator

For the past several years, one of the lurking gotchas to implementing VoIP was supplying enough power. In this tip, Tom Lancaster explores why, and how, to create a power budget prior to your VoIP implementation.

For the past several years, one of the lurking gotchas to implementing VoIP was supplying enough power. The problem was most noticeable in densely populated user areas where large, chassis-based switches -- which can offer hundreds of Ethernet ports for users -- were used. These switches typically run off two or three power supplies. Of course, the expectation is that at least one of these supplies power for redundancy, so that means we're really asking a single supply to provide enough juice for the switch itself and all its line cards and all the phones and other Power over Ethernet (PoE) devices that are attached to it, such as WAPs.

Reality has been that few of these power supplies had sufficient output to run a full compliment of phones at the specified ratings. (The different PoE specs have a variety of maximum outputs.) Fortunately, few have run into problems because few organizations fill up these chassis with phones on every port, and phones don't necessarily pull the maximum power all the time, so this issue, while real, was seldom discussed.

Recently, the situation has improved, as power supplies with obscenely high output are now available so that it may now be possible to get all the power you need (Important safety tip: Do not touch your tongue to a 4200W power supply).

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However, you should still do a power budget whenever you are planning a VoIP deployment, just like you do a budget for your voice trunks and QoS bandwidth and delay. To start, check to see if your switch vendor has a power calculator. For example, Cisco's very handy tool can be found here.

To do a power budget without access to a handy calculator, you would of course use the formula "Watts = Volts * Amps." Aggregate the power consumption for all the devices you intend to plug into the switch, and then compare that to the ratings on your power supplies. Don't forget to include the power required by the switch itself. But in any case, you should check with the manufacturer, because there can be a lot of little gotchas.

Since these new supplies can be expensive, you'll want determine which one you need before you buy. If you're rolling out VoIP on existing boxes, you should calculate your power requirements to determine if you need to upgrade your supply before you do your deployment.

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