Switching to VoIP

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Chapter 15: What Can Go Wrong?

I love the Cisco IP phones but they don’t accept 802.3af inline power. What

do I do?

There are a couple of things you could do. First, if your budget permits, the obvious

(though proprietary) answer is to use Cisco PoE switches to power the phones. Some

other switch makers, like Foundry Networks, also support Cisco’s proprietary PoE

standard. If you can’t afford to forklift your switches and instead want to power your

Cisco phones by way of power injectors, then you should consider Cisco PoE-com-

patible injectors like those made by PowerdSine (http://www.powerdsine.com). But if

you can’t do that either, do the next best thing: hack.

Hacking inline power will almost certainly void your IP phone’s war-

ranty, and probably your switch’s or power injector’s too. A short cir-

cuit could fry your switch and phone if you’re not careful. Proceed

with caution!

By flipping wires 4 and 7 and wires 5 and 8 on a standard UTP Ethernet patch cable,

you’ve basically made a “compatibility cable” that lets you plug Cisco IP phones into

an 802.3af source, as in Figure 15-2.

Make sure your switch lets you program, port by port, which ports get power and

which ones don’t. This is necessary because, in the Cisco PoE solution, Cisco IP

phone power requirements are autodetected, ...