Everything you need to know about Power over Ethernet

Ian Yates interviews a Power over Ethernet expert to explain how it works, what it does and how you can retrofit your network to take advantage of it!

Power over Ethernet (PoE) is a feature that is making its way into more and more networking devices, as users look to take advantage of its ability to power devices like IP phones, security cameras or Wi-Fi access points without the need for new electricity cabling.

But as Ian Yates discovers in this interview with Fotios Kotsiopoulos, Technical consultant at HP ProCurve Networking, the technology may not yet have caught on in the mainstream.

Kotsiopoulos: I believe that cost benefits provided by PoE are becoming more apparent these days. People are figuring out how to best utilise the power provided by these switches.

IY: Now that's something that people should realise. It is just a low DC voltage. I mean, obviously it wouldn't be 240 volts. You're not going to get zapped if you pick up your internet cable.

FK: No. That's right. I mean - and you can't just plug any device in there and expect your switch to power it. It has to be a device that complies to specific standard for power over Ethernet. It's really handy to have devices like wireless access points or IP handsets that can actually be powered via your switch.

IY: Otherwise what? More wall warts on your desk and, if it's in the ceiling for Wi-Fi, it's just plain tricky.

FK: Yeah, exactly. It becomes really handy for wireless access point deployment. Because most of the times, you need to place an access point to a certain location, the ceiling, when there's no AC power and having to get an electrician to climb up a ladder, install power points, isn't really a nice thing to do. Especially if you want to move that access point a couple of metres close by.

IY: Yeah. True. Of course, desks, as you say. IP phones getting more common. Instead of having to power them, they can just get their power from the switch.

FK: Yeah, absolutely. The most common deployment of PoE has been with IP handsets these days.

IY: Any limitations? I mean, there's probably a limit on how many phones you can plug into a switch, I guess.

FK: Yeah. It's quite important actually to do your homework when you work on a large PoE project with IP handsets, because most switches can provide a maximum of 15.4 watts through their switch ports. The device that you plug may utilise all that power or it might only require half of that power. So you have to actually do a quick calculation as to how many devices you're going to plug in and how many the switch can actually power at any point in time.

IY: Okay. Well you should have - people are used to doing that for working out their power in the data centre. But I guess now they've got to do to work out the power that can be drawn from their switch.

FK: Yeah. Absolutely. So in the past, you only had to worry about the number of data ports required and how much power your switch needed to forward [traffic] across the cable. But now you need to worry about what size UPS you need to put in to offer redundancy enough time for your PoE power.

IY: Yes indeed. Because these days, you can't let phones drop dead, can you? In the old days, your phone didn't drop dead, because the PBX usually kept it going.

FK: Yeah, exactly. Everyone's used to having access to the telephone when the power goes down. So if we want to provide that same type of functionality then, at the back end in the data centre, we've got to make sure that we've got redundancy for a power to our switches, but also redundancy for the PoE aspect as well.

IY: Yes indeed. Even if it's just so you can call the electrician, you still need that phone!

FK: Exactly. Failing that, you might have to use a mobile phone.

IY: It's not like we don't have any of those!

FK: That's right!

IY: Now what about retro-fitting? What if I got a lovely network and I didn't get any PoE? What do I do?

FK: Well you still have the option of injecting power into the cables.

There are devices which are called mid-span PoE devices. So you can add PoE at a later point in time. But it's not really an ideal solution. Most vendors these days including ProCurve network are integrating PoE into the switch port. It avoids having a cabling mess at your data centre.

IY: Right. Now if I've already bought some ProCurve and I forgot to get that, is that an option I can bolt in as well?

FK: You can. You can actually bolt in mid-span PoE into any switch port with a 100 megabit or gigabit. But if you do the sums, you'll probably find that you may actually be better off buying a new switch with integrated PoE.

IY: Okay. Send those old ones without it to somewhere that doesn't need power.

FK: Yeah, exactly. You've got to be careful as well, because depending on how old those mid-span PoE devices are, they may actually support pre-standard PoE power. So these days you want it to be an IEEE 802.3AS compliant device.

IY: Right. Okay. So that's pretty standard. Everyone does that now?

FK: Absolutely. Everyone does that now.

IY: Okay. So no risk there, as long as you look for that sticker.

FK: Exactly.

IY: Okay. Well thanks for bringing us up to speed with power over Ethernet. If you've not been using it, you should look at it. I mean, if some people have seen it, but they don't know what it means. But it just means you don't have to put 240 volt plug [warts] out there on those devices.

FK: That's right. It's the first time we've actually got a universal standard for power.

IY: Yeah. Wouldn't it be so good if all the other gadgets that use wall warts can all use the same one? Why can't they all just have a 12 volt supply?

FK: Who knows down the track what happens with the evolution of PoE? Let's see what the future holds.

IY: Well maybe perhaps, perhaps, perhaps, if we wait long enough, the consumer gear that you get in your home, your little home routers, might be able to do it, to power up these gadgets like IP phones for home.

FK: Definitely seems to be a possibility.

IY: Yes. I know these wall-warts are a major user of power. As people say, even when things are turned off, they sit there, drawing power. Everyone wants to be a bit greener these days.

FK: That's right. A lot of companies in the enterprise are actually considering this as far as their green IT path is concerned. So the use of PoE is quite important to them.

IY: Of course it's much more efficient to generate it centrally on that switch instead of having lots and lots of little gadgets.

FK: You can actually control the delivery of power through a centralised point on your network. So a network administrator could pick and choose which devices he can enable or disable power to.

IY: Alright. All things to think about and things to look forward to. They're already existing. Make sure your next switch has that option I would suggest.

FK: Absolutely. Definitely look into it.

This interview comes from A Series of Tubes, ITradio.com.au's weekly networking podcast. You can find the podcast, plus the Risky Business security podcast, in our live content directory.


Read more on Data centre networking