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Ethernet hub/switch powered from PoE

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December 13, 2009 10:14:52 PM

Does anyone know of an Ethernet hub or switch that can be powered from Power Over Ethernet? I have some networks to install over quite a large area that has no mains wiring. I suspect that the most cost effective solutiion will be to have one master hub powered from mains, and all the other hubs powered from the master via PoE. This will avoid the cost of having a substantial mains infrastructure installed by qualified electricians. I need to interface to up to 100 Ethernet devices distributed around the site, so there will be quite a few hubs. The devices themselves are DC powered, and have their own low voltage (probably either 18V or 48V - decision not made yet) DC power distribution system.

Another solution would be to power the hubs from the 18/48V DC, but this would presumably require special DC-to-DC power supplies for each hub, which is already sounding non-standard and expensive. Another issue is that the DC power bus is likely to be pretty noisy. I would therefore like to explore the PoE route first.

Thanks - Rowan
December 14, 2009 4:32:28 AM

Here is a possible solution

http://www.linksysbycisco.com/US/en/products/WAPPOE12

This limits you to the equipment listed on the page.

I was going to say I'd never heard of a switch being powered by another switch. There could be a POE device that splits all this from the start, but as Ethernet cables are thin, the only way I could see to keep it all going with data is by splitting. Looks like this can be done once and so you can't daisy chain this solution.
December 14, 2009 9:12:44 AM

Thanks for this suggestion. I think there are a number of "injector/spliter" solutions of this type, but it seems to me to result in a very messy solution. Lots of extra little boxes, cables, plugs, sockets, all of which need mounting, and can be a source of unreliability. And at the "master hub" (assumed 16 way), it seems to require 16 "injectors", each with its own mains socket, transformer and cable - surely a very inelegant solution. Much better would be that the master hub supplies the power to all its output ports, and the remote hubs are all powered directly from the downlink CAT5 socket without requiring a splitter. With some more Google searching I've found a couple of manufacturers who make such products: IMC Networks (http://www.imcnetworks.com/Products/product.cfm?family=...) and Level One (http://uk.level1.com/product_d.php?id=703).

You are right, of course, that you can't go on splitting the power indefinitely. Two levels may be enough for my application.

I'll continue to research this solution. The next question is of course cost effectiveness!

Rowan
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December 15, 2009 12:07:00 PM

The PoE feature is only good to the device. Each PoE switch will need to have dedicated power to it. Depending on the distance, you might be able to put 2 switches in, one on either end and branch off that way?

It doesn't cost that much to have additional power lines installed in another area. Actually, if you want to do it right, you should have another area that has power. On top of that with PoE you will want your switches running on a UPS. That's one thing very few people bother telling you, otherwise you could lose everything attached to your PoE switches.
December 15, 2009 5:11:23 PM

Thanks for your inputs.

>Each PoE switch will need to have dedicated power to it.

I'm thinking of having just one PSE switch at the centre, with a number of remote PD switches/hubs around the site, and connected to these my "leaf" devices. The "leaf" devices need their own power, but this is 18V DC and probably not suitable for supplying the hubs. This architecture seems to require only one 230V power point at the centre.

>It doesn't cost that much to have additional power lines installed in another area

Because of the nature of the site, the cost will be significant. Once I've identified candidate solutions in enough detail to cost them, I'll be doing a big cost comparison to see what comes out cheapest in the widest range of situations. I would like to have a solution that doesn't require additional 230V power points to compare against other solutions.

>you will want your switches running on a UPS

Not sure of the justification for this. If there's a power cut, the whole system is dead. If the switch itself has failed or been unplugged, then there's no data distribution, so the whole system is dead. It doesn't seem very likely that power will fail just to the switches (which seems to be the only situation in which a UPS would help) especially if plugs and sockets are protected against interference, overload trips are properly rated etc.

Thanks - Rowan
December 15, 2009 5:51:00 PM

You'll want to run on a UPS. It will regulate the power going across the PoE too. Losing power isn't a big deal for the most part... its the power spikes that can fry everything. That switch will get hit and send out a spike before it dies, potentially taking out any equipment attached to it.

Ask me how I know. :) 

Aside from that you could suffer cable damage if you have a lightning strike, etc. It doesn't have to be even a great battery, just something that can regulate the power. Its a small cost compared to replacing the equipment.
January 10, 2010 3:59:58 PM

I have a somewhat similar need. I want to set up a small network of laptops that will work even when power from the grid is off 50% of the time or more. (The setting is a hospital in rural Nepal; see www.nyayahealth.org for details.) I'm happy to use netbooks, so they remain perfectly functional even with the grid being off for a few hours. But the computers are too far apart for ad-hoc wireless networking (and more importantly are separated by concrete walls), and so it would be really nice to have an ethernet hub that could be powered whenever the laptops are running. The linked-to solution above appears to require a working mains outlet on at least one side of the connection, right? A UPS seems like the right idea, except I want it to work for several hours on the battery, not just 10-15 minutes. Finally, I would prefer an inexpensive solution if possible.
January 10, 2010 11:44:08 PM

Do you just need a single ethernet hub? In that case I don't think POE is particularly relevant. What you need is a rechargeable battery supply for the hub. It doesn't need to be a UPS with mains output, because most hubs can be supplied with low voltage DC. Something like the products shown here should do the job, if you choose a hub that will accept a DC input voltage that matches one of the voltages supplied by these units.

Hope this helps - Rowan
January 10, 2010 11:52:13 PM

Thanks! Those look perfect!
(And yes, one hub is enough for me. That's why it would have been so sad to need to run the generator for it.)
Anonymous
November 18, 2010 10:39:45 AM

I'm sort of in the same situation and I'm getting google eyes trying to find the solution.

I've got a PoE central switch and an office room with only 2 power outlets for two laptops.

I need to set up a switch in the room to connect the computers to the network.

The central switch supplies the PoE so all I need is a 4 port switch in the office get it's power over the ethernet so that I can connect the two computers to it.

You'd think it be easy but it seems impossible... anyone got some advice?
December 9, 2010 6:20:15 PM

There's a trick used for Video cameras that don't have POE. You split the POE power off before you feed the data to the camera and convert the POE to 12v DC. Many of the camera weatherproof enclosures have a little module in them to do this.

Here's a "box" version you could use for this: http://www.supercircuits.com/Video-Networking/Midspan-P... I suspect there are higher power units available now as well. You just have to find a POE box that matches your network switch input or visa-versa.

Dan Hoyer
DPH Network Services, Inc.
December 11, 2010 12:20:17 PM

Thanks Riser, never thought of it like that i will consider this next time i implement similar solutions.
February 6, 2012 11:37:11 PM

I'd go with a HP IntelliJack Switch.

Managed (web interface with support for VLANs, etc): NJ2000G
Unmanaged (standard "dumb" switch): NJ1000G
Anonymous
November 10, 2012 9:33:20 AM

This HP NJ2000G IntelliJack is a very good recommendation, easier to find, and in Belgium, less expensive.

Thank you!

Michel
November 12, 2012 4:47:06 PM

Panoptic Technology makes some PoE adapters with a variety of barrel connectors to power devices.
November 20, 2012 6:29:44 PM

007networkpro said:
Panoptic Technology makes some PoE adapters with a variety of barrel connectors to power devices.



Agreed. Ever since I saw the POE adapters from Panoptic, I've been trying to power everything I can on my network via POE (dummy switches, APs, IP phones, Cameras, etc.). They have a POE switch that supplies up to 60W per port, and I use it to power some LED Signs out in the front of my shop using one of the POE smart adapters. It works great, and if I ever need to reboot anything I can just log into the switch remotely to disable then re enable power to the port that it's plugged into.

The Panoptic switches do some pretty advanced VLAN management as well and they have a clean web interface. Overall, they make some really solid powerful devices.
January 16, 2013 1:32:42 PM

Hi,
Use the Netgear GS108T-200
!