Blog de AlterMundi

DIY PoE-enabled 100mbit router

Most CPEs and outdoor equipment support PoE out-of-the-box, and come with a PoE injector so that you can run a long length of UTP between the power source and the equipment.
Most SOHO routers don’t come with this feature, but with a few tools, minimal soldering skills, and 10 spare minutes, you can pull out this hack and obtain more flexibility than most CPE solutions offer.

Introduction

This procedure works for any 100mbit SOHO switch / router – it does NOT apply to Gigabit hardware! (since there are no “spare pairs” in gigabit ethernet).

Gigabit hardware is covered in another post.

Basically, you’ll connect blue and brown pairs, from 2 LAN ports and the WAN port, to the original 12v barrel plug socket.

PoE 100mbit router scheme

(+) = Pin 4 & 5 = blue pair
(-) = Pin 7 & 8 = brown pair

Then, you’ll have two basic use cases:

  • Power it up with the original barrel plug, and have 2 LAN and 1 WAN PoE-enabled ports, as if it was a PoE-enabled switch.
  • Send the power with an injector using a single, long, UTP cable, placing the router high on a rooftop, with one or two additional routers connected to the extra PoE-enabled LAN/WAN ports. Ubiquiti refers to this idea as “PoE passthrough”,

Video HOWTO

Footnotes:

Step 2: Not strictly necessary, you can use the cable without first removing the insulating plastic: the soldering iron will melt it and the solder will blend with the copper inside. Note that some people don’t like melting plastic with their soldering iron – bear this in mind if you borrowed the tool from someone else.

Step 3: After each solder point, I check the quality of it by pulling and moving the cable; if it wiggles (as can be seen on many cases) I correct it by reheating or adding a drop of solder. A weak solder junction might work at first, but fail randomly after weeks or even months, with strange reboots that are really hard to debug. Avoid headaches: assert every solder point is firm as metal, before doing the next one.

Step 4: There are safer ways to do this, but the only thing I had at hand was a plain knife. With the router in a vertical position (LAN ports face down on the table) I first pushed away the metal tab from the plastic casing, and then pushed down with significant force (but very carefully, to avoid damage to other components). A safer alternative is using a small flush cutter to cut the metal tab.

Step 5: Blue and brown pairs (4,5 / 7,8) are normally connected to ground by a resistor and condenser, to filter out currents coming down the line. Now, we will purposely inject voltage on those pairs, so we need to break that filtering circuit – removing the resistors is enough. If you forget to do so, they will simply overheat and blow out by themselves.

Step 6: First, PoE is applied to WAN, LAN1, LAN2 and original plug socket, and in all 4 cases the router turns on.
Then, 2 additional routers are cascaded in a “PoE passthrough” setup: power comes from a single ethernet cable connected to LAN1 of the center router, and then LAN2 and WAN give power to the other two routers. This simulates what happens in a tower, where you run a single, long, UTP cable up connected to a modded router, and then with short patchcords you add more equipment; might be other modded routers or even Ubiquiti hardware, since the PoE standard is the same.

Tagging PoE-enabled ports

Last modification date: Jan. 29, 2013, 9:34 p.m.
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