Knowledge Brings Fear

power to the roof

Power-over-Ethernet is something that most people doing WLAN links have allready played with. Since ethernet uses only the pins 1,2,3 and 6, you get four of the eight wires left inside the ethernet cable linking to your rooftop basestation for power supply. Usualy the basestation takes 12V. Depending on length of cable you need to put in more to compensate for the resistance of the cable. Usualy you end up with 16-20V going in and something like 12V coming out.

The drawbacks of power-over-ethernet are numerous. You need to be really carefull when crimping the cable. You need some sort of wall socket to hold the crimped setup.You need be carfull with the setting on the power supply.The original power supply will not do most of the times since the supplied voltage is to low for the length of the cable, so you need to buy a new, more powerfull and flexible power supply. If one wire in the ethernet cable breaks, you dont have any spare wires left since normaly you need all 4 remaining ones to get enought wire diameter for the power needed. Wire break does happen while laying the cable and from wind wear and tear on the cable. All in all, I ended up with putting two cables to the roof, one ethernet, one 230V power. This is of course not very concealed and a lot of work.

So today arrived a solution to the problem that I hope will solve the ethernet cable problem once and for all: Ethernet-over-Power. There is a industry standard called HomePlug that uses one of the currently most advanced modulation methots called Orthogonal Frequency Division Multiplexing (OFDM) to transmitt up to 14MBit (theortical) over plain old power lines in your home. I got ftp transfers of 720KByte/sec accross one floor, thats 5.76MBit real world transfer rate. A touch better then WLAN.

OFDM is the best way to show that Moores law translates directly into faster communications. The mathematics behind OFDM processing are rather heavy, I didnt understand the details completely on first try so I spare you the details. The HomePlug stuff uses 84 individual frequency bands that are spread out between 4 and 21 MHz. Continious measurement adapts the usage of the bands to the line noise from your electric appliances. Since some noises are rather spiky in nature, error-correction is also in heavy use.

Some testing in the company showed a remarkable reach of the coverage. The drawback of the ALLNET ALL1682 Ethernet Bridge Adapter I have is that you need a PC to do the initial setup. The setup consists basicaly in putting in a password (used by the DES line encryption as a seed for the key) and connecting the devices you want to use together into a network. Rather simple and straightforward, I actually needed longer then anticipated because I was looking for something complicated that I must have overlooked, not noticing that the network was allready up and running.

I am the first to admitt that it looks a bit counter-intuitive to plug a ethernet cable into something that looks like a power plug, but as expected it did not fry my computer.

So the setup for a rooftop basestation now is only to put power up there and use HomePlug to get the bits moving. Power cables tend to be much more solid then ethernet, dont suffer from broken wires, dont need to be crimped and - best of all - transport sufficient power to run also multiple devices on your roof. Great if your are running stuff in multiple directions. If you can use power allready aviable on the roof, your setup now looks like just an ordinary TV antenne distributor. You might be unlucky when the power avialbe on the roof is on a completele seperated phase, meaning that the HomePlug adapters cant see each other on the wire. The only way to find out is to try.

Tommorow I will put the stuff up on the roof, hopefully fixing my brooken home network link.

Posted by frank at July 7, 2003 09:24 PM | TrackBack