Friday, 31 December 2010

Wireless Link

Nearly a decade ago I built a wireless access network in my village, as BT had declared that the exchange wasn't viable for their ADSL broadband service.  As it turns out, there was sufficient interest in the wireless network that BT decided the exchange would be viable after all, so they put ADSL in and the wireless network never reached its full potential.

One remaining vestige of the old network is the access point attached to the farm house opposite mine, and the bridge that sits on the window sill in our front bedroom.  It is on this link that my neighbours still receive their Internet service - I share my broadband connection with them.

People have asked me how it works, so here's the detail.

The equipment used is cheap - really, really cheap.  They are entry-level consumer-grade D-Link WiFi access points.  The ones I have aren't made anymore but the nearest new equivalent is the DAP-1160 which can be had for as little as £20!

Here's the unit in my house, and you can make out the chimney stack of the farmhouse opposite where their one is located.

These units support "bridge" mode, which is what you'd normally use to run a link like this between two houses, however these were originally installed as a community access network so the far unit is actually running as a standard "access point" and the local unit is running as a "client".

Here we see a close-up of the access point, it's the same model D-Link housed in the white weatherproof box you can see in the eaves.  It is connected to a large external antenna (the white pole that sticks up from the chimney stack.)

This antenna is overkill for the simple point-to-point link that we are now using, but it made sense for the original design which would have seen a number of houses in the surrounding area connect using "client" boxes.

A "client" box isn't strictly necessary - you could connect to the access point using a wireless card in a PC, or a laptop's built-in wireless, if the signal was strong enough.

For a desktop PC, a USB type wireless adapter could be used with a USB extension lead to allow positioning of the adapter near a window.

One oddity of this arrangement is that the Internet comes in to the site where the "client" unit is located and the only client sits connected to the access point - but this is the distinction between the "layer 2" and "layer 3" of the network.  Wireless, like Ethernet LAN, is a "layer 2" protocol and Internet Protocol (IP) operates at "Layer 3" so neither particularly cares how the other works, as long as it does.

One other detail which is missing from the above is that the access point unit located in its weatherproof box is powered from the LAN cable, so there is only one lead run up the side of the farmhouse.  In the house, at the back of the PC, is a little "power injector" which mixes power with the data signal.  Very neat and not something you can do with fibre optics - take that you "fibre is the be-all and end-all" people :-)

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