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800m Phone Call

I just made some test phone calls between two Mesh Potatoes 800m apart. Quite a nice result as both MPs were using just their little 12cm omni whip antennas.

One MP (a V1.1) was placed on a mast on the roof of my house. The other MP (a V1.2) was “portable” – strapped to the aluminium handle of our pool vacuum! This was a cool mast as aluminium is very light and one section slid inside the other for transport. I strapped this temporary mast to a playground at a local park. Watching me make phones calls into a 6m pool vacuum handle with wires hanging off it was highly puzzling for the little kids and other parents at the park! Total height of one antenna was 11m, the other 8m. I would guess that the antennas had line of sight (just) but not a free Fresnel zone. The terrain where I live is dead flat so their are no hills or valleys to take advantage of. Masts are needed to get above the houses and trees.

This test actually took a few days part time work to prepare. I had to put the MPs into boxes and fashion a bracket to clamp the MPs to the poles. Working out how to get them high enough at either end took some thought. I built a power over telephone line injector for one end – so I only needed one cable up to the MP on the roof for phone and power. This works OK but the ohmic losses of the little phone cable conductors means the MP01 needs a 20V power supply, it wouldn’t boot off 12V with a 30m phone cable. At one end I connected the MP FXS port to an IP04 to automatically answer the call, play a prompt, then run the “Echo” Asterisk application so I could hear my voice coming back to me. I was manning the park end of the link, and the home end was answered automatically.

First I tested the link in my back yard then at 800m. The call quality at 800m was usable but not perfect. When the other end answered I would miss a few seconds of the prompt being played by the IP04 – I imagined a flurry of WiFi rate-adaption over these few seconds as the algorithms tried to work out what bit rate was acceptable for the call. However by the time the Echo application started I could hear my own voice just fine. Occasionally I could hear the call breaking up (a packet getting repeated) but not too often. I was using the ulaw codec. This result highlights the needs for some work on improving call quality on marginal links, the sort of ideas Alex has been talking about. It also shows the need for good line of sight links when setting up a Village Telco network.

I eventually killed the link by pressing a DTMF key. This started both ends sending continuous DTMFs in some sort of loop and the IP04 wouldn’t hang up! So time to climb off the playground equipment and head home.

A “iwlist ath0 scan” on the roof-mounted MP showed 22 signals, so it’s likely there was some interference. A bit of height sure does help with Wifi links.

This was a nice test to back up our other results that show the Wifi & RF side of the MP01 is working OK. One of the MPs was a V1.1 which used the prototype MP PCB design. Although the PCB design has been refined on the V1.2 boards, I suspect the frequency offset problem (along with calibration) was probably the biggest issue with the earlier MP boards – it caused packet error rate problems at the low bit rates which get selected for long distance links.

Sorry no pictures, my wife is out of town with our digital camera. I have some snaps on my phone but can’t work out how to upload them without a teenager nearby (also out of town). Maybe later.

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