One of the early mistakes we made with the Mesh Potato was not placing sufficient emphasis early on, on getting type approval for the Mesh Potato and indeed focusing on both European and U.S. type approval.  What is type approval you ask?  Type approval is the magic glue that makes unlicensed spectrum work.  Many people take the term unlicensed to mean unregulated but nothing could be further from the the truth.  Unlicensed spectrum succeeds because the devices that are permitted to use unlicensed spectrum are carefully regulated to ensure that they conform to strict standards in terms of power output and many other technical specifications that ensure that unlicensed devices “play nicely” with each other.

I am happy to say that this issued has finally been addressed in full and Mesh Potatoes now enjoy full compliance with both the standards of the Federal Communications Commission (FCC) of the United States and the European Union’s CE standard.  These are the two most common international standards for compliance and should ensure that the Mesh Potato can conform to almost any regulatory regime.

If you would like to get copies of the certification in order to apply for local type approval in your country, please get in contact with us via this website.

We headed for Tankwa Town with a full load and empty wallet and flashed & configured the Mesh Potatoes (MPs) in the desert using the good ol’ 192.168 range. The booths were different to last year: they had production MPs, new sleeves and no lighting. I set each node with a different Virtual Access Point (VAP) SSID based on the phone number because I found that using an identical VAP SSID caused SSH to die. This will be ideal in the future, when each MP owner has a WPA key on their router and both cabled and VAP connections are linked to the same account. So any device that wants to roam must either switch between VAPs or run batman itself.

Steve came up for 2 days to help set up and bring some much-needed supplies. I discovered that an Olmeca tequila bottle is not as sturdy as it looks. It broke in the trailer and softened up the Tetrapak milk until most of the cartons ruptured too. So a Nano, a few phones and other gear were swamped. Thanks for the rescue, Steve.

We set up the booths easily this year – far less wiring and better ground pegs. We put phones in the organiser’s caravan, medics and the gate, with a Nano to cover the 5km distance to the gate. However this year they had moved the gate to the end of the airstrip, so it was only 1km away.

I had brought a netbook with broken screen to act as gateway, redirecting all port 80 traffic to a phpbb3 bulletin board with the appropriate DirtyBoard2.0 skin – see http://swug.za.net/phpbb/. I also redirected port 53 to dnsmasq that spoofed the DNS of  Top Level Domains (TLDs) to the gateway IP too. These are the kinds of things that naughty people do, but I think it works well for our application. I set each router’s dns as 8.8.8.8 and SIP registration to villagetelco.org. When a mesh is connected to the Internet, DNS & SIP will work fine (once VT has a SIP server). If there is no Internet access, DNS/SIP/HTTP can be fielded by an offline gateway server. It does look a bit odd, but it works.

The spoof gateway was running fine for devices connected to the LAN but was not accessible from the WAN. In fact, no other device connected to a different MP could be seen except for the MP’s LAN address itself. This suggested that there was an issue with the dummy gateway setting on the LAN, required to allow Asterisk to run as per David & Elektra’s discussion

I was 4 days into the trip by then and it was time to party, so I left the gateway for another day – and Internet access. It was most helpful though in checking the booths. Instead of plugging in a netbook, one could just pick a nearby cosy camp, settle down and connect wirelessly. DHCP, LAN bridging, batman routing, Asterisk were all working fine.

Solar operation

This year I invested in a 90W monocrystalline solar panel and regulator for the camp – 90 watts when the sun is shining for the next 25-50 years. It will be part of my home solar office in a few weeks. I had 4 105AH tractor batteries from last year and connected the gateway netbook and the fixed one (20-40W each), plus about 20W of lighting and didn’t have a power problem for the whole week.

We used the 10W flexible solar panels and 12AH batteries on the booths as with last year, but this time there was no lighting in the booths. 10W kept the booth MPs up throughout the week, but the batteries drained progressively. I put this down to the number of phone calls being made – nearly all the time, day & night. One booth’s phone was left off-hook providing dialtone for a few hours. I disconnected it for a charge back at camp and it was sorted. Perhaps a dialtone hangup can be written into Asterisk for such events on battery-powered phones.

I also installed a phone in a friend’s art car, connected to the cigarette lighter adapter. It was great to be able to phone the art car from anywhere and ask for a pick-up. However next year, I’ll bring along some battery clips so that the phone stays on when the car is off – see whether the MP can “survive the crank”.

Conclusion

This year was tougher with just Steve and I setting up, but easier because of the work we could build on from last year. The most promising result from this year were that the phones worked and worked hard. It gave me some time to work towards the 4093-node, no-NAT VAP firmware I long for. Although the VAP was not feeding into the gateway, every person with a wifi-enabled laptop or phone saw hotspots – so hopefully they’ll be better prepared next time too. Comments from participants were different this year too. Last year the phones were a novelty; this year people were more interested in and supportive of the technology behind the phones.

This year I met some important people for next year. Adriaan Wessels is the technical chap on the organising committee. For next year, we’ll be able to plan carefully regarding gate link and possibly organiser car links. Organiser confidence in the network will mean that they will be able to pre-announce a “Tankwanet” and its services so that participants can be better prepared to take advantage. There are a lot of IT geeks at Afrikaburn. Perhaps some of them could be persuaded to offer services on the network.

Rod Bracher runs the Tankwa Town post office, Burning Mail. They have some old rotary-dialling phones that they connect with carrier but without dialling. I tried to coordinate with him to hook his phones up to the network, but ran out of time. We had a chat about doing so at this burn, so pulse-dialling is definitely on for next year.

The streaming services company antfarm.co.za were at AfrikaBurn this year. I couldn’t spot their VSAT dish, but made brief contact. They were streaming the burns live, which led me to figure a rule at AfrikaBurn: the isolation is an important part of the event, but the isolation need only exist one way – incoming. So theoretically, participants should be able to send SMSs, update their twitter accounts and post on a forum on the Internet – as long as they don’t see any response from outside the burn. Antfarm’s ISP only allows Internet connection on port 80, so a little port jiggling and a helpful Internet server should squeeze whatever we need through, except for multiport SIP.

Lastly and simply, Isigidimi was at AB2011. The continuity will help us attract more participants next year, and perhaps more of the VT-dev community. You can set up an Asterisk service, website, jabberd, tinker with pulse-dialling, or just sip on the Kool Aid and be inspired ;^)

Here are some more images from Afrikaburn 2011

You can see the final design of the Mesh Potato at the right.  Those of you are familiar with the Ubiquiti Nanostation II will know that we borrowed wholesale from their excellent enclosure design.  They say imitation is the sincerest form of flattery.  In this case, it is certainly true.  We love Ubiquiti gear.

What we’ve borrowed from Ubiquiti is the sealed-unit design which makes for a sturdy weather-proof enclosure.  What’s a little bit different is the back mounting which has a slip-on piece that can be screwed to a wall.  Without the screw-on piece there is a similar mounting to the Nanostation for mounting on a pole.

The best news of all is that Mesh Potatoes are now available for purchase by anyone.  Simply click on the Products link above to buy them from our webstore.  The cost of each Mesh Potato is USD 119 but for the next two weeks we’re making them available at the wholesale price of USD 89 per unit.  In order get that price, enter “afinemesh” as a coupon code on the Checkout page after you have selected the number of Mesh Potatoes that you’d like.  Shipping is calculated via DHL from Atcom in China.  Please contact us if you’d like to make other shipping arrangements.  Orders placed now will ship by the end of September.

Finally, I would just like to express ongoing amazement and appreciation of the many, many people who have contributed to making the Mesh Potato a reality in small ways and in big earth-shaking ways.  You know who you are.  You all seriously rock.  Open Software, Open Hardware refreshes the parts that other intellectual property regimes can’t reach.  Thank you.

However, constraint can be both a barrier and an enabler of innovation and this is certainly true of the Mesh Potato.  As it turns out, we are only just beginning to discover the potential of the voice interface to the Mesh Potato to improve user experience.  Here is a great example.

The first thing you want to do when you power up a Mesh Potato is make sure it has a good connection to its peers.  This is easy if you have a GUI or even a command line but what if you just have a telephone handset?  David Rowe developed a script for his deployment of the Dili Village Telco that continuously polled the quality of the mesh link on the MP.  Elektra has taken that one further and integrated the script into a voice interface that give continuous audio feedback on link quality making it easy to tune your WiFi connection with a simple handset.  Here’s a brief  video of me testing out this feature.

Tuning Mesh Potato WiFi Performance with a Voice User Interface from Steve Song on Vimeo.

Mesh-Potato smoke testing with 230 Volt AC from Elektra Berlin on Vimeo.

The list of tests included a reverse DC voltage test from a unfused 36 Volt source (consisting of three powerful 12 V lead acid batteries in series), excessive DC voltage tests and finally a really scary test involving 230 Volt AC (330 Volt peak) from mains. Please don’t try this at home. Even if the Mesh-Potato survives, there is a 50% chance that you might have mains potential on ground of all components connected to the MP. Also don’t try this with your alpha or beta series Mesh-Potatos, because the previous over-voltage protection circuit is not up to that challenge.

Technically inexperienced people can easily make mistakes, so our idea was to design the Mesh-Potato as robust as possible. In 2005 I was helping to set up a large scale WLAN network in the Sylhet area in Bangladesh. The network consisted of high towers (up to 100 feet tall) and strong directional antennas, interconnecting towns and a school with wireless long shots (up to 32km). One of the trainees damaged a important wireless relay on a tower by taking the open ends of a 12 Volt cable and plugging it straight into the mains socket. Of course the equipment (a Mesh-Cube from 4G Systems) subsequently looked like a lightning strike had hit it, which was actually what I supposed first. However there had been no thunderstorm in the night before. It took me a while to find the reason. The trainee either hadn’t realized what he had done, or he didn’t want to admit it. He watched me trying to find the problem without saying anything. It is common practice in Bangladesh to plug cables into sockets without plugs. The quality of sockets and plugs is miserable, so loose contacts are the rule, not the exception. Now a important relay was down and it was hard to get a replacement. The problem wasn’t so much the financial loss. Shipping and particularly customs can take weeks in Bangladesh. So during the first Villagetelco workshop I suggested to design the MP as robust as possible.

We want to use etched PCB Wifi antennas for the Mesh Potato. However we have heard that some companies have had problems with PCB antennas, such as variable results in production. So before committing to PCB antenna we wanted to do some more tests.

Joel is a local hacker here in Adelaide who happens to live at the top of a 4 story apartment block in a beach side suburb called Henley Beach. This gives him good line of sight to points on the ground several hundred meters away. Much easier than testing at my place which is on dead flat terrain and requires masts for any Wifi range testing.

Before heading out to Joel’s place I tried some tests in my backyard. I set up two MP01s about 10m apart. First I connected conventional sleeve dipole (rubber ducky) antennas and measured the signal strength. I then connected a couple of PCB antennas and repeated. To monitor signal levels I wrote a simple script to dump the Received Signal Strength Indication (RSSI) levels (in dBm) from MadWifi:

#!/bin/sh

while [ 1 ]
do
wlanconfig ath0 list | grep 56:ac:90 | awk ‘{ print $6 }’
sleep 1
done

The grep filters out the last few MAC digits of the MP we are interested in, otherwise you get RSSI measurements of all nearby Wifi devices.

However the results were inconclusive and after an hour I became frustrated:

• On a good test both MPs would receive about -30dBm, however if I moved a MP 20mm a level could drop to -44dBm. Lots of multipath in my back yard!
• One MP was consistently around -30dBm, whereas the other would move between -30 and -48dBm. Maybe they had different diversity antenna settings. Or maybe the RSSI measurements can’t be trusted.

I had much better (and less frustrating) results using the spectrum analyser to measure signal strengths, as explained in the previous post on PCB Wifi antennas.

In the end I figured it was sufficient to test from a system level rather than attempt more measurements of antenna performance and signal strength. The $64 dollar question is can we make phone calls over a reasonable distance with these antennas?

On the Beach

We placed one MP01 on Joel’s balcony. I then walked down to the end of Henley Jetty (Joel has previously managed to pick up his home Wifi there). There is (just) a line of site between the two points – through a gap in a couple of buildings. The distance was 375m. We used two calibrated V1.2 MP01s (same design as the Beta units). An “iwlist ath0 scan” showed 9 other Wifi networks in operation.

Google Map of Range Test - 375m

One MP01 on Joel's Balcony

I tried a bunch of antenna combinations, starting with regular sleeve dipole (rubber ducky) antennas then moving to PCB antennas first at one end, then both ends of the link. From our previous tests both the sleeve dipole and the PCB monopole antennas have an estimated gain of 2dBi.

We had a total of 4 PCB antenna samples (a mixture of the 17mm and 20mm monopoles) as we wanted to get a feel for performance spread over multiple antennas of the same design. Here are the results:

The final test was just to make sure we weren’t kidding ourselves.

MP01 on the end of Henley Jetty

At one stage a fisherman assisted in my propagation experiments by standing right in my line of site and lowering a metal net as I talked to Joel!

For each call I tried moving the MP01 (with PCB antenna attached) around. For example upside down, side to side, rotated it. No break up of signal, good audio in both directions. Only problem was wind noise. Joel suggested we add digital noise suppression but I figure there isn’t much wind noise inside village homes!

Here is the view of the far end from either side of the link. The arrow shows the location of the other end.

View of Jetty from Joel's Balcony

View of Joel's Balcony from Jetty That's him with the brown eyes.

The first Beta Mesh Potato

These early Betas are running revision 203 firmware, which has Afrimesh as the primary GUI with Luci available for “advanced” configuration options. Thanks Elektra and Antoine for your fine work on the Mesh Potato GUI.

Here is Joel’s experience – our #1 Beta MP01 tester! Joel is pretty geeky (he has contributed for several years to the OLPC project) but has never used a Mesh Potato before. I hope this information will be useful for other Beta testers who should be getting their Mesh Potatoes over the next few weeks. I stood back through most of Joel’s beta experience – I wanted to see how other people approach the Mesh Potato.

How Joel Configured His Mesh Potato

1. The default Ethernet IP is 192.168.1.20. Joel plugged a cross over Ethernet cable into the MP01, although a regular cable should be OK.
2. He then logged in via telnet, and set the password from the command line using the “passwd” command. Setting the password activated ssh and disabled telnet. This password also becomes the web admin password.
3. Joel then pointed his browser at 192.168.1.20 and the Web GUI came up.
4. He changed the 10.130.1.20 “IP Address” to a unique IP on the mesh. Joel selected 10.130.1.123. This is the only change you need to start making phone calls between Mesh Potatoes. This IP address becomes your phone number, for example dialing 123 on the mesh will make Joel’s phone ring. Note this only changes the mesh Wifi IP – the Ethernet IP is still 192.168.1.20. The Ethernet IP can be changed via the Luci Network Menu.
   

   Setting the Wifi IP on the Mesh Potato

   Note the settings are applied automatically – there is no “save” button, although I understand that this will be present on later firmware as out of habit we all look for a save button! To apply the settings (i.e. change the actual Wifi settings) we rebooted (power cycled) our MP01.

   We also changed the Wifi channel and BSSID to match the mesh settings of a couple of earlier Potatoes. This is not really necessary, the default Wifi channel and BSSID are probably OK for you.

That’s it – the main step is just change the mesh Wifi IP to something other than 10.130.20. Then reboot and you can start making calls between Mesh potatoes.

Easier Configuration

A slightly easier way is:

1. Point your web browser at 192.168.1.20, login with user/password root/admin.
2. Simply change the Wifi IP from 10.130.1.x to 10.130.1.yourchoice.
3. Power cycle your potato.

Configuration without a Web browser

1. Power up your Potato and connect a phone, after about 1 minute you will get dial tone.
2. Dial CONF (2663) and you will hear HAL 9000 talking to you!
3. Enter the new Wifi IP, e.g. 10*130*1*123

Joel asked a good question – what do all the LEDs mean? They are not labeled although from memory it’s power/Ethernet/Wifi activity etc. I asked Edwin@Atcom about this and the reason was the vinyl labels hadn’t arrived when the first few betas shipped. The rest of the Betas will have these labels for the LEDs.

Jeff designed three types of antennas which I laid out on PCB and had fabricated locally. The three designs were a dipole, a monopole, and a biquad (single loop). We made three versions of each PCB antenna with slightly different dimensions.

I also made a some wire antennas, a monopole, a biquad (dual loop), and a quad (single loop).

Our PCB and wire antennas

PCB biquad design

PCB Monopole Design

Checking the Antenna Impedance

When the PCBs came back the first step was to check the impedance of each antenna. We want roughly 50 ohms impedance to ensure the maximum amount of power is transferred from the Mesh Potato transmitter to the antenna.

A Standing Wave Ratio (SWR) bridge can be used to measure the SWR. I used a version of the design by Erwin Gijzen, a radio Ham and Wifi experimenter. I constructed the SWR head, and measured the DC voltage from the bridge using a multimeter. The bridge compares the impedance of the antennas to a known 50 ohms impedance. If they are equal then the DC output from the bridge should be 0V. Various degrees of mis-match give different output voltages.

SWR head, microwave PCB made with a Dremel tool

I constructed the bridge from double sided PCB and cut the microstrip (3mm wide on 1.6mm thick FR4) with a Dremel tool to save time. When tested it gave sensible results once I fitted a decent microwave detector diode. Unlike Erwin I couldn’t null it down to 0V with a reference 50 ohm load but it did give indicative readings that enabled me to compare our antennas to reference antennas and determine if they had a reasonable match to 50 ohms.

Here are some results:

The 17mm and 20mm monopoles look good, close to the reference 50 ohm load and commercial off the shelf router antennas (which have sleeve dipole construction internally). The wire antennas also look good. The PCB dipole and PCB biquads don’t look so great.

I tuned the wire monopole to a low SWR by snipping off bits of wire, 0.5mm at a time. I started with a length of 31mm (free space quarter wavelength at 2.4 GHz) but found a good SWR at 26mm. This is probably due to the dielectric constant of the insulation on the wire affecting the wavelength.

Antenna Test Range

I constructed a test range in my back yard, along the lines discussed by Erwin. I used a Nanostation 2 at the transmitter, sending continuous 802.11b broadcast pings as described here. The antenna under test was placed about 6m away and a spectrum analyser used as the receiver. It wasn’t a very good antenna range but after some experimentation we did get surprisingly repeatable results when we compared our antennas to several control antennas.

I fashioned a clamp on a tripod to hold the antennas:

Tripod and clamp

However the tripod and clamp didn’t work very well. When I swapped antennas the results differed wildly in exactly the same position. It’s hard to place a 17mm printed monopole in the same position as a 80cm colinear antenna as their sizes are so different.

So instead I moved each antenna around by hand until I found the peak amplitude, which was captured by the “max hold” function of the spectrum analyser. Sounds a bit rough but gave good repeatable results, and Jeff and I achieved similar results when testing.

Path Loss

The 802.11b signal peaked at about -30dBm on the spec an. Using the Wifi power measurement method described here this means a total received power of -20dBm.

The expected received signal is:

Pr = Tx power + Tx antenna gain – path loss + Rx antenna gain – coax loss

So we plug in the numbers from the Nanostation 2, a 6m path loss and the 8dBi gain Superpass omni reference antenna we get:

Pr = 16 + 12 – 56 + 8 -1 = -19dBm

which is pretty close to what we are measuring using the spectrum analyser. If only all my calculations came out this close!

Antenna Gain Results

We used the 8dBi Superpass as a reference. We would first measure the signals from the Superpass, then save that on the screen as signal A. We would then measure the test antennas and calculate the antenna gain based on the known Superpass gain. We moved each antenna around by hand until a peak was found (the max hold function made this straight forward).

We repeated these tests several times over the day, and while the absolute levels would change 1-2dB the relative levels were always similar.

The antennas are listed in order of gain, and I would estimate the measurements have a tolerance of +/- 1dB. The RF level is the peak of the 802.11b signal on the spectrum analyser.

Discussion

The location of the physical position where peak received signal was found was quite “sharp”. This may have been due to lobes in the signal from the Nanostation 2 or multipath.

Several commercial router antennas were tested (sleeve dipole construction), they all measured about the same. The internal design of these antennas is discussed here.

The results from the control antennas (15dB grid, 8dB Superpass, and nominal 2dB sleeve dipole commercial router antennas) are consistent with what we would expect, which gives us some confidence in the other test results.

The impedance match and gain results from the PCB biquad are poor, which suggests the antenna is not resonant at 2.4GHz. It would be nice to test this antenna on a network analyser to find out where they are resonant (please contact me if you have one – I will ship an antenna to you!) Jeff is working up a simulation of the PCB biquad to test the design. We aren’t pursuing the PCB dipole as we have a bunch of antenna candidates that perform just as well (2dBi).

The wire biquad performance with a reflector was remarkable, nearly as good as the grid antenna which is a much larger antenna. The measured gain (12dBi) is consistent with other peoples results for this antenna.

Jeff and I really liked the wire antennas due to their performance and simplicity. They are easy to make: in production they could be bent up on a jig on 10 seconds from stiff copper wire then soldered to the Mesh Potato motherboard. One small problem with the dual loop biquad wire antennas is a feed arrangement – a small piece of coax would be needed to reach the central feed point. We don’t want the antenna wire directly over the PCB, as this would affect performance. The single loop wire quad is simpler in this regard, as it could be attached at one corner to the PCB.

The PCB monopoles perform well and are very simple, just a 17mm x 3mm track on the PCB next to a good chunk of ground plane. Virtually zero cost to add to the Mesh Potato motherboard. Both the 17mm and 20mm versions worked well, which suggests a relatively wide bandwidth and a high tolerance to small variations in manufacture like dieletric constant of the PCB substrate. Antennas fabricated on PCB are physically smaller than their wire cousins as the signals travel slower which means a smaller wavelength for a given frequency.

Wire single and dual loop biquad/quad antennas had above average gain and some directivity, with both peaks and nulls evident as they were rotated. Is directivity a good thing for a mesh router? You might enhance the signal of one node but null out the signal from another. I am not sure.

The higher gains of some antennas look attractive but may not be useful in practical mesh networks. To achieve the highest gain required careful adjustment of the antenna position. This is fine in a traditional point-point Wifi link, but in a mesh network their are multiple nodes we want to talk to. So if you peak the response to one node, you may dip the response to another. I guess it depends on how many nodes you want to talk to.

The reflector was a piece of blank PCB about 20cm x 20cm. It was moved back and forth behind the antenna until a peak was found (usually at around 15-20mm). All antennas improved by at least 4dB with the reflector, the wire biquad improved by 6-8dB. David C has suggested a slide-in reflector arrangement to give a choice between omni and directional antennas. These tests confirm David’s suggestion is a good one, if a precise way to mounting the reflector can be found.

Here are some of the antennas tested grouped by gain.

Antennas grouped by gain, highest gain on the left

Most embedded web configuration interfaces make heavy use of shell-scripting. LuCI takes a different approach and uses the LUA programming language.

Let me show you how to quickly set up a new Mesh-Potato, so you can make your first phone call. Configure your workstation/laptop to use a IP address like 192.168.1.1 on the Ethernet interface. Power up the MP and connect it to your workstation with a Ethernet cable. If this is the first boot after flashing a new firmware it will take roughly three minutes for the MP to boot. It needs to perform all the steps needed during first boot (generating SSH keys, formatting and mounting the free flash space with the overlay file-system).  Now open your browser and point it at 192.168.1.20. Note that you should click on the “Save & Apply” button after making changes in each window.

You can re-flash the firmware image from your browser, while retaining the individual configuration. LUCI supports configuration backups, uploading SSH public keys and so on and so on…

In the next post about the web interface development we are going to introduce you to Afrimesh – the amazing B.A.T.M.A.N. mesh management web interface for the Potato! Stay tuned!