December 5, 2007
The diagram above details the logical configuration of the network we are setting up (some of the data has been intentionally modified). It includes two custom-built routers described in my last post, optimized for long-distance links, and two Linksys WRT54GL routers running Freifunk firmware 1.6.20. Freifunk is essentially OpenWrt with a modified implementation of OLSR mesh software, some testing and management tools, and a friendly GUI. I wanted to include Freifunk in my testing because a mesh network should work well in the mountainous terrain to provide reliable wireless connections between villages which do not have line of sight, and the WRT54GL is a low-cost, low-power, and flexible platform on which to build routers.
The network diagram was not just done for the benefit of this blog. During the setting up and testing, I found it an invaluable reference source--all of the numbers I needed in one place, and presented visually. It is done in Gimp, allowing easy rearrangement of elements and changes to the text blocks.
Note how the sum of corresponding elevation angles is not zero; this is due to the Earth's curvature. In fact, on the long link, both antennas are pointed slightly down. Similarly, compass directions for corresponding antennas are not 180 degrees apart due to differences in magnetic declination at the endpoints, although the differences are negligible. Radio Mobile computed all of these values for me, except for adjusting for declination.
After configuring all four routers, I connected node 10 to the Internet, connected the actual power and networking cables and antennas (but without the parabolic grid parts), and placed the three nodes a few meters apart. This part of the testing is very important as it verifies the operation of all of the hardware and software components as a whole while you still have access to spare parts, a good work area, electricity, Internet, etc. More information on staging can be found here.
On December 5, two of us drove up to the location for node 13. The plan was to set up the bases, poles, guy wires, and two antennas, and verify the connection between nodes 10 and 13 (named 1013). The equipment planned for node 14 was set up at the node 10 site (with the antenna aimed at node 13), and we had hoped to test the 1314 link over 23 km. The following day we would set up node 14 and test the 121 km link.
After arriving and unloading, the first problem we faced was pounding our fence posts into the ground for anchoring the guy wires. What looked like good soil all over was only a thin layer of dirt on top of fairly solid rock. We were fortunately able to work around this by using three sharpened angle-iron stakes (which we could pound into the rock) and wrapping wire around two existing rocks. The rocks happened to be nicely placed so that we could still avoid having a guy wire in front of either antenna. While trying to carry another large rock over to help brace one of the existing rocks, I injured my leg and wrist, but not so badly as to hinder the work that day.
The second problem was that we could not connect with node 10; there was no detectable signal at all. After talking with someone at node 10 (via cell phone), we realized that the node 10 router had not been plugged in that morning. However, plugging it in still did not allow us to connect. The signal (SSID 1013) was detectable near node 10 using a laptop, but not at our location (using the 24 dBi antenna). We then turned on the node 14 radio located at the node 10 site, which was already aimed at node 13. This signal (SSID 1314) was received at node 13 using the same 24 dBi antenna and Linksys router, and the signal level was reasonably strong. We therefore concluded that the problem was with the node 10 antenna and would have to wait.
I also took the opportunity to check for possible sources of interference with a Wi-Spy 2.4x spectrum analyzer. When connected to the 30 dBi antenna pointed at the site for node 14, there was a very strong and constant signal around channel 13 (which requires a license in the U.S.), but the other channels were reasonably quiet, especially around channel 1. When connected to the 24 dBi antenna pointed at node 10, it was also reasonably quiet. When connected to a short omni antenna, it was very quiet, so I was not concerned about interference from or interfering with existing equipment.
For anyone interested, here is my packing list (I like lists). Construction materials: routers; antennas (dish, feed horn reflector, feed horn, mounting hardware, instructions); pigtales; car battery (fully charged); power cables from battery; mast, base, guy wires, turnbuckles, cable clamps, guy wire anchors; cable ties (attach Soekris box to pole, secure cables, keep anchor cables from slipping); rain cover; Ethernet cables.
Tools for the Hyperlink 24 dBi dish: two 8 mm tools, 10 mm tool, 10 mm deep well socket; small phillips screwdriver.
Tools for the Hyperlink 30 dBi dish: two 17 mm tools, 14 mm deep well or wrench, 12 mm tool, two 25 mm or adjustable wrenches; small phillips screwdriver.
Tools for the mast: hammer or sledge hammer (stakes into ground); large pliers (stakes out of ground); small pliers; screw driver (tighten turnbuckle cable tensioners); shovel; work gloves; wire cutters.
Other tools: bin or tub for small parts while assembling; 1/4-inch and 3/8-inch nut drivers; volt meter; duct tape; driving directions; cell phone, numbers of all other parties; food, water (it will be a long day); camera, spare batteries; binoculars; GPS; compass; inclinometer (make pole vertical, aim antenna at proper angle); azimuth and compass bearing of neighbor nodes; string for aiming; laptop (Radio Mobile, Kismet, spectrum analyzer); laptop car adapter, cables; business cards (if asked about what we're doing); permission letter (node 13); gate key (node 13); boots, old jeans, jacket; spare router; extra steel cable, cable hardware, baling wire; rope (you never know); spare AA batteries; umbrella (shade laptop screen from rain or sun); spectrum analyzer; blankets for transporting antennas; pen and notebook.