January 17, 2008
We were finally able to make it up to the mountains again to finish what we had started, and I'll go ahead and say that the testing was successful. In the photo above of node 14, one of those distant bumps in the bright clouds must be the snow-capped mountain of node 13, 121 km (75 miles) away. You can see that we were able to get the weather to cooperate this time; the sun was nice.
There was plenty of snow on the road to node 14, and we would not have made it half way up the mountain without a 4x4 pickup. The road to node 13 was completely blocked by a snow drift near the top, so the test equipment had to be carried the last quarter mile by hand. Fortunately, the base, steel mast, and antenna had been left at the site. Another problem at node 13 was that the mast had fallen down and one of the cable tensioners was missing. It must have turned and come unscrewed. Fortunately, no damage was done.
After aiming both antennas with the compass and inclinometer again, the signal level at node 13 was -61 dBm. We tweaked both antennas vertically and horizontally while watching the signal level with iwconfig, but were only able to improve it slightly (-59 dBm). We did see -57 dBm once. At node 14, the received signal began at about -69 dBm and got as good as -63 dBm at one point. The level seemed to vary a little even when the antennas were not being adjusted, and weakened by about 4 dBm during the time we had the connection up.
We measured connection speed with iperf running on the actual TIER Linux routers. TCP throughput averaged about 585 Kb/s (one direction). We simulated VoIP traffic with small UDP packets, and achieved about 300 Kb/s in both directions simultaneously. This should support a good number of voice connections. Ermanno et al achieved 3 Mb/s over 279 km using very similar hardware and software (see my prior blog entry), so I know that much faster speeds are possible. This will need further investigation.
After completing the performance tests with the TIER Linux routers, we disconnected them and connected the Linksys routers which had been configured for nodes 10 and 13. No configuration changes were made; we just plugged them in. I was surprised that it actually worked, although throughput was very slow (about 91 Kb/s). We also figured out why the link between nodes 10 and 13 never worked--the wrong antenna connector (on the back of the Linksys box) was configured! I had tested this and was certain that the setting was correct, but the 121 km link only worked with the Linksys devices when the antennas were connected to the opposite connector than the one we had been using.
During the testing, the team at node 13 met a guy who maintains some of the professional equipment mounted on one of the towers there. He mentioned that he has given up on the grid-type antennas for high elevations because ice accumulations warp the parabolic dish and degrade performance. He recommended solid antennas with a cover (radome) to keep ice and snow off the dish, and specifically Andrew antennas, which the snow just slides off of. They cost about twice as much, but last much longer.
I am very glad to have this phase of research and testing completed, and to know that 121 km is feasible. It is a great alternative to VSAT for certain situations.