Wireless Hacks

Historical Link State Monitoring

Comprehensively track the performance of your wireless links over time.

The monitoring tools mentioned so far give you a good instantaneous reading of received signal and noise levels for a given radio link. While useful for proving a link and installing your gear, remember that radio conditions change over time. Doing the occasional “spot check” doesn’t really give you the full picture of what is going on.

For example, take a look at Figure 3-31. This displays radio data for a one-mile link, averaged over several days. You can see that in the middle of each day, the signal drops by as much as 6 dB, while the noise remains steady (remember that these are really negative numbers, so in this graph, a smaller number is better for signal). The repeating pattern we see indicates the effect of thermal fade . This particular link is a simple waveguide antenna mounted in the middle of a low sloping roof. As the roof (and the rest of the environment) heats up, the perceived signal is apparently less. At night, when things cool down, the perceived signal increases. The effect of thermal fade in this installation was later mitigated (by about 2 or 3 dB) by relocating the antenna, placing it closer to the edge of the roof. With less hot roof in the path, the effect of the day’s heat was reduced. Nothing can completely eliminate the effects of the sun on a long distance path, but without a historical graph, it would be difficult to account for the effect ...