cont'd from previous (linked) post to WTC, re: free space I-R / optical communications during adverse weather conditions:
While heavy rainfalls and snow storms resulted in higher error rates (which were usually recovered rather easily, or necessitated down shifting speeds - see below), sustained, dense fog proved to be the biggest problem, and the only type of weather condition that was severe enough to take a link down hard. Even here, it was only to the most remote site that total communications had to be shut down, after reducing speeds to that location yielded no improvement. However, I can count the number of times that this occured over a three year period on one hand.
During heavy rains and snows, a means of coping was to cut back on the speed of the link to fractional T, or even n X 64, which was a tradeoff, in itself.
Explanation: there reaches a threshold when error recovery times can no longer keep pace with the application time window's demand, beyond which you enter the point of no return at the higher speeds, because of "normal" signal erosion [mostly due to normal atmospheric dispersion] "plus" the effects of weather, which puts overall performance over the edge.
Since the relative effects of dispersion on error performance vary directly with the link's signaling rate, lowering the speed allows for fewer errors to be encountered than at the higher speeds. And when speeds need to be reduced substantially, applications which are deemed expendable or non-critical must be thrown overboard ["bumped"] for the duration of the weather anomaly, in order to support the more critical ones when operating at the lower speeds.
Well, I guess I did elaborate after all, didn't I? smile
Regards, Frank Coluccio |
