[jpmattia]

As a guy that has done some hardware for router interfaces, this was an interesting post regardless of source.

There are lots of built-in monitoring functions for the hardware interfaces. For example, the laser driver for the transmitter often drops in power before failure, so there is a hardware function built-in on the receiver side to report the avg laser power. If the power drops below a certain threshold, an alarm goes off on the monitoring software.

The author is pointing out an interesting failure mode not automatically caught. As a shot in the dark: In the receiver, the hardware is spec'd to receive 31 zeros in a row (iirc, which btw goes way back to early SONET specs). If the detector (eg) is degrading in a way that causes a particular string of zeros (or some other fixed pattern) to fail, you would see it show up much like the test that was run.

What's interesting is that the errors did not light up alarms on the monitoring. Perhaps crc errors have a threshold, and the bit pattern causing the failure was showing up statistically below the threshold.

Anyway, I'm sure there's some hardware failure-analysis guy out there who would love to look harder at that link.