Fax for the memory
Before the internet, DSL and VPNs swept all before them, companies would often link sites with leased lines. I worked for a comms manufacturer who made a mux that could merge voice and data onto a single kilo-stream link, saving the cost of separate voice lines. The voice links were compressed to save bandwidth and they had a fax relay mode that demodulated the fax locally and just transmitted the digital content to the other end. This was in use in small branch offices and one of these had a problem. If someone was on the phone and the fax was used, the voice call went into fax relay mode.
Each voice channel had its own DSP, so a software fault was ruled out and the finger of blame started pointing at a 1U pizza box that housed the analogue interface between the mux and a standard phone connection. I had never seen one of these before, but that did not stop it becoming my problem as two tech support engineers "demonstrated" by talking to a phone attached to one line while getting me to listen to another. I could not hear anything over the wall of 19" racks of comms equipment in their lab so I took it away for further investigation.
I injected signals and measure cross talk on the adjacent channel, but I had to put in one hell of a loud signal to get anything noticeable. I persisted. The PCB was just two layers so the grounding layout was bad. I added thick wires and managed to reduce what cross talk existed. How I could get such a mod into production without being lynched was a problem for later, so I too the box back to tech support.
Weeks passed and I heard nothing, then I bumped into one of the tech support guys and asked how the modified box worked. He looked a little sheepish. They had taken the field service report at face value, but when he went on site he had noticed that the fax monitor was turned up to full volume. It was basically acoustically coupling to the nearby phone. When they turned down the volume all was well.
Thinking back, I wonder what the BER was like, but now I'll never know.
Biting the hand that feeds IT
