Case in point. Had a client in the late 80’s with an X.25 connection between their office and a Mainframe across town. It was accumulating hundreds of CRC errors per minute. Couldn’t fix remotely so I flew out there. Had a scope hooked up on both sides.
We loaded the connection in varying patterns for about an hour. ZERO CRC errors. Scratching our heads, we decided to punt. Then suddenly the CRC error count started climbing on my scope. I shout into the speakerphone, excitedly, what did you all just do? They’re like “nothing but unplug the scope from the patch panel”. “Please plug it back in”. CRC error count stopped.
We loaded the connection in varying patterns for about an hour. ZERO CRC errors. Scratching our heads, we decided to punt. Then suddenly the CRC error count started climbing on my scope. I shout into the speakerphone, excitedly, what did you all just do? They’re like “nothing but unplug the scope from the patch panel”. “Please plug it back in”. CRC error count stopped.
Diagnosis: faulty patch panel.