Working Up

by Dwayne Phillips

I have given many demonstrations of systems. I have watched even more demonstrations of systems. One thing I have learned to do during a demonstration is to pull a wire to see if the demonstration still works.

Many years ago, I was involved in a 24-hour-a-day, seven-day test of a system. This was a communications system, and a common test in a communication system is the loop-back test. In a loop-back test, you send a message out of your system. At some point, you “loop” or feed the outgoing message back into input of your system. The message you send should appear on your system as an incoming message.

A current example of this is to send an email to someone and add your own email address on the address line.

We were happily testing away in the “oh hours” (sometime between midnight and “oh six hundred” 06:00) performing a series of loop-back tests. Every message we sent looped back to us successfully. We typed a message, hit the SEND button, and the message was printed on our system – yes, a real printer. As I said, this was many years ago.

The loop back of our testing was performed via a patch cable at a patch panel. We had a physical wire enabling the loop.

At some point in the oh-hours, for some reason, I pulled the patch cable from the patch panel, thereby breaking the loop back. The message we sent out could not possibly come back to us.

Ooops, the messages we were sending out kept returning to us. I showed everyone the broken loop back. We all scratched our heads. I don’t remember the answer to the puzzle, but we all concluded that the preceding hours of tests were invalid.

I refer to this practice as the pull-the-wire test.

When I watch a system demonstration, I find something that should break the demonstration. I state, “If I pull this wire or turn off this box, the demonstration will stop working. Right?” After the demonstrators agree, I pull the wire or turn off the box. I have encountered a few demonstrations where the system failed the pull-the-wire test, i.e., the demonstration was a phony.