When investigating, seemingly valid clues can be misleading

In the past days I learned that when you are looking for the cause of a problem and proceeding logically like a detective, clues can still be misleading. You can gather clues that are unrelated (coincidence) or related to a different problem. In the latter case, a problem can become quite nasty when another one intermingles with it.

I won’t go into detail about a case a few years ago where a graphics card I bought was faulty, and when I got a replacement from a different brand, that one was totally producing nothing but garbage on the screen on some systems while working perfectly on other systems, with no determinable common factor between them.

No, but I just discovered why the internet was so slow in the last few weeks. I didn’t change anything on my system, so I didn’t suspect it when bandwidth from many sources on the WWW was fluctuating like crazy, rarely ever giving me full speed. I did check many things on my system anyway, though.

It became more interesting when added to that, often the data transfer would momentarily drop to zero every few seconds.
I began investigating more thoroughly when my router lost connection and the DSL line quality became very bad for a few hours.

I tested DSL speed on another computer close to the router, without a switch in-between, and there I had the same bad download speed.

The problem source was so difficult to isolate here because THREE things were coming together!

The main problem was a gradually, over weeks, decreasing cable quality of my 20 meter ethernet cable. I’d never have expected that, but maybe it wore at some point, maybe at a doorway where people walk over the cable. This caused the bad transfer speeds, because the switch negotiated 100 Mbit/s, which the cable couldn’t handle anymore. In the end, the momentary dropouts were caused by the switch, and any other switch did the same. (Side note: My LAN adapter has a cable quality check and claims it’s fine, so this was useless. I had to manually tell it to use 10 Mbit/s to avoid trouble. I now have wonderful internet speed; no more anger over Youtube not being able to provide full speed. My, was I wrong.)

And here are the two deceivers:

1) The DSL line quality drop that my router indicated was unrelated.

2) The DSL speed test on the alternative computer SHOULD have been fine, since it is not using the faulty ethernet cable, yet it showed the same bad transfer speeds that my main computer did via the faulty cable. I have to assume that there simply actually was a speed issue with the test server at that time.

I keep having complex problems like this way too often.

When things just don’t add up, it might be because they’re multiplying instead.