The other day I noticed my laptop’s CPU widget was showing near 100% usage, but Task Manager was showing only 3%, and applications were crawling. I did the usual incantations to no avail. Safe Mode with everything off seemed OK, and a solid two days of virus scanning revealed nothing (as I would hope, given the safeguards in place). Driver updates led me eventually to a BIOS update. No change. SysInternals revealed nothing. Then Windows decided it was time to do its own updates. More delay.

Everything was running slow, including networking, which seemed to be a problem even when in Safe Mode. Connecting a second laptop into the gigabit switch revealed the same network bottleneck. “Aha!”, I thought, “the switch is kaput.” Bypassing the switch and attaching its uplink cable directly into either laptop didn’t help. “Problem with another gigabit switch further upstream,” I thought.

Bypassing the switch in the attic didn’t help either. Now I’m heading to the main router, reset, shuffling of cable connections, no improvement. Then I attach my problematic laptop to a separate line that is running directly down to the router. Joy!

Interestingly, not only did the networking go back to normal, so did the odd CPU readings.

The evidence was now pointing to a problem in the cabling from the downstream switch all the way to the main router. The worst case scenario was that the cable would have to be replaced, and most of it is behind walls that were put in when the house was massively reworked over a decade ago. I was not looking forward to that, and was envisaging needing some surface-mounted ducting as the existing cable is inaccessible.

However, there are other places that cables can fail, and these places are accessible. The ends of the cables, for example. I checked the cables at the main router, then at the first downstream switch and at the second switch closest to the laptop. No problems were found.

Then I remembered that the uplink cable of the second switch goes to a wall junction, buried behind a desk. On my hands and knees, with a torch held in my teeth, I got to the junction and tried to extract the connector. This thing had been in place, untouched, since the builders were here and it was not going to move easy. Some space would have to be cleared so I could access it easier. Climbing out from under the desk, I briefly rechecked the laptop and noticed the network had somewhat improved. No longer at 10% usual capacity, it was now at about 30%.

At this point I was thinking that either it’s the wall connector, meaning the desk would have to come out, and some work done on the wall to replace the connector, or maybe the short uplink cable between the wall and the switch. I hoped for the latter, grabbed a fresh cable from my supplies, clamped the torch in my teeth and went back to under-desk cave snorkelling.

One more tug on the old cable and the plastic lug snapped as the cable was withdrawn, turning the plastic into a blade-like weapon. My first thought was that I’d have to crimp a new connector onto the cable, but with a high-grade new cable in my other hand I decided the old one could be retired, forever. I clicked the new cable into place, and went back to the laptop.

100% network throughput.

Changing the cable took just a minute. Discovering that the cable was causing my odd CPU behaviour took THREE DAYS, a bruised shoulder (from crawling in the attic), a sore head (from hitting it while snorkelling), and a cut finger (blade-like plastic lug). All good reasons why we pay other people to do tech support.

Categorised as: Hardware, Networking

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