Tuesday, June 8, 2021

Strange Cat5 cable speed issues

We have a client who was not getting the full 400Mbps speed from their internet connection. Upon investigation, we found their Unifi Secure Gateway was auto-negotiating the WAN connection to the ISP router @ 100Mbps.

We put this down to a poor cable so attended the site to replace the cable.

I initially tested the ISP's router with just a short pre-made cable and whilst I wasnt getting the advertised 400Mbps we were getting 200+ Mbps.

So I made a 30-metre cable round to where our USG was and before connecting to our USG I tested with just my laptop to ensure I was still getting the 200+ Mbps. Nope, it was hard capped @ 94Mbps.

Went back to the ISP router with the pre-made and tested and sure enough, 200Mbps again.

Thinking it could be the quality of cable I was using I made a shorter 2m length of cable and tested just into the ISP router and again 200Mbps

Ok now that's weird

So I then thought the length of the cable, or something I have run it next to is causing issues, so I pulled the cable back, laid it across the floor and chopped about 10m off it, tested and again hard-capped @ 90Mbps.

Any thoughts as to what could explain this?

Just to rule them out:

  • I tested on 3 different speed testing sites each time and all showed the same results
  • It was the same port on the ISP router each time
  • The ISP router is a high-quality Cisco router


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