Full fibre download speed drops completely when uploading anything

EpsilonLima — Thu, 19 Aug 2021 14:40:32 GMT

I've recently had BT full fibre installed into my property and am having a strange problem in that whenever I start to upload anything my download speed completely drops and becomes unusable.

I'm on the Fibre 1 package and usually get 50Mbps down and 10Mbps up, but as soon as I start to upload to file to cloud storage or transfer something from my machine to another over a VPN the download drops to around 5Mbps and will stay there until the upload finishes. If I kill the upload the download speed will instantly return to normal so this is definitely the cause as far as I can tell.

I'm using a different router (Ubiquiti edgerouter) and am measuring the speed from there, and have also run speed tests from multiple devices, both wired and wireless. I have also used this edgerouter on 2 different connections in the past and only started having issues when moving over to the full fibre connection so I don't think this is where the problem lies.

Does anyone have any suggestions as when I tried to contact BT support they were less than helpful...

Re: Full fibre download speed drops completely when uploading anything

Glamdring — Sun, 22 Aug 2021 09:52:56 GMT

Downloading also requires upload bandwidth (For ACK Packets), so yes, saturating your upload bandwidth will also kill your download speed.
The only way around this would be to limit the upload speed. I'm not familiar with the Edgerouter, but in PfSense I have FQ_Codel set up with the limits set at 90% of both upload and download bandwidth. FQ_Codel will also equally share both upload and download bandwidth between all connections which will prevent this issue.

This topic

Full fibre download speed drops completely when uploading anything

Re: Full fibre download speed drops completely when uploading anything