FTTC Fibre 1 : Uploading causes the download speed to drop from 50Mbps to 3 Mbps

[cwl]

BT Technical Helpline agrees the Hub Speed drops to 3Mbps and the Ping goes up to 800ms when I am uploading using OneDrive Sync. They say as the line sync speed is still at 50Mbps so they are providing the guaranteed service even though they acknowledge the throughput is 3Mbps when OneDrive is uploading files and they will not look in it further.  I have only seen it at this location where I now live and had hoped to work. The BT Openreach Engineer checked out the line to the exchange and cannot find a fault with his equipment but he said that he could not access the logical far side of the exchange. To me is looks like there is congestion somewhere in the system but BT don't appear to have the technical experts to analyse what is causing the throughput to fall so dramatically. Some technical support people say it is due to OneDrive and other Technical support people say they don't think it can be due to OneDrive.

[licquorice]

Saturating the upstream will always impact the downstream due to the TCP ACKS being returned.

[Robmonster · Accepted solution]

This is normal behaviour. When you download something your systems are constantly sending messages back to the senbding server acknowleding receipt of data and asking for mre, essentially.

If you are uploading a file at full speed then these acknowledgement messages get delayed, which results in slower downloads.

The answer is to limit whatever software you sue for uploading to about 80% of your max upload speed

[cwl]

I'm only uploading at 1.1Mb/s it was set to 6Mb/s but as soon as I have any uploads it drops to 3.Mb/s Download and 1.5Mb/s upload. 3rd BT Openreach Engineer due out but he can only check the copper and possibly the FTTC modem and these always come out fine.

[craigj172]

Hi,

Its definatly something that has changed as for the 1st 3 years at this address I had no issues at all uploading a vid to youtube and still able to use my internet connection fine, now if i have a vid uploading i have almost no download speed at all until it has finished.