Uploading makes download incredibly slow

We are on an ADSL line and normally we get around 9 Mbps speed down 0.8 Mbps up. However, when I start a chunky upload (for example, a video) it seems to also completely saturate the downpipe, almost to the point of making the internet unusable. Is this normal? I haven't had ADSL for a couple of years but I don't remember it behaving this way.

Yep, swamp the upload, and the download falls over.

Demands upon upload have increased a lot over the last few years. Devices backing up to the cloud, etc etc.

A similar post to yours recently got a reply of tinkering with the routers QOS settings as a work around ... but if that’s not your thing, can you upgrade to FTTC ? Or how good is the 4G signal where you are ?

As a very broad rule of thumb, you need the equivalent of about 10% of your download speed in upload to send acknowledgement packets. So in your case, about 0.9Mb/s. So yeah, saturating your upload will kill your download - this becomes slightly less of a problem on FTTC as the upload is not the bare minimum required to satisfy your download speed and you probably won't notice your internet running at 20Mb/s instead of 40 for instance, in most circumstances.

Edited by severedsolo (Wed 27-Jan-21 15:53:17)

Some newer routers seem to handle it better than older ones, but generally, it's a problem. E.g. Skys old hubs would totally saturate everything when I uploaded, on their newer hubs, it has effectively QOS built in so things like my TV still seem to download (although it's still not perfect).

In my case, since FTTC, I might drop to say 30-40Mbps when uploading a couple of bits, from 80/20, which is much more manageable than having ADSL and seeing near nothing.

Funnily enough I created a thread along the same lines a few days ago. I have FTTC that runs at an effective 74/18 and kicking off an upload renders the connection unusable. Can't even browse the web. Luckily it's not something I do very often so I've not tried to enable QoS.

Sounds like a classic case of 'bufferbloat' to me, look it up.

Playing with QoS settings, particularly upload rate limits is likely to aid the situation though it does depend where the buffers are that are being bloated.

There are some nice solutions to this in the form of fq_codel + a shaper (to rate limit) and even more suitable something like CAKE.

As an addition to what others have said what are you using to do the upload? Most cloud upload clients have the ability to set a maximum rate for the upload so you could set this a little below your line speed upload rate which can then leave some spare to allow you capacity to download. When using things like OneDrive I normally use the OneDrive client to rate limit the upload to no more than 80% of my upstream bandwidth.

Thanks to everyone for the suggestions. I'll look into QoS settings.

In reply to a post by ian72:

As an addition to what others have said what are you using to do the upload?

This mostly happens with video uploads to Vimeo through a browser. I haven't found a way to limit the bandwidth, but there is one thing I want to try. Vimeo has an option to upload from Dropbox. Since I can limit the upload bandwidth to Dropbox, I want to try this to see how the transfer then happens from Dropbox to Vimeo. Assuming this happens cloud-to-cloud, it should be relatively quick and not add too much extra time.

Unfortunately I don't use Vimeo but as you say they seem to support a number of cloud services so you could use one of those and set the upload bandwidth within their tools. I am surprised Vimeo don't have a client to allow this to be set.

In reply to a post by beneix:

We are on an ADSL line and normally we get around 9 Mbps speed down 0.8 Mbps up. However, when I start a chunky upload (for example, a video) it seems to also completely saturate the downpipe, almost to the point of making the internet unusable. Is this normal? I haven't had ADSL for a couple of years but I don't remember it behaving this way.

It's probably best to change the QoS settings as other people have mentioned.

I just wanted to add my 2 pennies worth in that many years ago when Pipex ISP was around and I was on ADSL at some point what you describe started happening to me. I eventually phoned up Pipex as it was making it impossible to surf websites the moment anything uploaded. Very stranglely when I was on the phone to Pipex explaining the problem it suddenly went away. Then it happened again a few weeks later. I had changed nothing at my end, same setup from everything working to now having that problem.

It only got permanently resolved when I changed ISP.

In reply to a post by beneix:

Vimeo has an option to upload from Dropbox. Since I can limit the upload bandwidth to Dropbox, I want to try this to see how the transfer then happens from Dropbox to Vimeo.

This has helped - I set Dropbox to use around 75% of the theoretically available speed as per Speedtest. With this, my download speed seems not to be affected. Here's a good explanation on Quora.

I didnt think it could be fixed except by some form of QoS or throwing more bandwidth at it?

If you have 1mbps upload then anything syncing will naturally use the maximum allowed, in effect slowing everything else down. Same with downloads although not as bad as you have more room to play with