Broadband becomes 'symmetrical' when uploading

[Toonshorty]

If I start any kind of heavy uploading (youtube videos, linux torrents etc.) my ping shoots up to 750ms and my download speed slows to around 0.14-0.30Mb. My upload remains at 0.32-0.36Mb.

Once the upload finishes my connection restores to 36ms ping and my download goes back to 5.30Mb.

Why does it do that when I upload?

[Matt]

Mainly because of the asymmetric nature of ADSL but also, if you download at full rate, your latency will increase also.

Limiting upload to 25~Kb on a 448k connection, 50~Kb on a 832k connection and 70~Kb on a 1Mb connection should limit high latency but still allow a respectable download speed.

Matt

[Alan]

When you're downloading, things whilst uploading at full speed, due to the way that TCP works, you will see this sort of thing.

If we take it back a stage, when you download a file, using TCP/IP, for every packet that you download, you send back a small 'ack'nowledgement that you've received the packet correctly; the sender then knows to send the next packet of data, or if it doesn't receive the ACK, then it knows to send it again.

The problem comes, when you fully saturate your upstream by uploading at full speed, is that these ACK packets also have to queue and fight with the rest of the large files that you're sending up, and will slow down their transmission to the person that you're downloading from. Lots of servers adapt the rate that they send data based on how quickly they receive back the ACK - there's no point in sending the data faster than you can take it, so it can exasperate the issue.

As Matt has said, the best thing to do is to never fully saturate your connection, which will always leave room for the smaller packets; some routers handle this through QOS, others have QOS but don't handle this very well at all.

If whatever client your using can limit uploads, then this is the best way to handle it, but obviously with YouTube videos it's not that easy.

Unfortunately it's just the way the protocol was designed.