Download speeds anomaly?

[easybeat]

I've just gone to broadband but this is a phenomenon I noticed on my old 56kb connection as well. When starting a download for the first few seconds the speed is incredibly high compared to the average speed for the whole download, for instance:-

56Kb connection - average download speed 4.6kb/s at start 20+Kb/s
576Kb connection - average download speed 61.2kb/s, at the start I have seen it way over 200kb/s frequently and as high as 300kb/s.

Is there a reason for this? If so would it be possible to somehow spoof the connection/download manager so that it thinks it is downloading the first few seconds throughout the entire download?

Interesting!

[LigH]

This is just simple...

The timer starts at the first downloaded block.
The first block took - lets say, less than 1 second to download. So the datarate at the beginning is Blocksize/1 s.
Then it takes time to download the next block. The connection stalls because the server is sending blocks to many other clients besides you. The time counts up, but no new data comes in for a few seconds.
After the 2nd second, the average datarate is Blocksize/2 s.
After the 3rd second, the average datarate is Blocksize/3 s.
...
After 5 seconds, another block arrives.
After the 5th second, the average datarate is 2 Blocksizes/5 s.

Just an example, but it hopes to help you understand the term "average"; it is different to "current" or "maximum".

"Current datarate" is similar, it usually calculates like "incoming data divided by a time window duration". For example, in the 5th second, the average datarate may be "1 Blocksize during the last 2 seconds".

[easybeat]

Okay, so the original speed is not actually higher it's just down to how the download manager reports the speed.
The manager reports the block as being downloaded whereas it has only just started, therefore a second later when it has actually finished it starts the next one and so on.

Therefore if each block is 200kb
0 seconds - block starts speed registers at 200k/bs
3 seconds - 2nd block starts speed @ 400/3secs
6 seconds - 3rd block starts speed @ 600/6secs
9 seconds - 4th block starts speed @ 800/9secs

and so on till eventually you do get a correct value.

Hope I've understood your explanation.

Thanks

[LigH]

I think so.

You should in general not assume that a download always is a continuous stream with a constant bitrate, but instead may have interruptions sometimes.

[acassara]

Depending on your browser... MOST browsers start to download the file while you're choosing where to save it to. So it has to make up for how much it has already downloaded by showing you those high speeds, then it goes back to what it's at. Hope that makes sense.

-acassara