Why do dial-up downloads speed up at first?

Question

I've noticed that with dial-up downloads, the download speed at can be several times faster. But I though the fastest dial-up can ever throughput is 53 K. But I've definitely seen speeds over that at the beginning of the download.

Any wisdom to impart?

Edit: also to mention, I've noticed it happens with uploads, too.

Answer 1

After you get to the save file dialog in most browsers, your download has already started while you type in a file name. After you click save, a significant amount of data has already been downloaded and the "meter" if you will, has to catch up so the measurements will be higher for a few seconds then slow down to normal.

As for uploads, I suppose a similar method is taking place. Data is probably being accumulated before a connection is fully negotiated, that's why it seems to be a "burst" at first. Although I've never had this happen to me with uploads, probably because I don't upload enough to notice. Both of these oddities are not specific to dial-up though.

Answer 2

Because usually, the initial part of the data transfer has more regular structure than the actual data and can be compressed better. Try to download a large file which consists of zero bytes only - I believe the transfer speed will be faster than the max modem speed, because the data is compressed during transfer