Why does download rate slow?

Question I’ve not seen answered -

When downloading from the internet using a 56k modem, the download rate will initially start high, and then trail off.

Why is this so?

Does it always trail off or does it fluctuate up and down? One reason can be changing conditions across the internet. These can be anywhere along the download chain from increased traffic at your ISP to increased traffic at the file’s host.

One experiment you might want to try is pinging the host of the file at certain intervals (depending on the expected length of the download) to see if the actual latency increases during the download.

Somewhat of a guess…

Your browser may begin to download the file to a temporary folder as soon as you choose to save it, before you choose a location. So its already downloaded for 10 seconds or whatever prior to you seeing the download speed. If it takes you 5 seconds to choose a location and file name, it could download 20 KB @ 4 KBps already, so when the download box opens, it will show that you’ve downloaded 24 KB in 1 second (therefore you’re downloading at 24 KBps), 28 KB in 2 seconds (14 KBps), 32 KB in 3 seconds (~10 KBps), 36/4 (9 KBps), 40/5 (8 KBps), 44/6 (~7 KBps), etc… and after 30 seconds you’re down to about 4.5, and it continues until it’s almost indentical to your real speed.

I second this suggestion. I have experienced it myself, especially when using Mozilla.

It’s the compression used by your modem. On almost any file, the first portion is an uncompressed loader or header, which can be compressed fairly well. Since a sliding window is used to produce the download speed that’s displayed, it starts off high, then slips down to your average uncompressed throughput after a short time.

I think this is the answer.

I noticed this “slowdown” when i had a dial-up modem, and i still see it (although it’s less obvious) now that i have DSL.

Try a little experiment. Try downloading two similar files from the same site. With the first one, right click “save as…” and wait five seconds before hitting the Save button. Then do the same thing with the second file, but wait ten seconds, or even twenty, and see if the initial rate of downlaod, and the change in rate, is different.

The above poster are correct in that your file is getting placed in temp cache.

The download speed is just an average of the data received up to that given time.

This is why you’ll see initial connection speeds of 190kbps for a few seconds – quite an impossible feat for a little ol’ 56k modem.

Ah - thanks for that. The issues of caching sounds fair convincing. I’ll definitely check that out on the next download.