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Why Do MP3s Download Slowly?

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bostonman:
It's been a while since I've downloaded MP3s from places like Napster, torrents, etc... but recently questioned download speeds.

If my ISP is providing 15Mbps, that's (rounding up) 2MBps. A typical 128KHz sampled MP3 is roughly 1MB per one minute of music.

So a four-minute MP3 would be 4MB which means it should download within three-seconds.

Obviously other factors need to be considered, but I don't remember any MP3 taking under a minute or two before completing a download.

Am I missing something in the math?

CatalinaWOW:
I agree with your math, but there are lots of places where throttling can occur in the system.

At the source, at your hard drive, in your OS and in handshaking at various stages in the process.  Your results are not greatly different than mine, though I have admittedly lousy satellite internet service.

bostonman:
I never gave thought to download speeds and stuff until having to explain ISP commercials to others and what Mbps means.

So I began to question why an MP3 takes so long to download.

You basically answered my question and confirmed my assumptions. If a 4MB MP3 took five-seconds instead of two-seconds, then I wouldn't question my math, but thought I didn't account for something since it's a gross difference.

mariush:
Some download servers bandwidth limit the download speed on purpose.

For example, the highest bitrate for mp3 is 320 kbps so a server may limit the speed to 512 kbps or 1 mbps.
The reason for this is to save bandwidth : someone may start to listen an hour long podcast and stop listening after 10-15 minutes. By the time you get to 10-15 minutes point, you probably only downloaded half an hour worth of data, so the server saved half hour worth of bitrate.
512 kbps - 1 mbps is good enough to give you an initial burst of data, basically to buffer a few minutes of audio, and it's big enough to allow your music player / browser to recover (refill buffer) if there's some temporary network slowdowns.

SeanB:
As well a lot of ISP's do tend to bandwidth limit audio and video, so that you can buffer a few seconds to a minute at a time, and then slowly get the rest. Basically done to manage bursts of data in the aggregrate on the network, and often as well with larger sites like YT and other CDN's there are local cache servers to provide the actual data, instead of having a central server. thus the cache server gets the request, looks to see if it is in cache and then trickles it out, or goes upstream to get it, grabs a minute or so of it, then has the time to trickle it down to you, while getting the rest to cache.

Simple test is to have a encrypted and non encrypted file, and see that the non encrypted is going to be rate limited, while the encrypted file, as a big binary blob, is not going to be rate limited, but will come down at the maximum speed of the slowest point in the system, most likely the oversubscribed local link to the connection to you, closely followed by the minimum viable product ISP provided connection equipment if you are using it. Store both files as random name, and in the same directory on the same server.

Another thing is that most ISP's will deliberately prioritise data to the most common speed test sites, so that they will show an inflated number, as the traffic will be priority over all other users briefly, giving a false impression of fast speed. All others will be throttled, just how badly is a combination of rate limiting both locally and at peering points along the way where your traffic is passed from one network to another.

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