Explain this strange occurence involving a 56k modem

Back in the day, we had a 56k modem but our connection never really got any faster than 30k due to something with our phone line I never quite understood. My usual download rate was around 3.5kb/s but one day, I was downloading some file and it was going unusually fast - around 15kb/s, and it just kept going faster and faster. By the time it finished it was around 70kb/s. No, I didn’t misplace a decimal point, 70kb/s. That’s, what, 560kbps? A fair bit more than what the modem should have been capable of, especially considering the slow speeds we usually got. I wanted to investigate this phenomenon a little further, see if the speed was still there etc., but then my computer crashed right then because I was running Windows ME. On restarting, it was back to the usual snail’s crawl.


  1. How is it possible for a 56k modem to go that fast? What is actually regulating the speed?
  2. Why wasn’t I getting that sort of speed, or even 56kbps, before?
  3. Can you deliberately set up a 56k modem to go that fast, and will you be in legal trouble if you do? (I have no interest in doing this, I’m just wondering)

The most likely explanation is that the particular file you were downloading contained a large amount of redundant data, which allowed for a very high compression rate.

  1. Your modem wasn’t actually going that fast. I’m particularly suspicious about the crash that occured as well. I suspect something was screwing up ME’s timing and it was reporting the speed wrong. It could also be that you have some sort of data compression thing running. A lot of “high speed” phone lines work this way. You aren’t actually getting higher speed than what the modem is capable of, but because the data is compressed on the other end and decompressed when it gets to you, you can get a slightly faster data rate, assuming that the data is the type that compresses well.

  2. Unless you can look out your window and see the nearest telephone switching station, you’re not likely to get much above 30k. The way the phone company limits the signal bandwidth, you’re not even going to get 56k even under the best possible conditions, though you can get close. Back when I was using a 56k modem, I got connect speeds anywhere from 28 to 33, but never above that. Like I said above, I think the speed you got was either not reported correctly, or was due to data compression skewing the numbers.

  3. No, you can’t, legally or otherwise. It’s a signal limitation that you are fighting, not an artificial bandwidth limitation.

Hmm. The file was downloading much faster, though. It was fairly large (a few megabytes in size, but I can’t remember how many) and would have taken an hour or so, but finished in a few minutes. So even if the speed was being misreported there was something else going on too. Our phone line is not high-speed, do you suppose some weird glitch caused the high speed? (but as you said, there aren’t a whole lot of possible ways for that to happen…)

Also, this was during the old computer’s last days when it was crashing 2-3 times a day, so there’s really no way to tell if the crash afterwards (not IMMEDIATELY afterwards, but after a minute or so) was something out of the ordinary or just another crash.

I agree it was most likely due to compression, but there may be another reason, your dialup connection dropped and you connected via a wireless network - unlikely but it is possible if you have a wireless card and wasn’t paying too much attention.

Take this from a guy who actually worked on US Robotics 56K modem (X2): There is absolutely no way you were downloading at any rate greater than 53K, and that was under ideal conditions only. Best conditions saw 48K, and if your call landed in a T1 channel using robbed-bit signaling (1 in 6 chance on any call), you would lose between 1.3333K and 4KB. More typical analog runs to the CO would get you in the 30-40KB range; this would include most rural locations, though when I used dial-up to access email (suburban location) I’d routinely connect at 42KB.

You can check the rate of the last call via an AT command (assuming you can open a serial port to your modem). For USR/3COM modems, I believe ati6 gave you last link diagnostics, but don’t quote me…and I doubt they’d survive a system reset anyway.

As other posters have noted, if you were depending on the Windows thermometer to estimate download time, you were probably benefitting from file compression. If the file was, say, ASCII text, I wouldn’t be surprised…