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Re: [TCLUG:3624] A new subscriber...



> I use Xisp and it works great for my connection.  I do have a general
> question though.  I have my modem set for 115200 and it claims to
> connect at that speed, however, my download is anywhere between 3k/sec
> and 9k/sec.  I thought it was impossible to transmit at speeds over 56k
> due to limitations within the phone lines??? Does anyone know what the
> formula for converting k/sec into an actual baud speed?

Well, for one thing, the 115200 is the speed from your computer to the
modem, not the line speed.  There's probably an AT command somewhere
that will report the actual line speed when you connect (On my Zoom
Modem, I think it's something like 'ATS95=44' or something quite
similar..  Don't have my manual, though..)

Now, let's get to the math:
  57600 kbps is the (illegal) maximum of these modems.  It's the only
number I can remember off the top of my head, so I'll use that ;-)  Now,
first, we divide that by 8 to get the speed in kBytes/second.  57600/8 =
7200 <-- 7.2 kB/s!  But that's just the line speed.  There's overhead in
the PPP protocol, so probably multiply that by 0.9 or 0.8 to get the
actual amount of data going over the line.  7.2*0.9 = 6.5 kB/s  Of
course, there's some modem-to-modem compression going on, and that's how
you can occasionally get higher transfer rates..

Obviously, a file created by reading /dev/zero will compress *really
well* and you could probably get very high transfer rate with that ;-) 
I recall transferring an easily-compressible text file over my old
2400bps modem that had compression.  I actually managed a transfer rate
over 2kB/s!
-- 
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|               Mike Hicks | Linux User Since: 1.2.13
: http://umn.edu/~hick0088 | mailto:hick0088@tc.umn.edu
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