Re: [SLUG] download speeds

[rod@xxxxxxxxxxx]

> On Wed, 28 Aug 2002, Ken Caldwell wrote:
> 
> > <quote who="Wienand Ian">
> >
> > > Well, 56k bps = 56 * 1000 bits per second
> > > 56000 bps/ 8 = 7000 bps
> > > 7000 bytes / 1024 bytes (1kB)= 6.8 kB/s
> > Shouldn't it be 56000 bps /10 = 5600 bytes/sec. (allowing for start and
> > stop bits)?
> 
> Everybody forgets about them. :-)
> 
> DaZZa
Thats cause they don't apply.  The start and stop bits only apply in the serial 
connection between the DTE (PC) and DCE (Modem).  The modem to modem 
connection is sync and framed (LAP-M). 

So the data format between the modems ends up being Flag (8bits) + Header (8-
32bits??) + n byte payload + CRC (16?) + Flag?(8bits).

In the end you can ignore the overhead of flags and headers etc and just 
aproximate with connect bitrate / 8.  Compression is hard to factor in as its very 
variable. Particularly with the new V.44 algorithm.

Using an ESS internal I've had 22KBytes/sec (yes bytes!) sustained on a highly 
compressable file (top output to a file). Same modem usually gets a 3-5 sec burst 
of 15kbytes/sec on a self extracting archive also.

HTH
-Rod