>What I mean is that with compression the amount of data that actually
>comes through is less.

Unless you're talking about end-to-end compression, using gzip or
compress at the Unix hosts, that's simply not true.  Modem compression
is transparent outside of the modem-to-modem link.  When using modem
compression (like mnp5 or v.42), the data is _NOT_ compressed on the
RS-232c lines between the UARTs and the modems.


>                       So a higher speed plus less data to send through
>gives much much better ftp performance. Basically a 60K file takes
>2 minutes to come thorugh at 9600,

So how much data is going through the UART to the modem?  About 30K
bytes/minute, or 500 bytes/sec.  that comes out to about 5000
bits/sec, not 9600 (asuming 1 start, 1 stop bit).  You're not
sustaining 9600 baud through the UART to the modem at 9600.


>                                   but only 40 seconds when I kick the
>speed up and get compression to kick in.


That's still only  ~ 1.5k bytes / sec through the UART, which I
compute  to be about 15360 bits/sec (counting 1 start and 1 stop bit).
You're not getting anywhere near 19200 bits/sec through the modem.

You're about 50% below a "speed of light" throughput for the
9600 case, and you don't seem to be getting anything significant
from "compression" in the 19.200 case; at least not much more than
ten percent over an uncompressed 14.4.

I beleive you when you say you're getting overruns at 19.2.  I think
there's much more going on than your analysis suggests.  I'd need a
good datalogger snapshot and tcpdump trace to understand what's going
on here, though.

Are you doing this using PPP, or SLIP?  Since you say you're using
FTP, I assume it's not zmodem, or uucp or (gack) tip's send-file
command.

I'd try (a) disabling rfc-1323 enlarged TCP windows,
	(b) increasing the SLIP/PPP mtu, 
	(c) turning on  header compression.
if you haven't already.

PS: if the files you're FTPing are _alread_ Lepmel-Zif compressed,
with gzip(1) or compress(1), further compression in the modems
slightly _increases_ the amount of data being shipped between modems.