My friend uses a twin port serial card that purports to be based on 16550AFN
uarts and a V.34 modem with a DTE speed of 38k4bps. The modem reliably
connects at 28k8bps to his ISP. Using Win95 he routinely achieves ftp put
and get transfer rates that are close to expected values. While using Linux
kernels 1.2.13, 1.3.77 and now 1.3.85 he achieves something that very nearly
approximates 19k2bps. The 19k2bps is consistently repeatable. I've double
checked that he is indeed achieving a DTE speed of 38k4bps.
Interestingly, on a punt, we tried a simple terminal connection and used
lsz/lrz to test transfers in either direction. These too peg out at just
below 19k2bps. Again, a similar test under Win95 gives results much closer
to what would be expected for the hardware configuration.
It seems likely to me that the problem my friend is experiencing has little
or nothing to do with the tcp/ip networking code, but rather something more
closely akin to the serial driver code.
We are still working on further isolating the problem but are rapidly running
out of ideas. Is it possible that there is some problem with support of
some breeds of 16550 uarts ? We haven't yet tried swappping the serial card,
that will be my next suggestion for him.
Terry