ppp lost packets - 16850 uart

Hi there,

I'm trying to setup a test environment that consists of two linux
machines networked using a serial connection using ppp. I bought two
high speed serial cards using the 16850 uart to do this. I'm having
some problems achieving the throughput I'm expecting.

With an ftp transfer with the link running at 115.2K I get about
6kbytes/sec using the 16850 equiped cards. I expected closer to
13kbytes/sec. I can achieve this rate (13K) when I create a 115.2K
link using the inbuilt serial ports (16550 uart) in the two machines.

I did a tcpdump on the ppp connection using the 16850 equiped cards,
performed an ftp transfer, and looked at the results. I found every
5th packet sent was being lost. I then halved the mtu size of the link
and retried and still every 5th packet sent was being lost. I had
thought that if the loss was related to something like a buffer
overflow or time related I would have found approx. every 10th packet
lost with the halved mtu size.

Does anybody have any ideas as to what may be going on or what I
should look at ?

I am using the 2.6.7 kernel. I started off trying to use 2.4.20 but
had problems with it's serial driver and the 16850 uart serial cards.

Alan