Re: 28.8K modem, pppd and Multiface III card

[Bruce Drake <bruce%synxcti.com@localhost>]

On Tue, 20 Aug 1996, Jose Alburquerque wrote:

> 
> Does anyone know what's the fastest baud rate (or recommended rate) that my
> ppp connection can support with a 28.8K SupraFax modem and a Multiface III
> I/O card?  I've tried 115200 in my `/etc/ppp/options' file and I keep getting
> "/netbsd: mfcs0: fifo overflow".  I rarely got this message when I used my
> old 14.4 Practical Peripherals modem, but with my 28.8K Supra the message is
> repeated a lot more times than what I'm comfortable with for a 28.8K modem. 
> I have an Amiga 2000/030 with 2+13 MB and a 2MB Picasso II graphics card. 
> Any suggestions?  Thanks.
> 
> -Jose
> 

My personal experience with the same card in mu A3000/25 is similar to
yours.  However, as I understand it, there are several reasons why:

1) Although the USARTS on the Multiface III board contain hardware FIFOs,
the erstwhile author of the driver for NetBSD-Amiga may not have used
them well (thank you for making this card work!).  Or...

2) The NetBSD kernel is not responding to the interrupts quickly enough.
It may be too busy with something else (i.e. swapping) to respond to
serial interrupts quickly enough.  

The reason you didn't see the problem with your Practical Peripherals
14400 bps modem was that although the modem's port was at 115200, the
modem was delivering the characters to the port at the _equivalent_ rate
of 14400 bps (plus effects of any compression).  The same principle is in
effect with the 28800 bps modem, except that the effective data rate is
twice as fast as before.  It is primarily this effective data rate that is
causing the FIFO buffers to overflow.  However, setting the serial port to
lower rates of speed places a ceiling on the rate the modem will transmit
characters to you, giving you an effective work-around.

Actually, I favor the second reason above because there was some
discussion a few weeks ago about the performance of serial ports and new
kernel development.  Seems the problem got worse for a while, not better. 
And it was due to kernel interrupt issues, not device driver issues.  I
have not heard whether this problem was corrected or not.  I generally
don't load nor use the alpha and beta releases.

To answer your question more directly, I stick with 38400 bps to the
modem.  Most communications you are likely to have over PPP will not take
full advantage of the modem's compression anyway.  You may try 57600, but
I had problems at that speed myself.  Also, keep the cables short at high
speeds to minimize PPP & TCP/IP retransmissions.

Commentary: IMO, It is still better to use the Multiface III card, even at
38400 bps rates because the CPU isn't as busy trying to operate the
Amiga's internal serial port, which is somewhat more CPU intensive.  This
way, you will have the most CPU cycles available for whatever else you are
doing.

Are there any other experiences out there?  I wouldn't mind hearing from
you myself.

------------------
Bruce Drake
Synectics, Inc.
(919) 872-1275 USA