PPP or compressed SL/IP with v.42bis modems on Sun SLC

[Kelvin Don Nilsen]

The latest generation of high-speed modems available from several vendors
uses data compression in cooperation with full-duplex v.32 9600 baud lines
to achieve transmission rates of up to 38.4 kbaud.  Since not all data can
be compressed by the 4-to-1 ratio and since error correcting modem
protocols occasionally require retransmission of certain information, the
actual transmission rate is usually somewhere between 9600 and 38.4K baud.
Modem manufacturers recommend that the modem be connected to a computer at
38.4 kbaud and that RTS/CTS full-duplex handshaking be used to pace the
bidirectional communication dynamically.  Please note that this is not the
traditional use of RTS/CTS.  It is a newer full-duplex standard that is
apparently being standardized by an ANSI committee.  It works like this:

	the modem asserts CTS when it is ready to receive
		data from the computer and lowers CTS otherwise
	the modem listens to RTS and only transmits to the
		the computer if RTS is asserted.

	the computer asserts RTS when it is ready to receive
		data from the modem and lowers RTS otherwise
	the computer listens to CTS and only transmits to the
		modem if CTS is asserted

I do not have a SLC with which to experiment.  I'm trying to decide
whether to buy one.

I've been told that Sun serial ports support only the traditional
interpretation of RTS/CTS.  From experimentation with a Sun 3/50, this
appears to be true.  But I'm hoping some of you might be able to speak
with more authority and/or experience.  These are the questions that I
have:

1. Is it really true that the SLC (and SparcStation I+/II) do not support
   the newer full-duplex RTS/CTS protocol?

   a. If so, is this a hardware or software limitation?

   b. I've seen mention of software patches available from, e.g.  Telebit to
      support high-speed modem usage.  What do these patches fix?  this?

2. I'm aware of several 3rd party vendors of serial ports for the SBUS.
   All of these vendors tell me that their ports and drivers support the
   full-duplex RTS/CTS protocol.  Do any of you have good or bad experiences
   using these?

3. What is the "architecture" of the serial port devices in the
   Sparcstation family?  Does the CPU get an interrupt on every single byte
   transmitted or recieved?  or do the ports have some sort of FIFO?  if so,
   how large is the FIFO?

4. PPP and SL/IP have some small degree of robustness.  Are they robust
   enough to allow occasional loss of characters due to overflowing of
   buffers?  (This is a subjective question, to be answered possibly by
   someone who may have set one or two of these systems up in a configuration
   that simply allows transmitting devices to ignore "XOFF" requests from
   receivers)

5. Lastly, my main interest in this is to set up a computer at home that
   provides the same X/NeWS user interface that I'm using at work and
   provides access to all of the same data and executable files.  I do not
   need a lot of processing capability at home.  If the SLC won't do it,
   maybe I need to look for a cheap SBUS system like a used SparcStation I or
   the new CompuAdd clone so that I can install a 3rd party "intelligent
   port" board.  Any recommendations?

If there is sufficient information gathered and interest expressed, I will
summarize.  Thanks much for whatever help and information you are able to
provide.

Kelvin Nilsen/Dept. of Computer Science/Iowa State University/Ames, IA  50011 
 (515) 294-2259   kelvin at cs.iastate.edu  uunet!atanasoff!kelvin