Path: utzoo!mnetor!tmsoft!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!uunet!hayes!tnixon
From: tnixon@hayes.uucp
Newsgroups: comp.dcom.modems
Subject: Re: Clarification on MNP4 and Everex 24+
Message-ID: <3762.27aaeb7e@hayes.uucp>
Date: 2 Feb 91 16:40:30 GMT
References: <3760.27a77a47@hayes.uucp> <8294@ucdavis.ucdavis.edu>
Organization: Hayes Microcomputer Products, Norcross, GA
Lines: 58

In article <8294@ucdavis.ucdavis.edu>, dias@eecs.ucdavis.edu (Gihan
Dias) writes: 

> In article <3760.27a77a47@hayes.uucp> tnixon@hayes.uucp writes:
>> MNP4 has a fixed maximum frame size of 256 bytes, but LAPM can
>> negotiate frame sizes from 16 all the way up to 4096 bytes.
> 
> Does this mean that for an MNP4 modem, if the DTE sends <256 bytes and
> stops, the modem will time out and send a short frame, but if the DTE
> sends >256 bytes, the modem will send a 256 byte frame?

You have it basically correct.  It's difficult to predict how a 
particular error-control implementation will actually divide its 
frames, however.  Hayes modems, for example, start off sending 
shorter frames, then gradually increase the frame size to the 
maximum if the DTE continues sending data.  This significantly
reduces the propagation delay associated with using protocols such
as XMODEM and Kermit over and error-control modem link, since the
entire protocol frame need not be received before it can begin to be
delivered to the remote DCE -- while at the same time not impacting 
total overall throughput during long transmissions.  I don't know 
whether any other manufacturer uses similar techniques.

> When using a protocol like kermit on top of MNP4, what is the best
> packet size to use? Would just under 256 bytes be better or worse than
> just over 256?

It is best to use the maximum frame size possible whenever using 
file transfer protocol on an error-control modem link.  Since the 
modems take care of recovering from errors, you want to minimize the 
number of times the sender has to stop and wait for an 
acknowledgement from the receiver.  Don't be concerned with the 
frame size being used in the modems  -- you're not paying by the 
packet, after all.

> I was trying to use two Everex 24+ modems connected at 1200 b/s in MNP4 mode for a kermit file transfer. I was running the modem-computer interface at 1200
> baud with no flow control. As I assumed the modems will give me a clear

It is very bad to use any error-control modem without flow control!  
All it takes is a bit of line noise, and WHAMMO, you've lost data.  
An error control modem is pretty dang useless if you lose data 
anyway.

> ...
> This seems to imply that the 24+ has an input buffer of <256 bytes
> when MNP4 is enabled.

I don't know what size buffer they use, but it could be small.  
Error control modems provide flow control FOR A REASON, and you 
should use it.  Hayes modems have 256-byte buffers in error-control 
mode.  There are some modems that have much larger buffers, such as 
Microcom (I think).

-- 
Toby Nixon, Principal Engineer    | Voice   +1-404-449-8791  Telex 151243420
Hayes Microcomputer Products Inc. | Fax     +1-404-447-0178  CIS   70271,404
P.O. Box 105203                   | UUCP uunet!hayes!tnixon  AT&T    !tnixon
Atlanta, Georgia  30348  USA      | Internet       hayes!tnixon@uunet.uu.net