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From: ch@dce.ie (Charles Bryant)
Newsgroups: comp.dcom.modems
Subject: Re: Clarification on MNP4 and Everex 24+
Message-ID: <1991Feb7.155351.494@dce.ie>
Date: 7 Feb 91 15:53:51 GMT
References: <8294@ucdavis.ucdavis.edu> <3762.27aaeb7e@hayes.uucp>
Organization: Datacode Communications Ltd, Dublin, Ireland
Lines: 24
Supersedes: <1991Feb7.143619.29816@dce.ie>

In article <3762.27aaeb7e@hayes.uucp> tnixon@hayes.uucp writes:
[In error correctin protocols...]
>... 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.

With MNP4 this is not the case. The optimal distribution of packet sizes
is to start with big packets and finish with smaller ones. For example
with a standard setup (MNP4, tty->modem at 19200, modem<->modem at 9600)
the optimal distribution for sending 64 bytes is 45, 17, 2. The actual
sizes should vary according to the amount of fixed packet overhead (e.g.
MNP4 in use or not) and the amount of overhead that varies linearly with
the number of bytes in a packet (e.g. MNP2 = 10, MNP3 = 8). If the
terminal speed is less than the line speed, the packets might as well
send all pending data since there won't be much.
-- 
Charles Bryant (ch@dce.ie)
--
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