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From: dias@eecs.ucdavis.edu (Gihan Dias)
Newsgroups: comp.dcom.modems
Subject: Clarification on MNP4 and Everex 24+
Message-ID: <8294@ucdavis.ucdavis.edu>
Date: 1 Feb 91 18:59:09 GMT
References: <3760.27a77a47@hayes.uucp>
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Organization: E.E.C.S. Dept. -  U.C. Davis.
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I have a general question on MNP, and a specific problem.

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?

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?

Now for my problem.

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
channel, I assumed I would get best performance by using a large a kermit
packet size as possible which would not overflow the modems' buffers.
I tried a packet size of 512 bytes, (with a window size of 1) but it
didn't work, the packets wern't getting through. With a 250 byte packet
size, I managed to transfer data, but with lots of packet retries.
Things were better when I reduced the packet size to 100.
This seems to imply that the 24+ has an input buffer of <256 bytes
when MNP4 is enabled.

My question is, is this a known problem with the Everex 24+? (Everex
technical support were friendly, but couldn't help). Or am I doing
something wrong? Does anybody know what size buffer this modem has when
MNP4 is enabled? (I did some tests and it seems to have about a 1k
buffer when MNP is not enabled).

Any help or suggestions would be appreciated.

Gihan