NetNews Usenet Archive 1992 #19/NN_1992_19.iso/spool/comp/dcom/modems/12833

Internet Message Format | 1992-09-01 | 2.7 KB

Path: sparky!uunet!hayes!tnixon
From: [e-mail]
Newsgroups: comp.dcom.modems
Subject: Re: v.42bis in sync mode
Message-ID: <[e-mail]>
Date: 31 Aug 92 17:48:36 EDT
References: <[e-mail]>
Organization: Hayes Microcomputer Products, Norcross, GA
Lines: 44

In article <[e-mail]>,
[e-mail] (Peter Graw) writes:

> Is there any reason why v.42bis (compression is what I'm after) is not
> available on modems when running in sync mode? In particular on the
> Zyxel U-1496S.

For data compression to work, every bit of the compressed data must
be received without error, in sequence, without duplication.  On
noise-prone telephone lines, this means that an error-control
protocol must be used.  Existing error control protocols such as
MNP4 and LAPM are designed only for use with character-oriented
(start/stop) data; they are, in effect, alternatives to the V.14
non-error-control method of carrying async data on a synchronous
physical connection.  But you say, my synchronous connection
DOES use error control!  Unfortunately, that doesn't help, because
the protocol must be under the control of the MODEM for data
compression to possible.

There are two possibilities.  One is to "spoof" the terminal-to-host
synchronous protocol in the modems.  This might be effective in a
few limited applications, but is not general-purpose enough for me.
A second idea is to take the synchronous bit stream from each DTE,
convert it into a sequence of bytes that can be transported by LAPM,
and send it through the modems as though it were start/stop data
(including compression), recreating it as a synchronous stream on
the other side.  That way, the modems don't "terminate the protocol"
(spoof), but the do potentially introduce unexpected propagation
delay when retransmission as necessary due to line noise.  It may
also be necessary for the modems to exert flow control on the DTEs
during retransmissions, and there is no standard provision for this
on normal synchronous interfaces; one possibility is slowing down or
stopping the transmit clock.

This is a very interesting area of study, and one that I hope to
pursue more deeply during the 1993-1996 period in CCITT Study Group
XVII (in the V.42 rapporteur's group).

--
Toby Nixon, Principal Engineer      | Voice   [phone]  Telex 401243420
Hayes Microcomputer Products, Inc.  | Fax     [phone]  CIS   70271,404
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