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Path: utzoo!henry
From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: comp.dcom.modems
Subject: Re: 8-bit byte can be sent w/o xform ?
Message-ID: <8804@utzoo.UUCP>
Date: Tue, 20-Oct-87 14:27:18 EDT
Article-I.D.: utzoo.8804
Posted: Tue Oct 20 14:27:18 1987
Date-Received: Tue, 20-Oct-87 14:27:18 EDT
References: <167100015@uiucdcsb> <8215@sri-unix.ARPA>, <123@uwspan.UUCP>
Organization: U of Toronto Zoology
Lines: 24

> ALL modems send whatever bit pattern is sent to them by the computer.  If
> the computer sends 6 bit data to the modem, the modem will send 6 bit data.
> similarly, if the computer sends 8 bits, the modem does too.

Um, not entirely true.  300-baud modems did just faithfully send every bit
the user threw into them.  They just altered the state of the signal they
sent out on the wire every time the signal coming in changed.  This is
NOT TRUE for 1200-baud modems, and still less so for faster ones.  The
modem receives the data, pulls it apart and puts it back together in a
form more suited to transmission.  For example, "1200 baud" modems really
send 600 two-bit lumps per second, and the actual transmission is done
synchronously rather than asynchronously, with the modems hiding all this
from the users.  (This is why autodialing is normal in modems now -- the
fast modems have to be smart anyway, and adding autodialing costs little.)
So the modems do have to understand the data coming in, and hence there
are limits on its format.  Fairly broad ones, fortunately, for most modems.

If you want a gross case of this, consider that the Telebit Trailblazer
knows what uucp packets look like, and the actual data on the wire of a
Trailblazer uucp connection only vaguely resembles what the hosts see --
the modems are using a totally different packet scheme between themselves!
-- 
PS/2: Yesterday's hardware today.    |  Henry Spencer @ U of Toronto Zoology
OS/2: Yesterday's software tomorrow. | {allegra,ihnp4,decvax,utai}!utzoo!henry