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Path: utzoo!mnetor!uunet!tektronix!decvax!decwrl!pyramid!fmsrl7!mibte!ccd700!reh
From: reh@ccd700.UUCP (reh)
Newsgroups: comp.dcom.modems
Subject: Re: 8-bit byte can be sent w/o xform ?
Message-ID: <713@ccd700.UUCP>
Date: Tue, 13-Oct-87 12:31:27 EDT
Article-I.D.: ccd700.713
Posted: Tue Oct 13 12:31:27 1987
Date-Received: Thu, 15-Oct-87 21:17:41 EDT
References: <167100015@uiucdcsb>
Organization: Ford CCD, Dearborn Michigan
Lines: 32
Summary: 8 bits vs 7 bits - let me explain

In article <167100015@uiucdcsb>, kkim@uiucdcsb.UUCP writes:
>    as i understand, most 1200-bps modems send data in the unit of
> seven data bits preceded and followed by some start and stop bits.
> there is usually one parity bit which may or may not be used.
>    it seems that one 7-bit ascii character is sent as one unit.
> however, binary files which uses all 8 bits are usually transformed
> before being sent - that is, 3 8-bit bytes are transformed to 4 6-bit 
> units, slightly modified (adding some number to make it non-control
> character), and then sent as 4 units, not 3.
> kyongsok kim
> arpanet/csnet: kkim@b.cs.uiuc.edu
> usenet/uucp  : ... {seismo, pur-ee, ihnp4}!uiucdcs!kkim

Modems ussually send a start bit, 8 bits, stop bit, where the 8 bits
are whatever your computer sends to the modem.  Your computer can be
set to send 7 data bits and a parity bit, or 8 data bits and no parity bit.
(other combinations are possible but not as common)
No modem I know of is smart enough to try to take apart 3 8-bit bytes and
pack them into 4 7-bit characters, you would need a program on your
computer to do that (like uuencode). 
   The TYMNET network does convert 8-bit characters to 7 bit characters
and sends the 8th bits as an extra character after each 7 in it's internal
network, but that is transparent to the user and does not reach the user's
modem. (user runs normal 8 bits)  That is the only network that I know
that does that kind of conversion.
......................................................................
Bob Harold                      313-845-5404   
Ford Motor Co., DPTC room B-206 ...!ihnp4!mibte!ccd700!eed090!bob
17000 Rotunda Drive             Disclaimer: The views expressed might
Dearborn, MI 48121-6010         not be those of my employer or myself
Have questions about life?  Read the original design manual - the Bible.
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