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Path: alliant!steckel
From: steckel@alliant.UUCP (Geoff Steckel)
Newsgroups: mod.telecom
Subject: error correcting modems
Summary: Error correction without notification is a road to disaster
Keywords: modems protocols error correction
Message-ID: <430@alliant.UUCP>
Date: 23 Feb 87 00:43:35 GMT
Reply-To: steckel@alliant.UUCP (Geoff Steckel)
Distribution: na
Organization: Alliant Computer Systems, Littleton, MA
Lines: 24

A number of vendors have introduced "error correcting" modems recently.
I have heard of none that notify the user that correction has taken place.
Some manufacturers actively discourage users from using error correction
protocols above their modems!  None, of course, detail the manner in which
they provide this service.  From experience and reading in networking,
this is a disaster waiting to happen.

Simply: no error correction scheme is perfect.  If an uncorrectable error
occurs, the modem has only two alternatives: break the connection, or silently
pass on bad data.  Alternative (1) is used by the well-known ISO protocol X.25.
Alternative (2) is used by error correcting modems.

A naive user who 'knows' his file is perfect, since his modem corrects errors
and therefore doesn't check his data will be rudely surprised some day when
the local telco surpasses itself in line awfulness.

Anyone who doubts how much trouble a supposedly error-free link can get a
system into should study the ARPAnet (now internet) archives, especially every
time a new link protocol was introduced.  The ISO networking layers all have
the problem of 'you can't know' if any errors happen in lower layers.

Some sort of education of the world would help manufacturers and users, but
where do we start?
	Geoff Steckel (consultant troublemaker) steckel@alliant.UUCP