Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!utgpu!water!watnot!watmath!clyde!cbatt!cbosgd!ulysses!ucbvax!grodberg@seismo.CSS.GOV@kodak.UUCP From: grodberg@seismo.CSS.GOV@kodak.UUCP Newsgroups: mod.telecom Subject: Submission for mod-telecom Message-ID: <8703092348.AA23358@kodak.uucp> Date: Mon, 9-Mar-87 18:48:13 EST Article-I.D.: kodak.8703092348.AA23358 Posted: Mon Mar 9 18:48:13 1987 Date-Received: Thu, 12-Mar-87 22:44:48 EST Sender: daemon@ucbvax.BERKELEY.EDU Distribution: world Organization: The ARPA Internet Lines: 57 Approved: telecom@xx.lcs.mit.edu Path: kodak!grodberg From: grodberg@kodak.UUCP (jeremy grodberg) Newsgroups: mod.telecom Subject: Re: error correcting modems Summary: Error correction with notification is superfluous. Message-ID: <733@kodak.UUCP> Date: 9 Mar 87 23:48:12 GMT References: <8702230043.AA02476@alliant.Alliant.COM> Reply-To: grodberg@kodak.UUCP (jeremy grodberg) Distribution: world Organization: Eastman Kodak Co, Rochester, NY Lines: 44 In article XXX steckel@alliant.Alliant.COM (Geoff Steckel) writes: > >A number of vendors have introduced "error correcting" modems recently. >I have heard of none that notify the user that correction has taken place. >... From experience and reading in networking, this is a disaster waiting to >happen. From my experience with error correcting modems, notification of error correction would be pointless. First, at high speeds (4800 baud and up) over normal phone lines, there is error correction taking place very frequently (more often then every 1000 characters). Thus the notification would be so frequent as to be routinely ignored, and therefore pointless, totally leaving out the fact that such notification would necessarily make the modem non- transparent, which would greatly reduce it usefullness. Besides, notification that error correction has take place has nothing to do with passing along incorrect data. Incorrect data comes from so many errors ocurring in the same block that the error correcting scheme gets fooled into thinking some erroneous data was correct because "it checks." True, some good data may be trashed as well, but it is also possible that the mistakes will fall so that no error correction takes place at all. Certainly, the fact the error correction took place is not at all a good indicator that bad data was passed, and in many error correction schemes, it is not possible to know when an uncorrectable error has occurred (although there are some that can catch a high percentage of such errors, none can guard against all possible error sequences). It may be adviseable to use a low-power error correction scheme or error detection scheme with ECC modems, but I'm not sure. We have been routinely transmitting around 750K of binaries a day at 4800 baud (with error correction and data compression) via Motorola S-records, which provide very good error detection. We have been doing this 5 days a week for over two months, and have NEVER detected an uncorrected tranmission error, even though we frequently have error correction taking place in every block (512 characters). If an error correcting scheme can promise me no more than one uncorrected error every six months of use, I'll be happy. Nothing is perfect, and it is dangerous to make things almost perfect, because it leads to humans putting too much faith in such systems (It has to be right, the computer says so.). -Jeremy Grodberg Usenet: ...rochester!kodak!grodberg Arpa: grodberg@kodak or kodak!grodberg@rochester