steckel@alliant.UUCP.UUCP (02/23/87)
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
davidsen@steinmetz.steinmetz.UUCP (William E. Davidsen Jr) (03/05/87)
In article <KPETERSEN.12283065282.BABYL@SIMTEL20.ARPA> steckel@alliant.UUCP (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. 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. I agree with you premise, but I *think* you're working from a false starting point. The error correcting modems *seem* to work by sending a packet and doing a resend if it's munged in transit. I get the impression that you are assuming error correction ala Hamming or Fire codes. The prime bebefit of error correction is when using the modems in an interactive manner, eliminating the "babble" which so often goes with a connection these days. It is not a substitute for other methods currently in use. It also allows non-critical data to be sent by just blasting the data out, similar to cu text tranfers. This improves the throughput considerably for files which do not contain vital data. I would be willing to trust the modems on a news posting, or casual conversation, but would still use additional error checking for sending the draft of a contract or a buy order to my broker. Forgive me if you knew how these modems work and still think another level of error checking is evil. -- bill davidsen sixhub \ ihnp4!seismo!rochester!steinmetz -> crdos1!davidsen chinet / ARPA: davidsen%crdos1.uucp@ge-crd.ARPA (or davidsen@ge-crd.ARPA)
jhh@ihlpl.ATT.COM (Haller) (03/09/87)
In article <1264@steinmetz.steinmetz.UUCP>, davidsen@steinmetz.steinmetz.UUCP (William E. Davidsen Jr) writes: > In article <KPETERSEN.12283065282.BABYL@SIMTEL20.ARPA> steckel@alliant.UUCP (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. 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. > > I agree with you premise, but I *think* you're working from a > false starting point. The error correcting modems *seem* to work > by sending a packet and doing a resend if it's munged in > transit. I get the impression that you are assuming error > correction ala Hamming or Fire codes. See the Febrary, 1987 Data Communications, page 62. It is possible that if the software uses the same polynomial that the modem does that no further detection will be available by the software. The case described in the magazine had a modem whose scrambler had the same polynomial as the BISYNC protocol above it. It was a noisy link, and large amounts of undetected errors were being passed. I would not trust CRC-CCITT for error detection of that was what the modem used for its error detection. Maybe it would work, and maybe it wouldn't. The point is, when there is a noisy line, you probably want to know about it, so as not to run into cases where the CRC algorithms don't work. An occasional bit error is fine, but an extremely noisy line should not be used, expecting the CRC to find all errors. John Haller ihnp4!ihlpl!jhh AT&T Bell Laboratories