kaufman@shasta.stanford.edu (Marc Kaufman) (07/26/86)
Newsgroups: mod.protocols Subject: Re: PROB CRC-FAIL on VMSHARE Summary: Expires: References: <WANCHO.12224727681.BABYL@SIMTEL20.ARPA> Sender: Reply-To: kaufman@Shasta.UUCP (Marc Kaufman) Followup-To: Distribution: Organization: Stanford University Keywords: CRC-16 is extremely susceptible to even-number-of-bit errors. For example, the following error pattern will not change the CRC: 10000100000010001 (binary), where '1' indicates a bit error. The newer high-speed modems use 64-QAM or 256-QAM modulation schemes, so the right kind of error in 2 or 3 adjacent frames can cause a problem that still passes CRC. The solution is to post-scramble the bits before the modem sees them, so that adjacent bits are separated by many frames, thereby reducing the chances that a burst error of a few frames will allow the CRC to pass. Unfortunately there is no hardware to do this that I know of. Marc Kaufman (kaufman@shasta.stanford.edu)