Newsgroups: comp.dcom.modems Path: utzoo!henry From: henry@utzoo.uucp (Henry Spencer) Subject: Re: V.42 [was: Telebit in Byte's test, Ventel PEP, V.32] Message-ID: <1988Aug26.174021.23655@utzoo.uucp> Organization: U of Toronto Zoology References: <1313@neoucom.UUCP> <5916@bigtex.uucp> <6069@bigtex.uucp> <8808162000.AA04427@yorkville.csri.toronto.edu> <15303@shemp.CS.UCLA.EDU> <8808251940.AA00343@ellesmere.csri.toronto.edu> Date: Fri, 26 Aug 88 17:40:21 GMT In article <8808251940.AA00343@ellesmere.csri.toronto.edu> ray@csri.toronto.edu (Raymond Allen) writes: >provision for the use of a 32-bit CRC (CCITT calls it >the "Frame Check Sequence") was added to the V.42 specification. This is nice >from a technological standpoint since it will reduce the bit error rate to >about 10e-239 (note: :-) ) but from an implementation standpoint it is a real >b**ch because there is hardware in existence that will calculate the 16-bit >CRC but the 32-bit one would have to be done in software and this has to >be done for each character transmitted -- a rather large overhead. >Seems that (as usual) standards committies have reduced awareness of real- >world considerations. Ho ho. Not so. The standards committees are more aware of real-world considerations than you are, in this case. The fact is, 16-bit CRCs *ARE NOT ENOUGH* for some types of modern modems, and this problem will only get worse. Things like RF modems will often send a substantial number of bits as a single transition, which means that one noise hit can foul up rather a lot of bits. 16-bit CRCs fail with significant frequency in this sort of environment; there was an uproar in the IBM mainframe world a while ago when clear proof was produced that IBM's 16-bit CRC did not dependably detect errors in bulk transmission over high-speed networks. In this case, the standards committee is being farsighted, and is accepting implementation problems today for the sake of reliable functioning tomorrow. (Well, they may have *done* it because 32 bits sounded sexier, but the net result is favorable regardless of real motives.) -- Intel CPUs are not defective, | Henry Spencer at U of Toronto Zoology they just act that way. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu