fjs@nntp-server.caltech.edu (Fernando J. Selman) (12/06/90)
I am confused about the terminology. Is there a modem with real error correction scheeme, say, 2 errors detection one error correction? Or do they just retransmit after one error detection? Thanks, - Fernando
grr@cbmvax.commodore.com (George Robbins) (12/07/90)
In article <1990Dec6.115526.8595@nntp-server.caltech.edu> fjs@nntp-server.caltech.edu (Fernando J. Selman) writes: > I am confused about the terminology. Is there a modem with > real error correction scheeme, say, 2 errors detection one > error correction? Or do they just retransmit after one error > detection? Thanks, Conventional modems (protocols actually) rely on error detection and retransmission, since this has the lowest overhead over a a good channel. Special purpose modems/protocols such as those used for radio, satellite or other exotica also use forward error correction as you describe when channel reliablilty is more important than nominal thruput or when retransission is either impossible or requires too long a delay. The two may also be combined where errors too severe be corrected by the error correction coding cannot be tolerated. -- George Robbins - now working for, uucp: {uunet|pyramid|rutgers}!cbmvax!grr but no way officially representing: domain: grr@cbmvax.commodore.com Commodore, Engineering Department phone: 215-431-9349 (only by moonlite)
morelos@uhunix1.uhcc.Hawaii.Edu (Robert Morelos) (12/07/90)
In article <1990Dec6.115526.8595@nntp-server.caltech.edu> fjs@nntp-server.caltech.edu (Fernando J. Selman) writes: >I am confused about the terminology. Is there a modem with >real error correction scheeme, say, 2 errors detection one >error correction? Or do they just retransmit after one error >detection? Thanks, > > - Fernando Usually, only error-detection is used in most modems, by simply adding parity-check bits at the end of each transmiited data block. Modems for the CCITT X.25 recommendation for packet-switched data networks use a distance-4 cyclic Hamming code with 16 parity-check bits for error detection/correction. The polynomial used is either g(x)=x^16 + x^12 + x^5 +1 or g'(x)=x^16 + x^14 + x + 1 In theory, this code is capable of correcting any single bit error or detecting any 2 bit errors. Usually, as I mentioned before, they are used only for error detection. The latest trend in modem technology is to combine error control coding with modulation and equalization to obtain bit rates almost as close as the capacity of the channel. For example, CODEX's latest modem is capable of transmiting about 23,000 bits/sec over the telephone channel! Robert Morelos University of Hawaii robert@wiliki.eng.hawaii.edu
Norman_J_Gillaspie@cup.portal.com (12/09/90)
Does anyone know of a program that adds forward error correction to a file. What I am looking for would be like a Zip file compression program but instead og compressing a file the program would add FEC. Regards Norman Gillaspie 415-967-0833 fax 415-967-0772