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From: morelos@uhunix1.uhcc.Hawaii.Edu (Robert Morelos)
Newsgroups: comp.dcom.modems
Subject: Re: Error correction vs Error control
Message-ID: <10528@uhccux.uhcc.Hawaii.Edu>
Date: 7 Dec 90 04:17:38 GMT
References: <1990Dec6.115526.8595@nntp-server.caltech.edu>
Sender: news@uhccux.uhcc.Hawaii.Edu
Organization: University of Hawaii
Lines: 33

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