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From: mangler@cit-vax.UUCP
Newsgroups: comp.dcom.modems
Subject: Re: Network protocols questions
Message-ID: <2160@cit-vax.Caltech.Edu>
Date: Mon, 30-Mar-87 03:02:24 EST
Article-I.D.: cit-vax.2160
Posted: Mon Mar 30 03:02:24 1987
Date-Received: Tue, 31-Mar-87 04:45:58 EST
References: <12289499503.24.SY.FDC@CU20B.COLUMBIA.EDU>
Distribution: world
Organization: California Institute of Technology
Lines: 18
Summary: convolutional codes

In article <12289499503.24.SY.FDC@CU20B.COLUMBIA.EDU>, SY.FDC@CU20B.COLUMBIA.EDU (Frank da Cruz) writes:
> Does anybody know what kind of protocols and error control techniques are used
> to send back data from space (like from Uranus and Neptune), where corruption
> is very likely, and retransmission is very expensive?

They use convolutional codes, often with a rate of 1/2 (i.e. half of
the bits sent are data, the rest are error correction).  These are
serial codes, with the next error correction bit depending on the last
N data bits.  As N gets large, the code can handle longer error bursts,
and a higher percentage of errors, at immense decoding cost.  With such
codes you can approach the Shannon limit, which is why they're used in
deep-space communications, where transmission power is dear.

If you wanted to make a dialup modem with continuous full-duplex
9600-baud throughput, that's what you'd have to do.  ISDN should
eventually obviate the need for it, though.

Don Speck   speck@vlsi.caltech.edu  {seismo,rutgers,ames}!cit-vax!speck