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Re: CRC



In a message dated 01-06-02 11:25:08 EDT, you write:

> 
>  Could someone give me a reference /discussion on the meaning of CRC'c
>  referenced for the data content of AO-40 Telemetry?  thanks, Farrell 
Winder,
>  W8ZCF.

The CRC is a Cyclic Redundancy Check.  It's a way of verifying
that you received what the sender sent.  This is done using a
set of mathematical operations that takes as input the bytes
you are sending and outputs a "CRC" residue.   The sender
computes and sends this, and the receiver also computes it.
If they match, then you probably received what they sent (with
a high but not 100% probability).  Ok, I've greatly simplified,
but this is the basic idea.  It's actually a bit more complicated.

The AMSAT CRC as used on AO-40 is based on the 
polynomial x^16 + x^12 + x^5 + 1.

More details are available on Amsat's web site at:

http://www.amsat-dl.org/p3d/tlmspec.txt

in section 7.  If you're really interested in CRC
math, the calculations, the various CRC-16's, 
CRC-32, and computing lookup tables, I suggest
"C Programmers Guide to NetBIOS" by
David Schwaderer.  (Yes, it's a NETBIOS book, 
but the back quarter or so of the book is all 
about CRC's.)

Hope this helps,

Douglas KA2UPW
dquagliana@aol.com
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