James Cameron wrote:
>
> Assuming I'm going to be generating between two to eight words of twelve
> bits each in a data stream over a remote control UHF link, what sort of
> checksum or CRC is appropriate?
>
> I get the impression that calculating a checksum or a CRC on such small
> data streams might be ineffective, because of the probabilities
> involved.
>
> For example; sending 24 bits of data followed by a 12 bit arithmetic
> checksum ... the chances of corruption affecting one of the 24 bits and
> then also one of the 12 bits of checksum to mask the corruption seem to
> be higher.
>
> I'm not exactly sure how to express this ... but is there a concern?


James, CRC's (8,16 or 32) offers a high confidence to your data
transmission, doesn't matter if is only one byte or one million bytes.
If your transmission media corrupts one bit of data (or CRC), it means
you have a *corrupted* data, nothing else, and you need to retransmit
the thing.  Except, of course, if you use an inteligent protocol and
correction codes to *de-corrupt* the bit (or bits).

Your worries about to have a higher probability of data corruption
because transmitting just two data bytes followed by 2 CRC bytes
(CRC-16), meaning that you will double the corruption probability, is
not ... hmmm... nice, since your transmission media should be clean. You
can not think of it as a machine gun shooting gallery, while if your
data is skinny better the chances to survive across the bullets.

If you suspect your media is dirty and noise poluted, this is a strong
reason to use a good CRC system. This is the price to pay.

Some time ago I saw a check system, silly but functional, (I don't
recommend, but...), send every bit followed with its binary complement.
Noise could change just one or multiple bits, and probably, (I will
repeat), probably, noise will not change the complement bit to the right
level, so it will be easy to identify the discrepancy between the
original and the complement bit... it doubles the transmission data size
and not ensure a good check system.

I would stick to a plain and easy CRC16 (any variation of it).

--------------------------------------------------------
Wagner Lipnharski - UST Research Inc. - Orlando, Florida
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