On Sat, Jun 26, 2004 at 11:33:05PM +0100, Howard Winter wrote:
> Kyrre,
>
> On Sat, 26 Jun 2004 17:50:44 +0200, Kyrre Aalerud wrote:
>
> > Is this better or wose than the sum method ?
>
> If I remember rightly (it's late so I can't think
> clearly enough to check!) if the last byte transmitted
> is the checksum, the result of a rolling XOR of
> everything before, when you XOR it you will end up with
> zero if all was sent correctly.  So the receiver just
> XORs everything it receives and the result should be
> zero - easy!

Right. The question is how effective? Just conceptually XOR has a major
problem: 1) paired bit errors in the same column can result in an erroneous
packet with a valued checksum.

Consider the three byte stream ....1 ....1 ....1

When the LSB's are XORed the resumt is a 1. But when added the result is three.
Adding affects other columns in the checksum whereas XOR can only impact the
column it's in. Consider if there were two bit errors: ....0 ....1 ....0
The XORed result is still 1. Whereas the summed result is also 1, and would
indicate an error.

>
> > Maby I can do both ?
>
> No need - it doesn't improve anything.

Agreed. However it's best to pick the added result. Since adding and XORing
cost exactly the same, why not use the sum?

BAJ

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