Wagner:

  Say you have a 10-byte fixed length packet. Your receiving
routine would typically wait until 10 bytes are in its buffer, then
apply the CRC routine. If the CRC does not match, you might
empty the buffer, saying that there is garbage coming in, not
packets. Well, the routine may not ever synchronize if you do
this.

  As an example, say the transmitter is always sending out
valid 10-byte packets. What if the receiver wakes up in the
middle of a packet? It receives 5 bytes from the end of the
first packet, then 5 bytes from the start of the next packet,
applies the CRC, discards the data and then repeats the
whole process, staying out of sync, even though there are
good packets coming in.

  The trick is to only discard one byte instead of the whole
buffer or a whole packet's worth of bytes. Then the receiver
essentially has a  sliding 10-byte window, whose CRC will
only match up if there is a valid packet in the 10 bytes. Of
course, once you do get  valid packet, you take the whole
packet out at once.

  Maybe this is obvious to everyone, but it had me stumped
for a bit--once I got out of sync it would reject all packets.

At 06:57 AM 8/24/99 , you wrote:
>Brent Crosby wrote:
>[snip]
>> If you are going to throw data away when the CRC does not match,
>> remember that it is important to only throw away one byte, and then
>> re-apply the CRC criteria. This will guarantee that your algorithm
>> re-synchronizes without the need for timeouts or any other complex
>> code.
>
>Brent, I did not slept well last night, and probably this is the reason
>why I can't understand your text above. Can you explain it better,
>please?  I understand you are talking about the received data block in
>error, but what you mean with "throw away one byte"? strip it out from
>the data block?
>Wagner.


                                Sincerely,
                                Brent A. Crosby

| Brent A. Crosby
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