Date:    Tue, 24 Aug 1999 15:11:51 -0700
From:    Brent Crosby <brent@CRYSTALFONTZ.COM>
Subject: Re: [OT] CRCs on short packets

  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.

Brent and Wagner,
In my dealings with data packets, I have always included a synchronizing
byte. My receiving routine would wake up and then check every byte it
received until a synchronizing byte came in. Then it would start reading the
number of bytes required to make a packet.

In this case, my interpretation of the suggestion was to save the packet
that you received with an error and then call for another transmission.
Assuming the CRC failed on the second packet also, you would then compare
the two packets to determine which bytes were different. Assuming that the
bytes that matched were correct, you could mix and match the the
non-matching bytes until you got a complete packet.

My 2cents,
Paul Kolesnikoff