[Packet] Decode of incomplete packets?

[Dr. Gerald N. Johnson]

Monitor mode in my hardware TNCs does that. Its really difficult because 
the data is a synchronous string the only way to align characters all 
the way through is to detect the leading sync characters at the 
beginning. Then the tnc firmware has to keep receiving bytes in the 
right order until the end of the packet which has a check sum. Sometimes 
a few crashed bits make the recognition of the sync bytes fail, then 
there are no start and stop bits in the data string to synchronize 
characters and every byte is wrong. If the leading sync character(s) is 
destroyed, there's no hope for the rest of the packet. The format of the 
system allows for a connected station to request packets be resent. 
That's the error correction mechanism. Sometimes there are many 
collisions destroying second and third frames when the first frame was 
OK. Some early TNCs in radios can only handle MAXFRAME of 1. They don't 
have enough buffer for several frames and will blat a response packet in 
frame 2. Early software TNCs in slow PCs did that too.

The only way that I can think of receiving data with bad sync characters 
would be to capture all the bytes, string them end to end and shift all 
the bits (like about 700 for an 80 character transmission) and shift 
until there's detectable text in the result. That's hard to do realtime 
and has to be in the firmware. I don't know of any that does that 
because even though the text string might be the language spoken by the 
users, without sync characters at the beginning the check sum isn't 
going to compute correctly to allow the data to be accepted.

73, Jerry, K0CQ

On 12/22/2012 10:35 PM, KD7JYK DM09 wrote:
> Is there something out there that can decode, as best as it can hear,
> incomplete or corrupt packets?
>
> Sometimes I get bits and pieces of a packet.  I may lose part for whatever
> reason, but, is there a way to get what little data it contains, even if
> only a few good characters?
>
> Kurt
>