RE: [Discuss-gnuradio] lost bytes and reed-solomon

[Torell, Kent-P12255]

The receiver doesn't 'loose' symbols; it will clock through the correct
amount of data, assuming it is tracking the signal bauding rate.  The
symbols will be in error, though.  So, your length should be constant
for good packets as well as bad.  If the baud tracking loop is  having
trouble, you will typically stuff or delete a symbol (maybe), which then
garbles up everything until you re-establish time sync. 
Kent Torell

-----Original Message-----
From: address@hidden
[mailto:address@hidden On
Behalf Of Brett L. Trotter
Sent: Friday, May 25, 2007 2:11 AM
To: address@hidden
Subject: Re: [Discuss-gnuradio] lost bytes and reed-solomon

Brett L. Trotter wrote:
> I'm working on a self-educational project to replace the simple CRC in

> the tunnel apparatus with a very heavy reed-solomon encoding for very 
> lossy and low-bandwidth links. One thought just occurred to me.
> Originally, I was going to group up all of my data in 127 byte blocks,

> r/s encode it with a 255,127, and just shove each 255 byte block into 
> a new payload string which would basically be twice as long with reed 
> solomon parity stuff every other 127/128 bytes from each respective 
> block and then just grab each 255 bytes and run them through the 
> decoder on the receive and do the opposite, shoving each decoded 127 
> bytes into a decoded payload.
>
> But the thought was, what if a byte is completely obliterated- then 
> everything would be shifted over one (or more) and my groupings 
> wouldn't align so I'd be looking in the wrong places for parity and 
> not know that there was a total deletion rather than just a 
> corruption? Is there any way to prevent/identify this case, or can it
even happen?
>
>
>
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden
> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>   
Partially answering myself here (and expanding on my question/thought)

I suppose a simple answer would be to check the length of the received
payload and if not a multiple of 255, just discard the whole thing- then
we can at least identify the deletion. Since we can't pinpoint where it
was, we can't just fill it in with a 0 that would get repaired
(hopefully).. If this is the right move, then the question becomes, how
often can a byte be lost in transmission, and is it more often than only
corruption occurs? If it is rare enough, then the loss of the entire
packet is acceptable (and no worse than the current CRC), but if it
isn't so rare, would the reed-solomon do much good in the first place?


_______________________________________________
Discuss-gnuradio mailing list
address@hidden
http://lists.gnu.org/mailman/listinfo/discuss-gnuradio