Re: [Discuss-gnuradio] A Question on Packet Processing

[Tim Meehan]

Hi Achilleas,

I was interested in various length packets.  I am taking a fixed
length packet ( say 200 symbols ) and then bitstuffing it, resulting
in a packet >= 200 symbols.  I then receive this with the viterbi
algorithm and then finally unbitstuff.   So in this, case K = 200 +
number of stuffed bits.  I guess I could run N viterbi algorithms in
parallel for the N possible different number of stuffed bits.   I
guess I could also have the block that bitstuffs the data pad out to a
fixed block size.


Thanks again for you suggestions and help.

Tim



On 2/21/07, Achilleas Anastasopoulos <address@hidden> wrote:
> Tim,
>
> I think there are two parts in your question:
> 1) how to detect an incoming packet and do something with it.
> 2) how to operate the viterbi algorithm block in a packet to packet
> basis without generating the flow graph again.
>
> I only know how to answer the latter question:
>
> The viterbi algorithm (see all examples in
> gnuradio-examples/python/channel-coding/)
> processes the incoming stream in PACKETS by design.
> When you instantiate the block, ie using
>
> va = trellis.viterbi_s(f,K,s0,sK)
>
> then "K" is the block size to be processed (measured in trellis steps),
> with initial state fixed at "s0" and final state fixed at "sK"
> (or put -1 in the corresponding positions if you dont want to fix them).
>
> Thus is we send a stream of n x K symbols then the va block will run the
> algorithm n times with the set initial/final states.
> If you send a continuous stream of data to the input of the VA it will
> break it into blocks of K symbols and process each block separately.
>
> Hope this helped clarify the use of the viterbi block.
> Achilleas