Re: [Discuss-gnuradio] Lost packet problem

[Tom Rondeau]

On Thu, Feb 18, 2010 at 8:25 AM, abbasi9999 <address@hidden> wrote:
>
> I'm facing a problem, when the packet is lost in the channel.
>
> I've tried to create my own program with multiple ACKs between transmitter
> and receiver (both of them works as transceiver) to retransmit the lost
> packets. However i faced some problems:
>
> 1. the CRC32 cannot recognize the error packets (missed packets, larger
> packets and smaller packets cannot be recognized).
> 2. It seems that the (PN) access code cannot work probably. how can i know
> whether it works probably or not (its purpose is for synchronization, but
> how can i check it)?

Which modulation are you using? Are you using the
digita/benchmark_*.py files or is this OFDM. I'm not quite getting
what you mean when you're talking about the PN code (which makes it
sound like the OFDM code we have in the examples).

> The receiver will send LOST-ACK with the indexes of the lost packets (which
> identified by the CRC32, when there is error in the payload).

Sorry, I'm really not understanding what you're doing. Are you
checking the CRC and sending an ACK if the CRC doesn't check out? If
so, that requires that you get enough good bits of the packet through
to first detect that it's a packet and then be able to parse it. If
you miss the header and length fields, you won't be able to see the
CRC.

This is usually solved, like in TCP, with a sequence number. You can
tell if you missed a packet because the sequence numbers from two
consecutive packets received will not match up. This way, the receiver
can completely drop a packet and still know it after then next packet
arrives.

Also, in any of the digital mods and OFDM examples we've created, we
don't use any channel coding, so you're definitely going to lose
packets. A single bit error can cause the loss of the entire packet.
Secondly, if you're using OFDM, there are still some problems that
Matt and I have identified but have not had a chance to work out. The
problem(s) will cause the receiver to misfire on the preamble and miss
the entire packet.

Tom


> did any body tried to solve the lost packet problem before, if yes can you
> give me some tips?
>
> Is it possible in GNU radio to use some lower complexity protocol than tcp,
> for the error packet correction?
> what kind of build-in protocol ?
>
> regards,