Hi!
On Sun, 27 Sep 2009, bo zhang wrote:
> Hi.
> I'm simulating multi-hop application in tossim.
> I requested acknowledgement using PacketAcknowledgements for each packet
> sent. I found some strange things happened in the simulation. For some
> packets,
> (1). the sender gets negative ACK even though the packets was successfully
> received;
This is the case when the ACK is lost.
> OR
> (2). the sender gets positive ACK even though the packet was lost.
>
In real life you can get and ACK if there is other 802.15.4 traffic
because the sequence number is only 8 bits and there are no addresses
fields. Figure 23 from page 42 from here shows this:
http://focus.ti.com/lit/ds/symlink/cc2420.pdf
> Then, I found this:
> http://mail.millennium.berkeley.edu/pipermail/tinyos-help/2009-January/037710.html
> which basically said that such false acknowledgement may happen at a chance
> of .1% in TOSSIM.
>
> Can anyone let me know how to ensure correct acknowledgement, since my
> application cannot proceed without reliable acknowledgement. In my code, I
> checked the acknowledgement as follow:
>
> event void AMSend.sendDone(message_t* bufPtr, error_t err) {
>
> if(call PacketAcknowledgements.wasAcked(bufPtr)) {
> // go to next step
> }
> else
> // resend the message at application level
> }
>
You should probably use application level acks. :-) You can also disable
the .1% but doing this will make the simulation less real. The code that
produce the false acks is in tos/lib/CpmModelC.nc and the lines are the
following:
341 if (RandomUniform() < 0.001) {
342 dbg("CpmModelC,SNRLoss", "Packet was technically lost, but
TOSSIM introduces an ack false positive rate.\n");
343 if (mine->ack && signal Model.shouldAck(mine->msg)) {
344 dbg_clear("CpmModelC", " scheduling ack.\n");
345 sim_gain_schedule_ack(mine->source, sim_time() + 1, mine);
346 }
347 else { // Otherwise free the receive_message_t*
348 free_receive_message(mine);
349 }
--
Razvan ME
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