Hmm...interesting. Why would #2 happen? Message overrun? If it was only #1 it would seem that you would have at worst a 1::2^16 chance of a falsely correct CRC and then an even smaller chance of it being the type field that was corrupted.
That said, the chance of missing a valid ACK is much greater than getting a false one. MS Philip Levis wrote: > On Dec 23, 2008, at 2:00 AM, Pratibha S wrote: > >> Hi All, >> >> We are running an algorithm in TOSSIM over 200-nodes and we are >> experiencing packet losses due to improper ACKs. i.e. The sender >> would receive the packet.wasAcked() signal, but the receiver wouldnt >> have got the message. Is this a bug? Any help on this issue would be >> appreciated. >> > > This isn't a bug. > > Experimentally, I've found that motes receive false positive > acknowledgements. This is rare, but it does happen. There are two > possible reasons: > > 1) The packet was corrupted, but passed CRC. If the AM type is > different, then unless you have a handler for that type the stack will > just discard it. > > 2) The packet is read properly by the radio and passes CRC, but the > software stack flushes the receive memory before reading it out. > > Because net2 has observed this to happen experimentally on pretty much > every testbed we've used, I put it into TOSSIM. That way, your code > doesn't make incorrect assumptions about the behavior of hardware. > Remember the end-to-end argument: you can't depend on link-layer acks > to be sure that the packet arrived successfully at a network layer. > > IIRC, TOSSIM issues false positive acks .1% of the time (1 in 1000), > which is approximately what net2 observed in CTP tests on telosb and > micaz hardware (CC2420). > > Phil > _______________________________________________ > Tinyos-help mailing list > [email protected] > https://www.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help -- Platform: WinXP/Cygwin TinyOS version: 1.x, Boomerang Programmer: MIB510 Device(s): Mica2, MicaZ, Tmote Sensor board: homebrew _______________________________________________ Tinyos-help mailing list [email protected] https://www.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help
