[Tinyos-help] CRC in CC2420

Philip Levis pal at cs.stanford.edu
Thu Feb 19 20:44:25 PST 2009


On Feb 19, 2009, at 8:36 PM, Avinash Sridharan wrote:

> Hi Phil,
>  Thanks for the explanation. That was quite helpful. I agree with your
> explanation. I agree that to have perfect reliability we have to
> implement end-to-end reliability (or at least network layer
> reliability). I do not have the exact numbers for the  false
> acknowledgment rates, but at high data rates was starting to see
> around 10-20% of packets failing to make it through.

You mean, you get an ack, but the data packet didn't arrive  
successfully? This sounds like a high number.

> Hence was trying
> to  see if I can improve the reliability at the MAC layer. The only
> problem with implementing acknowledgments at the network layer is that
> it will introduce latency and also consume capacity (since the data
> packets are slightly larger). Hence was trying to take the shorter
> route by improving reliability at the MAC layer (without much success
> though :) ).
>
> I also wasn't aware of the limitation caused by the DSN bits in the
> ACK's, in a dense deployment this can definitely cause problems.
> Thanks again for your explanation.
>

CTP, under reasonable but not super-heavy load, sees around 0.1%  
false positive acks. It actually ends up becoming a limiting factor  
for end-to-end reliability.

Phil


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