[Tinyos-help] CRC in CC2420

[Philip Levis]

On Feb 19, 2009, at 6:42 PM, Avinash Sridharan wrote:

> Ah ok,
> Thanks a lot. The specs makes sense, I guess need to look at other
> places to understand the false acknowledgments. Any idea as to why
> this might be occurring ?

What false positive acknowledgement rate are you seeing?

802.15.4 suffers from false acknowledgements in part because  
acknowledgement packets do not have source addresses. Since the DSN is  
only 8 bits, it is possible that a node hears an acknowledgement for  
another transmission and thinks it is for its own.

Also, it's possible that a packet is corrupted but passes CRC. The  
receiver will issue an ACK. But, if, say, the AM type is different,  
your network protocol may never receive the packet.

Link-layer acknowledgements are not perfect -- no acknowledgements  
are. This is the end-to-end argument. If you want to know that a  
packet has arrived at the network layer, you need the network layer to  
send acknowledgements.

The problem is a little more pronounced in 802.15.4 because CRCs are  
only 16 bits and there aren't checks at many layers (layer 2, layer 3,  
layer 4, etc.).

Phil