[Tinyos-help] TOSSIM Problem

Philip Levis pal at cs.stanford.edu
Sun Sep 4 15:29:13 PDT 2005


On Sep 4, 2005, at 3:00 PM, Brett Parsons wrote:

> Hi All,
>
>          2 -- 3
>        /          0 -- 1            6
>        \           /
>         4 -- 5
>
> Notice all I have done is remove the bidirectional link between 3  
> and 6.  My guess is that nodes 3 and 5 are attempting to broadcast  
> the message at the same time, resulting in corrupted packets.   
> However, the TOSSIM documentation states that this is a very minor  
> possibility.
>
> Does anyone have an idea what is going on here?  I'd appreciate any  
> suggestions you could offer!

You are seeing the hidden terminal problem. Where in the TOSSIM  
documentation does it say that this is very unlikely? It does say  
that the chances of two nodes *that can hear each other* transmitting  
at the same time is very unlikely (a 25us race condition between  
listening for a free channel and transition into the transmit state).  
If they can't hear each other, all bets are off.

This topology was actually part of one of the early TOSSIM tutorials,  
in that it was an example of a topology that would cause the hidden  
terminal problem. The issue is made more pronounced by your immediate  
rebroadcasting: it means that the only randomness in timing between 3  
and 5 transmitting is the initial random MAC backoffs IB(x): 3  
transmits at time IB(2) + IB(3) while 5 transmits at time IB(4) + IB(5).

Phil


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