[Tinyos-help] Could anyone clarify a small piece of code

[Tie Luo]

I am confused by the following code in CC2420TransmitP.nc (T2):

 

void attemptSend() {

       bool congestion = TRUE;

...

1     status = m_cca ? call STXONCCA.strobe() : call STXON.strobe();
2     if ( !( status & CC2420_STATUS_TX_ACTIVE ) ) {
3           status = call SNOP.strobe();
4       if ( status & CC2420_STATUS_TX_ACTIVE )
5          congestion = FALSE;
6      }

7     atomic m_state = congestion ? S_SAMPLE_CCA : S_SFD;

8     call CSN.set();

9    if ( congestion ) {
      releaseSpiResource();
      congestionBackoff();
    }
#ifdef PLATFORM_MICAZ
    else {
      startBackoffTimer(CC2420_ABORT_PERIOD);
    }
#endif

}

questions:

line 2,3: if RF transmission is not active, why use line 3 to detect status again?

To my understanding, congestion means channel busy. So if I can turn on RF transmission, it means no congestion, am I right?

 

line 9 & below: no matter congestion or not, micaz will do backoff. This looks wield. Really?

 

--
Regards,
Tie

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