CC2420 Clear Channel Assessment
By default, the CC2420 radio stack performs a clear channel assessment (CCA) before transmitting. If the channel is not clear, the radio backs off for some short, random period of time before attempting to transmit again. The CC2420 chip itself provides a strobe command to transmit the packet if the channel is currently clear.
To specify whether or not to transmit with clear channel assessment, the CC2420CsmaP requests CCA backoff input through the RadioBackoff interface on a per-message basis. By default, each packet will be transmitted with CCA enabled.
If layers above the CSMA layer wish to disable the clear channel assessments before transmission, they must intercept the RadioBackoff.requestCca(...) event for that message and call back using RadioBackoff.setCca(FALSE).
