Hi Vítor, I hope you're doing well!
SMAC is mounted on top of an 802.15.4 physical layer. This means that Clear Channel Assessment can be done by a request from the upper layers to the PHY layer, by creating a message to the lower layer.
However, the way that it's implemented in our Stack, the CCA is done automatically by the PHY layer.
You can read more about SMAC and 802.15.4 in the documentation included with the KW41Z's SDK.
The documentation can be found in the SDK's path, here:
CCA is a necessary process for the 802.15.4 communications protocol to make sure that no other message is being sent at the same time as a new message, so, when trying to transmit data, the PHY layer checks the transmission medium to see if the channel is clear and no other data is being transmitted.
If no data is being transmitted, and the channel is clear, then, a new message can be transmitted.
You can get more information about the 802.15.4 wireless communication standard and the NXP implementation of the stack in the documentation included with the KW41Z's SDK, in the following path: