[Tinyos-help] A question about radio state change

[Yinying Yang]

I am studying to use Tossim as the simulator for an energy efficient 
protocol. 
I initialize 5 motes in one second, and then after 1 second, let mote 1 
send a message to mote 0.
I check the radio state change when mote 1 begins sending: mote 1 is 
Tx, other motes are Rx; Then other motes become Tx, and mote 1 is Rx; 
Then other motes Tx twice again, mote 1 is Rx; and then again other 
motes Tx, mote 1 Rx; again others Tx, mote 1 Rx; and finally all motes 
become Rx, and motes receive the message. 
I am wondering why there is so many state changes during sending and 
receiving, does it have some relationship with MAC protocol?  
Is there any state machine in tinyos?
And when a mote is Tx, dose it mean that it must send a message or it 
is just a state without sending?

Any help will be very appreciated.

Yinying Yang