I'm trying to implement S-MAC protocol on waspmote xbee sensors and i know it has its own CSMA/CA. So first of all I need to understand the basic of xBee collision avoidance. Two senders set up in api mode in libraries and both periodically sending single bytes to a common receiver. I reduce the delay and many changing in libabries to make collision and to see how algorithm works. But when i monitor data at the receiver all looks as expected at the receiver .. byte1, byte 2 .. byte1, byte2. Do u have any idea how can i make collision? and is it possible to modify xbee module firmware?