XBee, how does it deal with collisions?
The radio being used by these modules (MAC &amp; PHY layer) is defined by the IEEE 802.15.4 standard which specifies the use of Carrier sense multiple access with collision avoidance or abbreviated as CSMA/CA.

A simple summary, a device will do a quick scan of the channel that its PAN network is on and if it senses that it is being utilized it will wait and retry using a random exponential backoff scheme. Using this method, you can think of the channel as a shared resource that is "taken" by a device to do a burst transmission and then released for others to use.

Further questions about reliable transport can be answered by reading about it in the Zigbee spec or IEEE 802.15.4 spec.