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This paper introduces X-MAC/CA protocol combining X-MAC protocol with CA (Collision Avoidance) algorithm with an aim to maximally randomize transmissions in overcrowded networks. X-MAC protocol, one of typical asynchronous MAC protocols for WSNs (Wireless Sensor Networks) allowing each node to independently and periodically sleep, is characterized as sending a sequence of short preambles until an early acknowledgement comes back. This feature is designed to save more energy by permitting the receiver to notify the sender that it is awake. X-MAC/CA protocol is designed to reduce collisions, especially when a large number of nodes in densely populated WSNs simultaneously deliver data once they sense some events to report. For its accurate measurement, this paper also extends a conventional mathematical model for X-MAC protocol to include the effect of CA algorithm. The extended model …