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A large number of the MAC protocols proposed for establishing wireless sensor networks are based on the 802.11 standard. The trade-off in these protocols is the control packet overhead and the retransmission cost due to collision without it. In this paper we evaluate the collision probability of a CSMA/CA MAC protocol in an unsaturated situation as a function of nodes' sampling and transmission rates. We provide an accurate and comprehensive analytical model in which a finite number of nodes exist. We assume an ideal channel condition, independent collision probability of packets as well as infrequent communication between sensor nodes. We will demonstrate that the collision probability changes from 0 to 0.22 as the sampling rate changes from 0 to 0.94Mbps. Moreover, we will demonstrate that collision only begins after the sampling rate reaches 0.31Mbps, which implies that for a sampling rate below this …