The greatest current challenge in wireless sensor networks consists in building a completely adaptive network without fixed infrastructures and with the smallest energy resources. Applications built upon such networks are various: from telemetry to medical follow-up and from intrusion detection to infrastructure maintenance. Nevertheless, their design requires a large effort to guarantee quasi unlimited lifespan of the network with a minimum QoS (Quality of Service). In this work, we propose a radio interference model which is combined with a markovian model of CSMA/CA [1] and permit to estimate power consumption and packet success rate in an IEEE 802.15.4 network.