Transmission delay, throughput and energy are important criterions to consider in wireless sensor networks (WSN). In this way, IEEE 802.15.4 standard was conceived with the objective to reduce resource's consumption in both WSN and Personal Area Networks (WPAN). In such networks, the slotted CSMA/CA of still occupies a prominent place as a channel control access's mechanism with its inherent simplicity and reduced complexity. In this paper, we propose to introduce a network allocation vector (NAV) as an extension of the fragmentation mechanism presented in [24] to reduce energy consumption and collisions in IEEE 802.15.4 networks. A Markov chain-based analytical model of the fragmentation mechanism, in a saturated traffic, is given as well as a model of the energy consumption using the NAV mechanism. The obtained results show that the fragmentation technique improves at the same time the throughput, the access delay and the bandwidth occupation. They, also, show that using the NAV allows reducing significantly the energy consumption when applying the fragmentation technique in slotted CSMA/CA under saturated traffic conditions.