The low-energy characteristics of wireless sensor networks pose a great design challenge for MAC protocol. The cluster-based scheme is a promising solution for MAC protocol designing. In this paper, an intra-cluster communication Binary Countdown Medium Access Control protocol (BCMAC) is proposed. BCMAC is intended for event-driven applications that is, sensor nodes forward data to the cluster head only if significant events are observed. The scheduling of BCMAC can change dynamically according to the unpredictable variations of sensor networks. In terms of energy efficiency, BCMAC protocol reduces energy consumption due to idle listening and collisions. Analytic energy models for BCMAC & Bit Map Assisted (BMA) MAC protocols are also developed. Analytic results show that, in terms of energy efficiency, BCMAC performance heavily depends on the sensor node traffic offer load, the number of sensor nodes within a cluster, the data packet size and, in some cases, the number of sessions per round. Index Terms- Wireless sensor networks; MAC protocols; BMA & BCMAC, LEACH, TDMA.