Wireless sensor networks represent a key technology enabler for enhanced health care and assisted living systems. Recent standardization efforts to ensure compatibility among sensor network systems have produced the IEEE 802.15.4 standard, which specifies the MAC and physical layer behavior. Energy and network life time are two critical factors for WSN performance. IEEE 802.15.4 is a standard used for low data rate Wireless Personal Area Networks (WPANs). Medium Access Control layer (MAC) of IEEE 802.15.4 plays an important role in the performance of Wireless Sensor Network (WSN).