IEEE 802.15.4 is a communication standard specifically designed targeting low power personal area networks. It provides MAC and Physical layer specifications for resource constrained networks like Wireless Sensor Networks(WSN). IEEE 802.15.4 MAC specification targets fairness as one of the design considerations. That creates a problem when IEEE 802.15.4 MAC is used for event driven WSN applications. For an example, in a WSN deployed to detect a forest fire an event of fire requires higher preferences than routine updates. To target this limitation, we have extended channel access mechanism of IEEE 802.15.4 MAC in the form of prioritized channel access mechanism namely Explicit Prioritized Channel Access Protocol (EPCAP). EPCAP adds different priority levels to IEEE 802.15.4 MAC to handle critical events. Further, we have modeled EPCAP as M=G=c multi-server queueing system. To show the effectiveness of our mechanism we have simulated and compared it with standard protocol. © 2013 IEEE.