Just as in traditional wireless networks, Medium Access Control (MAC) in Wireless Sensor Networks (WSNs) continues to attract the attention of many researchers. For a variety of proposed medium access solutions, ensuring good throughput while conserving energy is a real challenge. Asynchronous MAC protocols are distinguished by their short cycle time and low communication cost. Recently, Improved Receiver Initiated MAC Protocol (IRI-MAC), a receiver oriented protocol, has been proposed to reduce collisions in RI-MAC by generating a unique Clear Channel Assessment time (CCA_time) per node. In this book, after showing the possibilities of collisions in IRI-MAC, we propose eIRIMAC, an IRI-MAC based Balancing mechanism to improve it. We show that the proposed solution generates a lower probability of collisions, and reduces Idle-listening through a backoff-based balancing mechanism.