Wireless Sensor Networks (WSN) is a very emerging technology since they find application in many different areas including multimedia transmission which require high voluminous data transfer. An important issue related to WSN is limited availability of energy, which need much attention during network design. This paper focuses on network design issues in the MAC layer of WSN and proposed a simple analytical model to find Quality of Service (QoS) metric for evaluating packet loss in terms of probability of blocking. By implementing a simple buffer state in the MAC layer process, a minimum loss of packet can be archived, thus minimizing the energy use for retransmission of data packets. Other QoS metrics like waiting time for data packets, maximum system utilization are also determined to evaluate system performance.