Delay, one of the most important parameters in the wireless control networks, is largely dependent on the packet length when using contention based medium access control (MAC) protocol in single hop networks. In this paper, we have developed a three-state discrete time Markov-chain based model for the performance evaluation of MAC protocol. Using this Markov-chain model, we analyze the carrier sense multiple access (CSMA) MAC protocol for its delay and throughput characteristics in the presence of transmitter power control. Using simulations, we measure the probability of capture (which gives a measure of the effectiveness of transmitter power control due to the capture effect.) and validate these simulated data experimentally. To analyze the effect of transmitter power variation, the Markovchain model is modified by incorporating the probability of capture. Numerical results are obtained to measure the performance improvement.