We present a framework to study the performance of networked control systems where the feedback control loops are closed over a shared wireless network. In particular, we study the effects of the wireless network medium access control (MAC) protocols on the performance of the networked control systems. We consider a joint performance index of all the systems sharing the wireless channel. We compare several wireless network MAC protocols with a numerical example of two inverted pendulum systems. We show that the control performance degrades due to distributed medium access control in addition to the performance degradation due to communication faults including limited data rates, random delay and packet losses.