This paper addresses the time synchronization problem in IEEE 802.11a OFDM wireless systems. To enhance the coarse time synchronization mechanism, recent methods exploit not only traditional training sequences as specified by the standard but also additional knowledge available when the Carrier Sense Multiple Access with Collision Avoidance mechanism (CSMA/CA) is triggered. In this case, additional information can be used as training sequences based on the protocol knowledge training sequence known by the receiver. This step is followed by a time synchronization and channel estimation which results in the smallest Channel Estimate Errors (CEE) according to the selected criterion (e.g. LS, MAP). However it was found that the synchronization failure probability heavily depends on the channel estimate quality. Therefore to improve the performance of this class of algorithms, we propose an optimal time synchronization metric that minimizes the average of the transmission error over all CEE. Simulation results show a strongly improved performance in terms of synchronization failure probability in comparison with the existing algorithms.