With the recent advances in wireless communications and networking area, it has been a great challenge for researchers to improve a medium access protocol for providing an efficient service differentiation in wireless ad hoc, sensor and wireless Local Area Networks among different priority class applications that require high goodput, short term fairness, low delay and delay jitter. IEEE 802.11 Static MAC protocol assigns different Arbitration Inter Frame Space (AIFS) durations for different priority classes to provide service differentiation. However, it is subject to a significant goodput degradation when the high priority class is low. On the other hand, IEEE 802.11e Dynamic MAC protocol employs different AIFSs, (CW min ,CW max ) pairs and contention window expansion factors (PFs) for different classes to support differentiated quality of service levels. Yet, it can not protect high priority class traffic from greedy sources in case of a heavy network load. The proposed protocol SD-MAC, Service Differentiation via CMAC, uses Cooperative MAC protocol (CMAC) as a basis protocol and can easily be implemented using IEEE 802.11e and CMAC. In SD-MAC, each node has to change its back-off counter based on both its own packet’s priority level and the priority level of the transmitted packet. If a node hears an ongoing transmission of a packet of higher priority than its own, it has to increase its back-off counter linearly. Besides, if it hears an ongoing transmission of a packet of lower priority than its own, it has to decrement its back-off counter exponentially. The simulation results indicate that, SD-MAC is a short term fair protocol which performs well in all network scenarios providing high network goodput, short term fairness and large admissible region.