The Cognitive Radio (CR) technology has emerged as a hot research area recently due to its high potential in addressing the spectrum underutilization problem. In this paper, the CR is applied to unauthorized CR users (CRUs) such that these nodes can access the 802.11 spectrum opportunistically to flexibly use the detected "spectrum holes," with the ultimate goal of improving the 802.11 spectrum utilization. In , the authors proposed a bi-directional CR MAC protocol such that the CRUs can smartly negotiate and reserve bandwidth either uni-directionally or bi-directionally, to better support both the TCP and UDP flows. However, the protocol design is inadequate in supporting traffic demands in more complicated network cases. To address this problem, we proposed a CR MAC protocol with a dynamic bandwidth allocation capability, which can well support various types of network traffic, particularly asymmetric two-way traffic flows. We used the NCTUns network simulator to conduct the study. Our simulation results show that the proposed protocols can further utilize the "spectrum holes" in a more effective way while guaranteeing QoS support to both the surrounding CRUs and authorized 802.11 nodes.