Quality of Service (QoS) guarantees plays a critically important role in mobile wireless networks. For the real time services like multimedia video conference, the key QoS metric is to ensure a stringent delay-bound rather than high spectral efficiency. In this paper, a cross-layer approach is proposed to investigate the impact of physical-layer infrastructure on data-link layer QoS performance over wireless links. At the physical-layer a Single-Input-Single-Output (SISO) diversity scheme as well as Adaptive Modulation and Coding (AMC) techniques is taken into account.