In this paper, the provision of quality-of-service (QoS) for real-time traffic over a wireless channel deploying automatic repeat request (ARQ) error control is investigated. By introducing the concepts of ARQ capacity and effective capacity, an analytic model has been derived to evaluate the loss probabilities in both the network layer and the physical layer. In contrast to the previous results, this model quantifies the interaction between the network and physical layers. As shown by the simulation experiments, our analysis can predict the real metrics under a wide range of conditions. This enables the call admission controller in wireless networks to control and optimize traffic QoS using instantaneous channel status information.