Next-Generation Networks (NGNs) will support Quality of Service (QoS) over a mixed wired and wireless IP-based infrastructure. A relative model of service differentiation in Differentiated Services architecture is a scalable solution for delivering multimedia traffic. However, considering the dynamic nature of radio channels specifically, it is difficult to achieve the target quality provisioning working at the IP and lower layers separately as in the classical approach. In this work, an IP cross-layer scheduler able to support a Proportional Differentiation Model (PDM) for delay guarantees also over wireless is described. The key idea is to leverage feedbacks from the lower layers about the actual delays experienced by packets in order to tune at run-time the priority of the IP service classes with the objective of supporting a PDM at the network node on the whole across multiple layers. A simulation analysis demonstrates the prominent improvements in reliability and robustness of the proposal in the case of highly time-variant performance of the MAC and PHY layers with respect to the classical approach. Furthermore, considerations on the required functionality and likely deployment scenarios highlight the scalability and backward compatibility of the designed solution.