Next-Generation Networks (NGNs), comprising for example, 4G and B4G mobile systems, will support Quality of Service (QoS) over a heterogeneous 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, the dynamic nature of radio channels makes it difficult to achieve the target quality provisioning working separately at the IP and lower layers, as in the classical approach. In this work, an IP cross-layer scheduler able to support a Proportional Differentiation Model (PDM) for delay guarantees is introduced. The key idea is to leverage feedback from the lower layers indicating the actual transmission delays experienced by packets in order to dynamically tune the priority of the IP service classes with the objective of supporting the 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 with respect to the classical approach. Considerations on the required functionality and possible deployment scenarios highlight the scalability and backward compatibility of the designed solution. Therefore, it addresses the requirements and challenges for NGNs.