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Providing service differentiation in wireless networks has attracted much attention in recent research. Existing studies so far have focused on the design of MAC layer protocols to achieve distributed priority scheduling. While these works have been shown to achieve certain differentiation (higher throughput or lower delay for higher priority packets) at MAC layer, they did not consider the problem of service differentiation from end-to-end perspective. Moreover, these approaches did not present a formalized service differentiation model that quantifies the degree of differentiation, which is critical for service class selection and provision. This paper addresses above problems by introducing a clear service differentiation model, proportional service differentiation, to the domain of wireless network which targets at providing proportional delay differentiation in wireless LAN. Due to the unique characteristic of distributed medium sharing, the scheduling algorithm employed in wireline networks can not be applied directly to the context of the wireless network. We argue that delay differentiation in wireless LAN can be only achieved through a joint packet scheduling at network layer and distributed coordination at MAC layer. Hereby, we present a distributed waiting time priority scheduling (DWTP) algorithm which is deployed on a cross-layer architecture. DWTP consists of two tiers: an intra-node predictive WTP scheduler at network layer and an inter-node priority-based distributed coordination at MAC layer. These two tiers coordinate via cross-layer priority mapping to achieve proportional delay differentiation. Simulation results show that the cross-layer DWTP algorithm can effectively achieve proportional delay differentiation in wireless LAN.