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The Differentiated Services architecture (DiffServ) is a scalable and economic solution which provides relative QoS to different kind of services. Due to the critical QoS constraints of conversational multimedia services (including voice and video-telephony), a QoS mapping between those services and DiffServ classes must be well defined. Voice and video-telephony services are always mapped within the same EF Premium class to provide them with the best service. However, voice and video have divergent stochastic characteristics. Consequently, this standard mapping leads to heterogeneous traffic multiplexing within the same queues in the aggregating routers, which may deteriorate the best service always provided to voice. Thus, we have proposed a refined mapping which differentiates them. In order to carry out a performance study of voice/video packet scheduling, we have considered two different traffic models for voice and video. Simulation results show that WFQ scheduling with a well chosen weight performs better than FIFO and PQ. In our case, a weight of 0.25 for the voice class gives the optimal overall delay performance for multimedia telephony.