In this article, we provide an analytical framework to assess network level QoS measures for differentiated non-saturated traffic in infrastructure mode 802.11 wireless local area networks (WLANs) with distributed coordination function (DCF) access mechanism. We build on a general analytical framework that takes into account the MAC access mechanism, the MAC layer packet buffer and the characteristics of the offered load to obtain the probability of collision with M classes of traffic and derive analytical expressions for the throughput, the end-to-end packet delay and the packet delay outage probability for differentiated traffic. A case study for voice over IP (VoIP) traffic in 802.11b/g networks is used to validate the theoretical framework. The analytical results are in good agreement with simulation results showing that, although the collision probability for packets transmitted at the access point (AP) is lower than for packets transmitted from the clients, the end-to-end delay in the downlink is much longer than in the uplink because of the large queuing delay due to the multiplexing of several VoIP connections at the AP. We also compare the maximum number of VoIP connections than can be admitted in an 802.11 network, while respecting their QoS constraints, computed with the proposed theoretical model, a ns-2 simulation model, and other schemes previously proposed in the litterature. The results indicate that our approach is more accurate over a wide range of parameter values which demonstrate the validity, flexibility and robustness of the proposed theoretical framework.