This paper describes a quality of service (QoS) provisioning scheme for IEEE 802.11 infrastructure based networks. Our proposal, called "slide and translate" (SNT), adapts EDCF contention parameters to suit the varying traffic load in a basic service set (BSS). The aim of the SNT algorithm is to: 1. Protect high priority access categories (AC) from increasing background traffic. 2. Maintain efficient channel utilization over wide range of traffic conditions. SNT adaptively configures backoff intervals (BI) of each AC based on the QoS advertisements from the access point (AP). We evaluate through simulations the performance of SNT and compare the results with plain EDCF. Results from the simulation tests show that SNT is able to provide a high degree of isolation for high priority ACs. Additionally, SNT attains a 9% higher peak utilization value when compared to EDCF. Finally we present the delay and frame loss characteristics for SNT and EDCF. Based on our findings, we conclude that SNT performs comparatively better at providing QoS than EDCF and remains robust against congestion.