Several active queue management (AQM) mechanisms have been proposed in the literature to provide better support for congestion control. However, their performance is examined mainly in best-effort networks. In this paper, we present an empirical study of the effects of AQM mechanisms on the performance of the Assured Forwarding (AF) Service in the non-oversubscribed Differentiated Services networks. The AQM mechanisms to be studied include some existing mechanisms and a new AQM scheme, proposed in this paper. The study is carried out in the cases of the standard traffic conditioner and intelligent traffic conditioners. Extensive simulation results show: (i) when the standard traffic conditioner is employed at the ingress routers, the transient and steady-state behaviors of an AQM scheme in controlling the queue length not only affect the achievement of bandwidth assurance, but also affect the attainment of excessive bandwidth and link throughput; (ii) the behaviors of an AQM affect the ability of an intelligent traffic conditioner in improving bandwidth assurance; (iii) self-tuning AQM schemes perform better than AQMs with fixed-gains in terms of the achievement of bandwidth assurance and link throughput.