In the last years, DVB-RCS has emerged as a flexible technology offering broadband Internet access to a large community of users at a relatively low cost. At the same time, the spreading of networked multimedia applications has highlighted the need to investigate mechanisms that guarantee a certain level of Quality of Service (QoS) to the end users. In particular, the DVB-RCS standard specifies different capacity request categories to support QoS at the link layer. We describe Tdma-bod, an ns-2 improvement that implements generic bandwidth-on-demand allocation in TDMA satellite systems; the patch is available as free software. This simulator has been validated through experimental tests performed on the Skyplex satellite system. Specifically, we run CBR UDP flows to measure the characteristics of the satellite link in terms of throughput and delay and to verify that the simulative model output matches the experimental dynamic throughput and one-way delay behaviour. The simulations and experiments show that bandwidth-on-demand allocation mechanisms may cause large delays when sudden variations in the incoming traffic rate occur, a behaviour typical of multimedia flows.