This paper presents a comparative study of two well-known network simulators: OPNET Modeler and Ns-

2. Other studies in this area have generally been confined to just one simulator. The motivation of this paper is to

provide a guide to researchers undertaking packet-level network simulations. The simulator outputs were compared to

the output from a live network testbed. The experimental comparison consisted of deploying both CBR data traffic,

and an FTP session, both on the network testbed and the simulators. CBR data traffic was used due to its simplicity of

modeling. A custom analysis tool was employed to examine the behavior of the network in different background

traffic scenarios. The same scenarios were then recreated in the simulators in order to ascertain their realism. The

results show the necessity of fine-tuning the parameters within a simulator so that it closely tracks the behavior of a

real network.