On the Processor Sharing Properties of File Transfers in a WLAN Testbed

802.11-based WLAN deployments have become a commodity to provide today's wireless Internet access. In this paper, we conduct a practical study on the performance of FTP file transfers over real WLAN equipment. To this end, we propose a new analytic model that translates the highly complex dynamics of the FTP/TCP/IP/MAC-stack, and their interactions, into a single parameter, which will be called the effective load. The effective load is used to describe the flow-level behavior of FTP-based file transfers over WLANs without admission control as a Processor-Sharing (PS)-model. Next, despite the fact that PS models are heavily used in modeling flow-level performance in WLAN networks, an extensive validation of such models with real equipment has not been conducted. Motivated by this, we validate in the present paper our analytic model by comparing the model-based response times against the outcomes obtained from a testbed environment. The results show (a) that the obtained mean download times are fairly insensitive to the file-size distribution, as suggested by the PS-model, and (b) that the model leads to accurate predictions over a broad range of parameters combinations, including different file-size distributions and light- and heavy-load scenarios.