We study the problem of finding a characterization for the channel that results when a queue is operated under multiple access conditions. In such systems, the mechanism by which different sources gain access to the channel plays a fundamental role in defining what is the channel available to each source. In this paper therefore we study the structure and properties of these control devices in some detail. Under some (mild) technical conditions, and under modeling assumptions inspired by TCP/IP's flow control (the standard control algorithm in the current Internet), we are able to characterize the optimal controller for this problem. We also present some numerical simulations, to help develop an intuition on what exactly this control box does.