Input control policies had been proposed previously (e.g. in Lam and Kleinrock, 1975; and Fayolle et al., 1977) to improve the delay-throughput performance of Aloha access schemes. Through simulation results, Lam and Kleinrock observed that the optimal control policies were in general of control limit type. The authors discuss input control policies for one such Aloha access scheme and prove analytically that the optimal policy is indeed of the control limit type. The optimal policy is found to be extremely simple for the access scheme discussed. This policy is to accept the newly arrived packet if the present system state is either zero or one, and to reject the packet otherwise.>

The Urn scheme is known to perform better than optimal ALOHA and TDMA for all ranges of traffic rates. In this paper we discuss the dynamic behavior of the Urn scheme to show that it possesses bistable behavior in a manner similar to ALOHA schemes and that dynamic control procedures can be applied to improve the system performance effectively. In particular, an input control procedure (ICP) is presented that gives a delay-throughput characteristic very close to optimal (perfect scheduling) for a wide range of throughput rates. The improvement is obtained at no extra cost in terms of information acquisition and the complexity introduced is minimal. An analytical method is described to calculate the expected delay, throughput, and the probability of packet rejection. Numerical results are shown for various values of user population and compared with corresponding results for other schemes.