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In this paper, we investigate issues in the discrete-time implementation of passivity based control of bilateral teleoperators. The usual scattering formalism which, in continuous time, guarantees passivity for any constant delay, is extended in several important ways to the discrete domain, in particular to the case where communication between the master and slave robots occurs over a packet-switched network. We first show that passivity can be maintained in the face of varying delay and packet loss but that it depends fundamentally on the mechanism used to handle missing packets. Passivity alone is not sufficient to guarantee good performance. Therefore, we also introduce a novel buffering and interpolation scheme which not only preserves passivity but has been shown through simulation and experiments to improve tracking performance and transparency in a single-degree-of-freedom teleoperator system.