We consider the binary consensus problem where each node in the network initially observes one of two states and the goal for each node is to eventually decide which one of the two states was initially held by the majority of the nodes. Each node contacts other nodes and updates its current state based on the state communicated by the last contacted node. We assume that both signaling (the information exchanged at node contacts) and memory (computation state at each node) are limited and restrict our attention to systems where each node can contact any other node (i.e., complete graphs). It is well known that for systems with binary signaling and memory, the probability of reaching incorrect consensus is equal to the fraction of nodes that initially held the minority state. We show that extending both the signaling and memory by just one state dramatically improves the reliability and speed of reaching the correct consensus. Specifically, we show that the probability of error decays exponentially with the number of nodes N and the convergence time is logarithmic in N for large N . We also examine the case when the state is ternary and signaling is binary. The convergence of this system to consensus is again shown to be logarithmic in N for large N , and is therefore faster than purely binary systems. The type of distributed consensus problems that we study arises in the context of decentralized peerto-peer networks, e.g. sensor networks and opinion formation in social networks -our results suggest that robust and efficient protocols can be built with rather limited signaling and memory.

Abstract-Given that wireless communication occurs in a shared and inherently broadcast medium, the transmissions are vulnerable to undesired eavesdropping. This occurs even when a point-to-point communication is sought, and hence a fundamental question is whether we can utilize the wireless channel properties to establish secrecy. In this paper we consider secret communication between two special nodes ("source" and "destination") in a wireless network with authenticated relays: the message communicated to the destination is to be kept information-theoretically (unconditionally) secret from any eavesdropper within a class. Since the transmissions are broadcast and interfere with each other, complex signal interactions occur. We develop cooperative schemes which utilize these interactions in wireless communication over networks with arbitrary topology, and give provable unconditional secrecy guarantees.

In this paper we find properties that are shared between two seemingly unrelated lossy source coding setups with side-information. The first setup is when the source and sideinformation are jointly Gaussian and the distortion measure is quadratic. The second setup is when the side-information is an erased version of the source. We begin with the observation that in both these cases the Wyner-Ziv and conditional ratedistortion functions are equal. We further find that there is a continuum of optimal strategies for the conditional rate distortion problem in both these setups. Next, we consider the case when there are two decoders with access to different side-information sources. For the case when the encoder has access to the sideinformation we establish bounds on the rate-distortion function and a sufficient condition for tightness. Under this condition, we find a characterization of the rate-distortion function for physically degraded side-information. This characterization holds for both the Gaussian and erasure setups.

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DeecI XY x Ri X Ene Dee2-y En~R 2 Fig. 1. Cooperative source coding with encoder breakdownAbstract-This paper provides an inner bound to the ratedistortion region of a source coding setup in which two encoders are allowed some collaboration to describe a pair of discrete memoryless sources. We further require some robustness in case one of the encoders breaks down. This is modeled by having a second decoder, observing the messages from only one of the encoders. We prove the tightness of this inner bound for two special cases. In the first, one of the sources is required to be recovered losslessly if there is no encoder breakdown. In the second, the robustness requirement is dropped and only one of the sources is to be represented. For the second case, we explicitly compute the rate-distortion region for the quadratic Gaussian and binary Hamming problems.