Distributed computing models typically assume reliable communication between processors. While such assumptions often hold for engineered networks, e.g., due to underlying error correction protocols, their relevance to biological systems, wherein messages are often distorted before reaching their destination, is quite limited. In this study we aim at bridging this gap by rigorously analyzing a model of communication in large anonymous populations composed of simple agents which interact through short and highly unreliable messages. We focus on the rumor-spreading problem and the majority-consensus problem, two fundamental tasks in distributed computing, and initiate their study under communication noise. Our model for communication is extremely weak and follows the push gossip communication paradigm: In each synchronous round each agent that wishes to send information delivers a message to a random anonymous agent. This communication is further restricted to contain only one bit (essentially representing an opinion). Lastly, the system is assumed to be so noisy that the bit in each message sent is flipped independently with probability 1 2-e, for some small Ae >0. Even in this severely restricted, stochastic and noisy setting we give natural protocols that solve the noisy rumor-spreading and the noisy majority-consensus problems efficiently. Our protocols run in O(log n e2) rounds and use O(n log n e2) messages bits in total, where n is the number of agents. These bounds are asymptotically optimal and, in fact, are as fast and message efficient as if each agent would have been simultaneously informed directly by the source. Our efficient, robust, and simple algorithms suggest balancing between silence and transmission, synchronization, and majority-based decisions as important ingredients towards understanding collective communication schemes in anonymous and noisy populations.
Our paper falls within the scope of natural algorithms, a recent attempt to investigate biological phenomena from an algorithmic perspective @cite_53 @cite_22 @cite_2 @cite_27 @cite_37 @cite_52 . Within this framework, many works in the computer science discipline have studied different computational aspects of abstract systems composed of simple and restricted individuals. This includes, in particular, the study of population protocols @cite_26 @cite_36 @cite_13 @cite_44 @cite_23 , which considers individuals with constant memory size interacting in pairs (using constant size messages) in a communication pattern which is either uniformly at random or adversarial, and the beeping model @cite_53 @cite_11 @cite_5 , which assumes a fixed network with extremely restricted communication. However, despite interesting results obtained in such models, the understanding of their fault-tolerance aspects is still lacking @cite_13 @cite_44 . Here, we study basic distributed tasks in a model that includes highly restricted noisy communication.
@cite_37 @cite_26 @cite_22 @cite_36 @cite_53 @cite_52 @cite_44 @cite_27 @cite_23 @cite_2 @cite_5 @cite_13 @cite_11
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Distributed computing models typically assume reliable communication between processors. While such assumptions often hold for engineered networks, e.g., due to underlying error correction protocols, their relevance to biological systems, wherein messages are often distorted before reaching their destination, is quite limited. In this study we aim at bridging this gap by rigorously analyzing a model of communication in large anonymous populations composed of simple agents which interact through short and highly unreliable messages. We focus on the rumor-spreading problem and the majority-consensus problem, two fundamental tasks in distributed computing, and initiate their study under communication noise. Our model for communication is extremely weak and follows the push gossip communication paradigm: In each synchronous round each agent that wishes to send information delivers a message to a random anonymous agent. This communication is further restricted to contain only one bit (essentially representing an opinion). Lastly, the system is assumed to be so noisy that the bit in each message sent is flipped independently with probability 1 2-e, for some small Ae >0. Even in this severely restricted, stochastic and noisy setting we give natural protocols that solve the noisy rumor-spreading and the noisy majority-consensus problems efficiently. Our protocols run in O(log n e2) rounds and use O(n log n e2) messages bits in total, where n is the number of agents. These bounds are asymptotically optimal and, in fact, are as fast and message efficient as if each agent would have been simultaneously informed directly by the source. Our efficient, robust, and simple algorithms suggest balancing between silence and transmission, synchronization, and majority-based decisions as important ingredients towards understanding collective communication schemes in anonymous and noisy populations. Numerous massive data sets, ranging from flows of Internet traffic to logs of supermarket transactions, have emerged during the past few years. Their overwhelming size and the typically restricted access to them call for new computational models. This thesis studies three such models: sampling computations, data stream computations, and sketch computations. While most of the previous work focused on designing algorithms in the new models, this thesis revolves around the limitations of the models. We develop a suite of lower bound techniques that characterize the complexity of functions in these models, indicating which problems can be solved efficiently in them. We derive specific bounds for a multitude of practical problems, arising from applications in database, networking, and information retrieval, such as frequency statistics, selection functions, statistical moments, and distance estimation. We present general, powerful, and easy to use lower bound techniques for the sampling model. The techniques apply to all functions and address both oblivious and adaptive sampling. They frequently produce optimal bounds for a wide range of functions. They are stated in terms of new combinatorial and statistical properties of functions, which are easy to calculate. We obtain lower bounds for the data stream and sketch models through one-way and simultaneous communication complexity. We develop lower bounds for the latter via a new information-theoretic view of communication complexity. A highlight of this work is an optimal simultaneous communication complexity lower bound for the important multi-party set-disjointness problem. Finally, we present a powerful method for proving lower bounds for general communication complexity. The method is based on a direct sum property of a new measure of complexity for communication complexity protocols and on a novel statistical view of communication complexity. We use the technique to obtain improved communication complexity and data stream lower bounds for several problems, including multi-party set-disjointness, frequency moments, and Lp distance estimation. These results solve open problems of Alon, Matias, and Szegedy and of Saks and Sun. In Part I of this paper, we proposed and analyzed a novel algorithmic framework for the minimization of a nonconvex (smooth) objective function, subject to nonconvex constraints, based on inner convex approximations. This Part II is devoted to the application of the framework to some resource allocation problems in communication networks. In particular, we consider two non-trivial case-study applications, namely: (generalizations of) i) the rate profile maximization in MIMO interference broadcast networks; and the ii) the max-min fair multicast multigroup beamforming problem in a multi-cell environment. We develop a new class of algorithms enjoying the following distinctive features: i) they are across the base stations (with limited signaling) and lead to subproblems whose solutions are computable in closed form; and ii) differently from current relaxation-based schemes (e.g., semidefinite relaxation), they are proved to always converge to d-stationary solutions of the aforementioned class of nonconvex problems. Numerical results show that the proposed (distributed) schemes achieve larger worst-case rates (resp. signal-to-noise interference ratios) than state-of-the-art centralized ones while having comparable computational complexity. Networks form an indispensable part of our lives. In particular, computer networks have ranked amongst the most influential networks in recent times. In such an ever-evolving and fast growing network, the primary concern is to understand and analyse different aspects of the network behaviour, such as the quality of service and efficient information propagation. It is also desirable to predict the behaviour of a large computer network if, for example, one of the computers is infected by a virus. In all of the aforementioned cases, we need protocols that are able to make local decisions and handle the dynamic changes in the network topology. Here, randomised algorithms are preferred because many deterministic algorithms often require a central control. In this thesis, we investigate three network-based randomised algorithms, threshold load balancing with weighted tasks, the pull-Moran process and the coalescing-branching random walk. Each of these algorithms has extensive applicability within networks and computational complexity within computer science. In this thesis we investigate threshold-based load balancing protocols. We introduce a generalisation of protocols in [2, 3] to weighted tasks. This thesis also analyses an evolutionary-based process called the death-birth update, defined here as the Pull-Moran process. We show that a class of strong universal amplifiers does not exist for the Pull-Moran process. We show that any class of selective amplifiers in the (standard) Moran process is a class of selective suppressors under the Pull-Moran process. We then introduce a class of selective amplifiers called Punk graphs. Finally, we improve the broadcasting time of the coalescing-branching (COBRA) walk analysed in [4], for random regular graphs. Here, we look into the COBRA approach as a randomised rumour spreading protocol.