Parameterized Complexity of Broadcasting in Graphs

The task of the broadcast problem is, given a graph G and a source vertex s, to compute the minimum number of rounds required to disseminate a piece of information from s to all vertices in the graph. It is assumed that, at each round, an informed vertex can transmit the information to at most one of its neighbors. The broadcast problem is known to NP-hard. We show that the problem is FPT when parametrized by the size k of a feedback edge-set, or by the size k of a vertex-cover, or by k=n-t where t is the input deadline for the broadcast protocol to complete.Comment: Full version of WG 2023 pape

Heuristic Algorithms For Broadcasting In Cactus Graphs

Broadcasting is an information dissemination problem in a connected network, in which one node, called the originator, disseminates a message to all other nodes by placing a series of calls along the communication lines of the network. Once informed, the nodes aid the originator in distributing the message. Finding the broadcast time of a vertex in an arbitrary graph is NP-complete. The problem is solved polynomially only for a few classes of graphs. In this thesis, we study the broadcast problem in a class of graph called a Cactus Graph. A cactus graph is a connected graph in which any two simple cycles have at most one vertex in common. Equivalently, it is a connected graph in which every edge belongs to at most one simple cycle. We review broadcasting on subclasses of cactus graphs such as, the unicyclic graphs, necklace graphs, k-cycle graphs, 2-restricted cactus graphs and k-restricted cactus graphs. We then provide four heuristic algorithms that solves broadcasting on a k-cycle graph. A k-cycle graph is a collection of k cycles of arbitrary lengths all connected to a central vertex. Finally, we run simulations of these heuristic algorithms on different sized k-cycle graphs to compare and discuss the results

Approximation Algorithms for Broadcasting in Flower Graphs

Over the last century, telecommunication networks have become the nervous system of our society. As data is generated and stored on varied nodes, effective communication is imperative to ensure efficient use of the network. Our ever-growing reliance on these increasingly large and complex networks make ineffective communication strategies evermore apparent. Broadcasting is a fundamental information-dissemination problem which models communication across a connected graph in the following manner: a single vertex, the originator, seeks to pass some message along to all other vertices in the graph. In general, research on broadcasting can be grouped in roughly two categories: Firstly, given some particular graph and some particular vertex chosen to be originator, what is a broadcast scheme that informs the entire graph in the minimum time possible? Secondly, given some number of nodes, how can we arrange them in a particular network topology such that we can achieve minimal broadcast time from any vertex? This thesis focuses on problems of the first category. Finding the minimum broadcast time of any vertex in an arbitrary graph is NP-Complete, but efficient algorithms have been found for particular graph families. In particular, polynomial time algorithms have been found for trees and some tree-like graphs: unicyclic graphs, tree of cycles. Such algorithms have also been found for some graphs with no intersecting cliques, such as fully connected trees and trees of cliques. Finally, graphs containing cycles with particular restrictions were also studied, and efficient algorithms for necklace graphs and k-restricted cactus graphs were also found. The question still stands however, of whether these restrictions may be too conservative, and that efficient algorithms exist on broader classes of graphs. In particular, significant research has been made towards finding an efficient broadcasting algorithm on cactus graphs, which has not been found so far. This thesis studies the broadcasting problem on Flower graphs, which capture the difficulty of cactus graphs in a simple graph family. Flower graphs, or k-cycle graphs, are graphs composed of k cycles all joined on a single central vertex v_c. The contributions of this thesis for broadcasting on flower graphs is two-fold: it first improves the approximation ratio for broadcasting on flower graphs. It then provides a heuristic which performs significantly better in practice than the current best heuristic. We also demonstrate that our heuristic finds the optimal broadcast time for particular subcases of flower graphs

Broadcasting in highly connected graphs

Throughout history, spreading information has been an important task. With computer networks expanding, fast and reliable dissemination of messages became a problem of interest for computer scientists. Broadcasting is one category of information dissemination that transmits a message from a single originator to all members of the network. In the past five decades the problem has been studied by many researchers and all have come to demonstrate that despite its easy definition, the problem of broadcasting does not have trivial properties and symmetries. For general graphs, and even for some very restricted classes of graphs, the question of finding the broadcast time and scheme remains NP-hard. This work uses graph theoretical concepts to explore mathematical bounds on how fast information can be broadcast in a network. The connectivity of a graph is a measure to assess how separable the graph is, or in other words how many machines in a network will have to fail to disrupt communication between all machines in the network. We initiate the study of finding upper bounds on broadcast time b(G) in highly connected graphs. In particular, we give upper bounds on b(G) for k-connected graphs and graphs with a large minimum degree. We explore 2-connected (biconnected) graphs and broadcasting in them. Using Whitney's open ear decomposition in an inductive proof we propose broadcast schemes that achieve an upper bound of ceil(n/2) for classical broadcasting as well as similar bounds for multiple originators. Exploring further, we use a matching-based approach to prove an upper bound of ceil(log(k)) + ceil(n/k) - 1 for all k-connected graphs. For many infinite families of graphs, these bounds are tight. Discussion of broadcasting in highly connected graphs leads to an exploration of dependence between the minimum degree in the graph and the broadcast time of the latter. By using similar techniques and arguments we show that if all vertices of the graph are neighboring linear numbers of vertices, then information dissemination in the graph can be achieved in ceil(log(n)) + C time. To the best of our knowledge, the bounds presented in our work are a novelty. Methods and questions proposed in this thesis open new pathways for research in broadcasting

Broadcasting in Hyper-cylinder graphs

Broadcasting in computer networking means the dissemination of information, which is known initially only at some nodes, to all network members. The goal is to inform every node in the minimal time possible. There are few models for broadcasting; the simplest and the historical model is called the Classical model. In the Classical model, dissemination happens in synchronous rounds, wherein a node may only inform one of its neighbors. The broadcast question is: What is the minimum number of rounds needed for broadcasting, and what broadcast scheme achieves it? For general graphs, these questions are NP-hard, and it is known to be at least 3 - ε inapproximable for any real ε > 0. Even for some very restricted classes of graphs, the questions remain as an NP-hard problem. Little is known about broadcasting in restricted graphs, and only a few classes have a polynomial solution. Parallel and distributed computing is one of the important domains which relies on efficient broadcasting. Hypercube and torus are the most used network topology in this domain. The widespread use is not only due to their simplicity but also is for their efficiency and high robustness (e.g., fault tolerance) while having an acceptable number of links. In this thesis, it is observed that the Cartesian product of a number of path and cycle graphs produces a valuable set of topologies, we called hyper-cylinders, which contain hypercube and Torus as well. Any hyper-cylinder shares many of the beneficial features of hypercube and torus and might be a suitable substitution in some cases. Some hyper-cylinders are also similar to other practically used topologies such as cube-connected cycles. In this thesis, the effect of the Cartesian product on broadcasting and broadcasting of hyper-cylinders under the Classical and Messy models is studied. This will add a valuable class of graphs to the limited classes of graphs which have a polynomially computable broadcast time. In the end, the relation between worst-case originators and diameters in trees is studied, which may help in the broadcast study of a larger class of graphs where any tree is allowed instead of a path in the Cartesian product