Jon Crowcroft

Bio: Jon Crowcroft is an academic researcher from University of Cambridge. The author has contributed to research in topics: The Internet & Multicast. The author has an hindex of 87, co-authored 672 publications receiving 38848 citations. Previous affiliations of Jon Crowcroft include Memorial University of Newfoundland & Information Technology University.

Effective Capacity Analysis of HARQ-Enabled D2D Communication in Multi-Tier Cellular Networks

Abstract: This work does the statistical quality-of-service (QoS) analysis of a block-fading device-to-device (D2D) link in a multi-tier cellular network that consists of a macro-BS ( $BS_{_{MC}}$ ) and a micro-BS ( $BS_{_{mC}}$ ) which both operate in full-duplex (FD) mode. For the D2D link under consideration, we first formulate the mode selection problem—whereby D2D pair could either communicate directly, or, through the $BS_{_{mC}}$ , or, through the $BS_{_{MC}}$ —as a ternary hypothesis testing problem. Next, to compute the effective capacity (EC) for the given D2D link, we assume that the channel state information (CSI) is not available at the transmit D2D node, and hence, it transmits at a fixed rate $r$ with a fixed power. This allows us to model the D2D link as a Markov system with six-states. We consider both overlay and underlay modes for the D2D link. Moreover, to improve the throughput of the D2D link, we assume that the D2D pair utilizes two special automatic repeat request (ARQ) schemes, i.e., Hybrid-ARQ (HARQ) and truncated HARQ. Furthermore, we consider two distinct queue models at the transmit D2D node, based upon how it responds to the decoding failure at the receive D2D node. Eventually, we provide closed-form expressions for the EC for both HARQ-enabled D2D link and truncated HARQ-enabled D2D link, under both queue models. Noting that the EC looks like a quasi-concave function of $r$ , we further maximize the EC by searching for an optimal rate via the gradient-descent method. Simulation results provide us the following insights: i) EC decreases with an increase in the QoS exponent, ii) EC of the D2D link improves when HARQ is employed, iii) EC increases with an increase in the quality of self-interference cancellation techniques used at $BS_{_{mC}}$ and $BS_{_{MC}}$ in FD mode.

Enabling opportunistic resources sharing on mobile operating systems: benefits and challenges

Abstract: The intense use of hardware resources by mobile applications has a significant impact on the battery life of mobile devices. In this paper we introduce a novel approach for the efficient use of mobile phone resources, by considering the coordinated sharing of resources offered by multiple co-located devices. Taking into account the social behaviour of users, there are frequent situations where similar resources are available by co-located mobile phones. In this work we discuss the feasibility of sharing such resources in an opportunistic way, the possible benefits and the research challenges that need to be addressed in order to implement a reliable and robust solution.

Lazy Susan: dumb waiting as proof of work

Abstract: The open nature of Internet services has been of great value to users, enabling dramatic innovation and evolution of services. However, this openness permits many abuses of open-access Internet services such as web, email, and DNS. To counteract such abuses, a number of so called proof-of-work schemes have been proposed. They aim to prevent or limit such abuses by demanding potential clients of the service to prove that they have carried out some amount of work before they will be served. In this paper we show that existing resource-based schemes have several problems, and instead propose latency-based proof-of-work as a solution. We describe centralised and distributed variants, introducing the problem class of non-parallelisable shared secrets in the process. We also discuss application of this technique at the network layer as a way to prevent Internet distributed denial-of-service attacks.

A Combined Admission and Congestion Control Scheme for Variable Bit Rate Video

Abstract: An approach is developed to maximize the bandwidth utilization of a network for continually transmitting sources, such as video sources. We assume that the sources use hierarchical encoding and can adapt their source coding rates in response to requests from the network congestion control algorithm, and we use this adaptability as the controlled input for a proportional control algorithm. We model flows entering or leaving the network as step inputs, and use the algorithm as the basis for admission control, using very little intelligence in the switches. We then present simulation results that demonstrate the algorithm working in a number of scenarios.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Community detection in graphs

Abstract: The modern science of networks has brought significant advances to our understanding of complex systems. One of the most relevant features of graphs representing real systems is community structure, or clustering, i. e. the organization of vertices in clusters, with many edges joining vertices of the same cluster and comparatively few edges joining vertices of different clusters. Such clusters, or communities, can be considered as fairly independent compartments of a graph, playing a similar role like, e. g., the tissues or the organs in the human body. Detecting communities is of great importance in sociology, biology and computer science, disciplines where systems are often represented as graphs. This problem is very hard and not yet satisfactorily solved, despite the huge effort of a large interdisciplinary community of scientists working on it over the past few years. We will attempt a thorough exposition of the topic, from the definition of the main elements of the problem, to the presentation of most methods developed, with a special focus on techniques designed by statistical physicists, from the discussion of crucial issues like the significance of clustering and how methods should be tested and compared against each other, to the description of applications to real networks.