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 …

I and i

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

Imagined communities: Reflections on the origin and spread of nationalism

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.

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Community detection in graphs

Opportunistic networking largely relies on human-carried devices as relays to move data across the network. Human altruistic behavior is an important factor in the feasibility and performance of such a system. In this paper, we study the impact of different altruism distributions on the system throughput and delay of opportunistic communications. We evaluate the system performance by utilizing social network topological models and experimental human mobility traces with different communication patterns. We also discuss several observations from the result of the study. To the best of our knowledge, this is the first study of the impact of altruism on opportunistic communications.

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Impact of altruism on opportunistic communications

The recent work on COPE by Katti et al. demonstrates a practical application of network coding to wireless multihop networks. We note, however, that the opportunistic nature of COPE leaves it at the mercy of higher and lower layer protocols to create coding opportunities spontaneously. In this paper, we go one step beyond COPE's opportunism and study how to create coding opportunities in a more deterministic, yet still practical way. We start from the insight that in two-way traffic the existence of coding opportunities can be guaranteed through carefully co-ordinated packet scheduling, and establish general properties of protocols that are able to achieve this. We then propose Near-Optimal Coordinated Coding (noCoCo), a cross-layer scheme that integrates per-hop packet scheduling, network coding, and congestion control in a novel way. Extensive simulations show that noCoCo significantly outperforms standard non-coding approaches as well as COPE in terms of network throughput, delay and transmission overhead.

/pdf/near-optimal-co-ordinated-coding-in-wireless-multihop-4abhxwynw5.pdf

Near-optimal co-ordinated coding in wireless multihop networks

A chargeable session on the Internet may consist of more than one underlying chargeable service. Typically there will be two, one at the network layer and one at the session layer. Since different applications can have different demands from the network, a generic charging scheme has to separate the service provided by the network from the service provided by an application/service provider. In this paper we propose a pricing model which is session-based and we look at the impact of this on real-time multimedia conferencing over the Internet. In this model, we are trying to allow for the optional integration of charging at the network layer with charging at the session layer, while keeping the underlying technologies still cleanly apart. This paper also highlights the fact that the main problem of pricing application on the Internet is not just a simple case of analyzing the most technically feasible pricing mechanism but also making the solution acceptable to users. We take the position that session-based pricing is easier for end users to accept and understand and show why this is the case in this paper.

A charging model for sessions on the Internet

So what is all this DTN research about anyway? Sceptics ask: "Why are there no DTN applications?", or "Why is DTN performance so miserable?" This article attempts to address some of these complaints. We present suggestions of expectations for applications, and metrics for performance, which suggest a more tolerant view of research in the area.

/pdf/promoting-tolerance-for-delay-tolerant-network-research-brdg4wucxa.pdf

Promoting tolerance for delay tolerant network research

There is much in common between the models of traffic management being devised by the IETF and ATM Forum working groups. The authors' goal is to demonstrate that because of this convergence, there are a number of ways that each can take advantage of the other. The authors discuss service models, link sharing, network provision models, traffic service class hierarchies, borrowing and replacement strategies, delays and throughput, ordering and routing with particular application to the Internet. >

Jon Crowcroft

Papers

Impact of altruism on opportunistic communications

Near-optimal co-ordinated coding in wireless multihop networks

A charging model for sessions on the Internet

Promoting tolerance for delay tolerant network research

A rough comparison of the IETF and ATM service models