Computing with Tangles

Tangles of graphs have been introduced by Robertson and Seymour in the context of their graph minor theory. Tangles may be viewed as describing "k-connected components" of a graph (though in a twisted way). They play an important role in graph minor theory. An interesting aspect of tangles is that they cannot only be defined for graphs, but more generally for arbitrary connectivity functions (that is, integer-valued submodular and symmetric set functions). However, tangles are difficult to deal with algorithmically. To start with, it is unclear how to represent them, because they are families of separations and as such may be exponentially large. Our first contribution is a data structure for representing and accessing all tangles of a graph up to some fixed order. Using this data structure, we can prove an algorithmic version of a very general structure theorem due to Carmesin, Diestel, Harman and Hundertmark (for graphs) and Hundertmark (for arbitrary connectivity functions) that yields a canonical tree decomposition whose parts correspond to the maximal tangles. (This may be viewed as a generalisation of the decomposition of a graph into its 3-connected components.

Context and semantics in exchange networks

The NETSEED project deals with strengthening management of agro-biodiversity through social networks using an interdisciplinary method for analyzing how local seed systems impact the diversity of domesticated plants. The goal of the project is to study the structure of seed exchange networks among farmers, i.e. the fluxes of 'seeds' (generic term for all plant propagation vectors, including true seeds and vegetative propagules) and provide comprehensive models relating plant biodiversity to socio-cultural and geographic factors. The general question is to assess how SEEN structure - the significations, directions and intensities of seed fluxes among individuals or groups exchanging seeds, and the distribution of genealogical, socio-cultural or geographical distance among these individuals or social entities - impacts agro-biodiversity.The project gathers 12 datasets from 10 socio-ecological contexts (societies/countries) collected by different specialists from distinct disciplines: anthropology, ecology, ethnobiology, geography ... Producing a meta-analysis from the database supposes that a common collection method was adopted. Since this is not the case, the composition of databases requires the use Knowledge Management, i.e. methodologies and tools enabling a formal representation of knowledge in order to identify, create, represent and distribute insights and experiences in an organization, e.g. NETSEED. Referring to the distinction between Data, Information and Knowledge, the NETSEED datasets must be supplemented by a description of the study protocols to establish a link between Data and Knowledge. The semantic associated with the information has to be considered in the construction of this link. The purpose of this presentation is to illustrate the semantic distances between case studies described during the project workshops. (Résumé d'auteur

Abstract unordered and ordered trees CRDT

Trees are fundamental data structure for many areas of computer science and system engineering. In this report, we show how to ensure eventual consistency of optimistically replicated trees. In optimistic replication, the different replicas of a distributed system are allowed to diverge but should eventually reach the same value if no more mutations occur. A new method to ensure eventual consistency is to design Conflict-free Replicated Data Types (CRDT). In this report, we design a collection of tree CRDT using existing set CRDTs. The remaining concurrency problems particular to tree data structure are resolved using one or two layers of correction algorithm. For each of these layer, we propose different and independent policies. Any combination of set CRDT and policies can be constructed, giving to the distributed application programmer the entire control of the behavior of the shared data in face of concurrent mutations. We also propose to order these trees by adding a positioning layer which is also independent to obtain a collection of ordered tree CRDTs

A Design Methodology for Space-Time Adapter

This paper presents a solution to efficiently explore the design space of communication adapters. In most digital signal processing (DSP) applications, the overall architecture of the system is significantly affected by communication architecture, so the designers need specifically optimized adapters. By explicitly modeling these communications within an effective graph-theoretic model and analysis framework, we automatically generate an optimized architecture, named Space-Time AdapteR (STAR). Our design flow inputs a C description of Input/Output data scheduling, and user requirements (throughput, latency, parallelism...), and formalizes communication constraints through a Resource Constraints Graph (RCG). The RCG properties enable an efficient architecture space exploration in order to synthesize a STAR component. The proposed approach has been tested to design an industrial data mixing block example: an Ultra-Wideband interleaver.Comment: ISBN : 978-1-59593-606-

Modeling soil-water dynamics for diverse environmental needs

Accurate representation of the top soil matrix with special attention to the scales present is important to the dynamics of water flow and fate of pollution at field, farm and watershed scales. It is also important from an agronomic stand point since agriculture still constitutes the main source of pollution at a time when many agronomic models still use empirical notions of soil hydraulic properties. The soil dynamics literature describes soil hydraulic properties independently from the soil-water medium hydro-structural dynamics. This leads to an empirical approach to represent and estimate soil hydro-structural properties such as shrinkage, water potential, field capacity, available water, hydraulic conductivity, etc. This study presents a computer model of structured soil-water medium in which the thermodynamic equilibrium is characterized by its internal hydro-structural changes. (Résumé d'auteur

Transport congestion events detection (TCED): towards decorrelating congestion detection from TCP

TCP (Transmission Control Protocol) uses a loss-based algorithm to estimate whether the network is congested or not. The main difficulty for this algorithm is to distinguish spurious from real network congestion events. Other research studies have proposed to enhance the reliability of this congestion estimation by modifying the internal TCP algorithm. In this paper, we propose an original congestion event algorithm implemented independently of the TCP source code. Basically, we propose a modular architecture to implement a congestion event detection algorithm to cope with the increasing complexity of the TCP code and we use it to understand why some spurious congestion events might not be detected in some complex cases. We show that our proposal is able to increase the reliability of TCP NewReno congestion detection algorithm that might help to the design of detection criterion independent of the TCP code. We find out that solutions based only on RTT (Round-Trip Time) estimation are not accurate enough to cover all existing cases. Furthermore, we evaluate our algorithm with and without network reordering where other inaccuracies, not previously identified, occur