Pablo G. Madoery

On the Design and Analysis of Fair Contact Plans in Predictable Delay-Tolerant Networks

Delay-tolerant networks (DTNs) have become a promising architecture for wireless sensor systems in challenged communication environments where traditional solutions based on persistent connectivity either fail or show serious weaknesses. As a result, different routing schemes have been investigated that take into account the time-evolving nature of the network topology. Among them, contact graph routing has been proposed for space environments with predictable connectivity. In order to evaluate routing decisions, DTN nodes need to know the contact plan in advance, which comprises all communication links among nodes that will be available in the future. Since not all potential contacts can belong to the contact plan, its design requires analyzing conflicting contacts in order to select those that meet an overall goal. In this paper, we consider the design of contact plans that can maximize fairness requirements while still maximizing the overall capacity as well. To this end, we propose to formalize the problem by means of an optimization model and evaluate its performance in terms of different fairness metrics. Since this model can be computationally intractable for a large number of contacts, we also propose to tackle it as a matching problem, resulting in algorithms of polynomial complexity, and compare these results with those of the original model. We show that fairness can be properly modeled to design contact plans and that efficient algorithms do exist to compute these plans quite accurately while also improving overall network routing metrics for a proposed case study.

Traffic-aware contact plan design for disruption-tolerant space sensor networks

Delay and disruption tolerant networks (DTNs) are becoming an appealing solution for extending Internet boundaries across challenged network environments. In particular, if node mobility can be predicted as in space sensor networks (SSNs), routing schemes can take advantage of the a-priori knowledge of a contact plan comprising forthcoming communication opportunities. However, the design of such a plan needs to consider available spacecraft resources whose utilization can be optimized by exploiting the expected data which is largely foreseeable in typical Earth observation missions. In this work, we propose Traffic-Aware Contact Plan (TACP): a novel contact plan design procedure based on a Mixed Integer Linear Programming (MILP) formulation which exploits SSNs predictable properties in favor of delivering efficient and implementable contact plans for spaceborne DTNs. Finally, we analyze a low orbit SSN case study where TACP outperforms existing mechanisms and proves to be of significant impact on enhancing the delivery of sensed data from future space networks.

Congestion modeling and management techniques for predictable disruption tolerant networks

Delay and disruption tolerant networks (DTNs) are becoming an appealing solution for extending Internet boundaries so as to embrace disruptive communications. In particular, if node trajectory can be predicted as in space networks, routing schemes can take advantage of the a-priori knowledge of a contact plan. Despite mechanisms such as Contact Graph Routing (CGR) exist, they might derive in harmful overbooking of forthcoming contacts, also known as congestion. In order to tackle congestion, we initially formulate the problem by means of a linear programming (LP) model so as to establish an upper theoretical bound of performance. Next, we survey existing congestion mitigation mechanisms for predictable DTNs to later contribute with a CGR extension named PA-CGR. Finally, in the pursuance of an optimal congestion avoidance approach, we also propose and evaluate in a realistic scenario a novel multi-graph technique (MG-CGR) that outperforms existing solutions by exploiting traffic predictability.

Leveraging routing performance and congestion avoidance in predictable delay tolerant networks

Delay or Disruption Tolerant Network (DTN) architecture has become a promising solution for challenged environments where communications cannot be assumed persistent. In particular, if node dynamics can be predicted like in space-borne networks, routing schemes such as Merugus Floyd Warshsall (MFW) and Contact Graph Routing (CGR) can take advantage of the a priori knowledge of the DTN topology. In this work we compare and leverage these popular schemes and propose Cache-CGR: a computationally efficient version of CGR that reduce processing requirements while preserving its features such as local congestion avoidance. We demonstrate the performance gain of C-CGR both by means of simulation and an on-board computer test-bench with Interplanetary Overlay Network (ION) DTN implementation. Finally, we discuss CGR open issues and lay the foundations for future research on DTN global congestion avoidance mechanisms.

On the design of fair contact plans in predictable Delay-Tolerant Networks

Delay-Tolerant Networks (DTNs) have become a promising solution for challenged communication environments. As a result, different routing schemes have been investigated that take into account the time-evolving nature of the network topology. Among them, Contact Graph Routing (CGR) has been proposed for environments with predictable connectivity. In order to evaluate routing decisions, DTN nodes need to know the contact plan in advance, which comprises all communication links among nodes that will be available in the future. Since not all potential contacts can belong to the contact plan, its design requires analyzing conflicting contacts in order to select those that meet an overall goal. In this paper, we consider the design of contact plans that can maximize fairness requirements while still maximizing the overall capacity as well. To this end, we propose to formalize the problem by means of an optimization model and evaluate its performance in terms of different fairness metrics. Since this model can be computationally intractable for a large number of contacts, we also propose to tackle it as a matching problem, resulting in algorithms of polynomial complexity, and compare these results with those of the original model. We show that fairness can be properly modeled to design contact plans and that efficient algorithms do exist to compute these plans quite accurately.

Assessing Contact Graph Routing Performance and Reliability in Distributed Satellite Constellations

Existing Internet protocols assume persistent end-to-end connectivity, which cannot be guaranteed in disruptive and high-latency space environments. To operate over these challenging networks, a store-carry-and-forward communication architecture called Delay/Disruption Tolerant Networking (DTN) has been proposed. This work provides the first examination of the performance and robustness of Contact Graph Routing (CGR) algorithm, the state-of-the-art routing scheme for space-based DTNs. To this end, after a thorough description of CGR, two appealing satellite constellations are proposed and evaluated by means of simulations. Indeed, the DtnSim simulator is introduced as another relevant contribution of this work. Results enabled the authors to identify existing CGR weaknesses and enhancement opportunities.

Internetworking approaches towards along-track segmented satellite architectures

The Argentinian Space Agency (CONAE) has been pursuing the development of segmented satellite architectures as a novel strategy to reduce the cost and improve responsiveness of space access. Among the key enablers for these architectures, wireless communication is likely to be the most critical as it requires to operate on a highly dynamic, sparse and extreme environments. In this work, we analyze and derive the main specifications that a communication system for segmented architectures (CSSA) must support. In particular, we provide an abstract model to predict the access time to segmented systems to later propose a layered arrangement of the main specifications for the CSSA. Finally, we analyze by simulation a particular CSSA configuration in a simple yet representative scenario with an along-track flight formation.

Congestion management techniques for disruption‐tolerant satellite networks

Summary Delay and disruption-tolerant networks are becoming an appealing solution for extending Internet boundaries toward challenged environments where end-to-end connectivity cannot be guaranteed. In particular, satellite networks can take advantage of a priori trajectory estimations of nodes to make efficient routing decisions. Despite this knowledge is already used in routing schemes such as contact graph routing, it might derive in congestion problems because of capacity overbooking of forthcoming connections (contacts). In this work, we initially extend contact graph routing to provide enhanced congestion mitigation capabilities by taking advantage of the local traffic information available at each node. However, since satellite networks data generation is generally managed by a mission operation center, a global view of the traffic can also be exploited to further improve the latter scheme. As a result, we present a novel strategy to avoid congestion in predictable delay- and disruption-tolerant network systems by means of individual contact plans. Finally, we evaluate and compare the performance improvement of these mechanisms in a typical low Earth orbit satellite constellation.

Preliminary results of an evolutionary approach towards Contact Plan design for satellite DTNs

Delay and disruption tolerant networks (DTNs) are becoming an appealing solution for extending Internet boundaries so as to embrace disruptive communications. In particular, if node trajectory and orientation can be predicted as in satellite networks, routing schemes can take advantage of the a-priori knowledge of a contact plan comprising the forthcoming communications opportunities. However, the design of such a plan need to consider both available spacecraft resources and the expected traffic which is largely foreseeable in space applications. In this context, the existing Traffic-Aware Contact Plan (TACP) procedure exploits this properties, but the computation complexity of its theoretical formulation results prohibitive for real satellite applications. As a result, we propose CPD-EA: a genetic algorithm to provide sub-optimal yet efficient and implementable contact plans in reasonable time. In particular, we describe the algorithm strategies and evaluate its preliminary performance in a realistic Low Earth Orbit (LEO) scenario demonstrating it usefulness for planning future DTN-based satellite networks.

Analysis of Communication Strategies for Earth Observation Satellite Constellations

Earth observation satellite constellations are arising as a new paradigm with important advantages in comparison with traditional monolithic systems. In particular, the concept of Segmented Architecture proposes to transform these constellations into a distributed and connected system in order to exploit resource sharing. However, this imposes severe technological challenges such as packetized space networking communications. In general, and due to the limited availability of data transmission opportunities, these communications can be modeled as delay and disruption tolerant networks (DTN). In this article, we propose to evaluate a particular case study of several flight-formation satellites intended for the observation of the Earth. To this end, we consider both the availability of Earth-to-space links and their combination with cooperative inter-satellite links. In the latter, different routing schemes such as Contact Graph Routing (CGR) and its extension Multi-Graph CGR (MG-CGR) are analyzed and compared.