Juan A. Fraire

Design challenges in contact plans for disruption-tolerant satellite networks

During the past 20 years, space communications technologies have shown limited progress in comparison to Internet-based networks on Earth. However, a brand new working group of the IETF with focus on DTN promises to extend todays Internet boundaries to embrace disruptive communications such as those seen in space networks. Nevertheless, several challenges need to be overcome before operative DTNs can be deployed in orbit. We analyze the state of the art of effective design, planning, and implementation of the forthcoming network communications opportunities (contacts). To this end, different modeling techniques, system constraints, selection criteria, and methods are reviewed and compared. Finally, we discuss the increasing complexity of considering routing and traffic information to enrich the planning procedure, yielding the need to implement a contact plan computation element to support space DTN operation.

Routing-aware fair contact plan design for predictable delay tolerant networks

Delay tolerant networks (DTNs) have become a promising solution for extending Internet boundaries to challenged environments such as satellite constellations. In this context, strategies to exploit scarce communication opportunities, while still considering device and application constraints, are still to be investigated to enable the actual deployment of these networks. In particular, the Contact Graph Routing (CGR) scheme has been proposed as it takes advantage of the contact plan, which comprises all future contacts among nodes. However, resource constraints can forbid the totality of these contacts to belong to the contact plan; thus, only those which together meet an overall goal shall be selected. In this article, we consider the problem of designing a contact plan that can provide fairness in link assignment and minimal all-to-all route delay; therefore, achieving equal contact opportunities while favoring end-to-end traffic latency. We formalize this by means of a multi-objective optimization model that can be computationally intractable for large topologies; thus, heuristic algorithms are proposed to compute the contact plan in practice. Finally, we analyze general results from these routines and discuss how they can used to provision valuable contact plans for real networks.

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.

Assessing DTN architecture reliability for distributed satellite constellations: Preliminary results from a case study

Networked small satellites constellations can yield, in general, not only higher revisit rates but new mission opportunities with important cost and risk saving by means of successive small launches and distributed functionalities such as payload, storage, processing, or data downlink. Nevertheless, as this networks operates in challenged environments, they usually face resources constraints; moreover, orbital dynamics might impose sporadic channel availability. As a result, these intermittent inter-satellite communications challenges existing networking protocols as they assume persistent connectivity. To this end, Delay Tolerant Networking (DTN) has emerged as an automated store-carry-and-forward communication architecture capable to cope with contact disruption. In order to assess DTN reliability, we generalize the communications disruptions to also include transient and permanent component faults so as to demonstrate that DTN architecture result inherently fault tolerant as failures no longer implies a service outage but an overall system capacity degradation. To this end, we developed a network model encompassing DTN communication protocols, routing algorithms, and satellite failure models to measure the system capacity degradation under specific constellation topologies.

OpenCL Overview, Implementation, and Performance Comparison

High performance parallel computing was something exclusive for expensive specialized hardware some years ago. But now we can find powerful parallel processors in many home graphics card whose interface has been recently opened by many manufacturers for general purpose computing. OpenCL, created by the world most important processors manufacturers, went a little further, aiming for a platform and manufacturer independent parallel language. However, understanding this new processing paradigm is challenging and critical for future computation demanding applications. The first approach of this document is to provide a deep technical background of OpenCL architecture. Second, we propose an implementation of a matrix product calculation OpenCL kernel directly implemented in C++ without wrappers so as to describe in detail the OpenCL programming flow. Thirdly, different platforms and algebraic scenarios are created for this program concluding that the improvement of calculation performance can reach up to 3 orders of magnitude over the same algorithm in plain C++.