Esther Jennings

Enhancing Contact Graph Routing for Delay Tolerant Space Networking

When designing routing protocols for space-based networks, we must take into consideration the unique characteristics of such networks. Since space-based networks are inherently sparse with constrained resources, one needs to design smart routing algorithms that use the resources efficiently to maximize network performance. In Space Exploration Missions, the trajectories and orbits of spacecraft are predetermined, thus communication opportunities are predictable. This a-priori knowledge can be used to the advantage of scheduling and routing. In this paper, we focus on analyzing Contact Graph Routing (CGR) for space-based networks. CGR makes use of the predictable nature of the contacts to make routing decisions. Mars and Lunar mission-like scenarios were used in our simulations to gather statistics on routing protocol performance in terms of delay and buffer usage. We provide improvements to the underlying cost function of CGR to avoid routing loops and suggest applying Dijkstras shortest path algorithm for path selection. The cost function change was incorporated into the latest Internet Draft posted for CGR. Dijkstras shortest path algorithm was successfully implemented and tested in NASAs Interplanetary Overlay Network (ION) implementation of the DTN protocols.

Space Communications and Navigation (SCaN) Network Simulation Tool Development and Its Use Cases

In this work, we focus on the development of a simulation tool to assist in analysis of current and future (proposed) network architectures for NASA. Specifically, the Space Communications and Navigation (SCaN) Network is being architected as an integrated set of new assets and a federation of upgraded legacy systems. The SCaN architecture for the initial missions for returning humans to the moon and beyond will include the Space Network (SN) and the Near-Earth Network (NEN). In addition to SCaN, the initial mission scenario involves a Crew Exploration Vehicle (CEV), the International Space Station (ISS) and NASA Integrated Services Network (NISN). We call the tool being developed the SCaN Network Integration and Engineering (SCaN NIE (2) to optimize system configurations by testing a larger parameter space than may be feasible in either production networks or an emulated environment; (3) to test solutions in order to find issues/risks before committing more significant resources needed to produce real hardware or flight software systems. We describe two use cases of the tool: (1) standalone simulation of CEV to ISS baseline scenario to determine network performance, (2) participation in Distributed Simulation Integration Laboratory (DSIL) tests to perform function testing and verify interface and interoperability of geographically dispersed simulations/emulations.

Spacecraft data and relay management using delay tolerant networking

NASAs demonstration of the successful transmission of relay data through the orbiting Mars Odyssey, Mars Global Surveyor, and Mars Express by the Mars Exploration Rovers has shown not only the benefit of using a relay satellite for multiple landed assets in a deep space environment but also the benefit of international standards for such architecture. As NASA begins the quest defined in the Vision for Exploration with robotic and manned missions to the Moon, continues its study of Mars, and is joined in these endeavors by countries world-wide, landed assets transmitting data through relay satellites will be crucial for completing mission objectives. However, this method of delivery of data will result in increased complexity in routing and prioritization of data transmission as the number of missions increases. Also, there is currently no standard method among organizations conducting such missions to return these data sets to Earth given a complex environment. One possibility for establishing such a standard is for mission designers to deploy protocols which fall under the umbrella of Delay Tolerant Networking (DTN). These developing standards include the Bundle Protocol (BP) which provides a standard, secure, store and forward mechanism designed for high latency and asymmetric communication links and the Licklider Transmission Protocol (LTP) which is used to provide a reliable deep space link transmission service.

A network architecture for precision formation flying using the IEEE 802.11 MAC protocol

Precision formation flying (PFF) missions involve the tracking and maintenance of spacecraft in a desired geometric configuration. Autonomous control of the distributed spacecraft requires inter-spacecraft communications with guaranteed performance. We present a network architecture that supports PFF control across the various phases of mission operations, ranging from initial random deployment to precision formation. The architecture incorporates the IEEE 802.11 MAC protocol and utilizes both its distributed control function (DCF) and point coordination function (PCF) modes as appropriate to the PFF operational phase. A proactive routing protocol provides timely topology status. A new application layer protocol is incorporated which provides a simple interface between the PFF control application and the underlying communications network

Communications architecture for space-based sensor networks

Numerous planned and proposed future space exploration missions employ multiple spacecraft that perform multipoint sensing. Distributed space-based sensing missions can significantly benefit from incorporation of cross-link communications capabilities, thereby forming space-based networks, by enabling continuous access to any/all spacecraft via a single ground contact, real-time coordinated observations, and autonomous in situ processing within a spatial neighborhood of spacecraft. We present a communications architecture for space-based sensor networks. Because of the large inter-spacecraft distances, directional antennas are used, with a single half-duplex transceiver per spacecraft to achieve low cost. Orbital motion induces a dynamic albeit predictable geometry (and topology) among the spacecraft. Primary offered traffic is sensor telemetry destined to the Earth ground station, although other traffic patterns are also treated. We present a technique that derives the link activation schedule (transmit/receive mode and communications neighbor selection) and routes used for efficient traffic relay through the network, leveraging the Florens and McEliece algorithm for tree networks. An illustrative example is presented, and throughput and latency performance are evaluated. An extension to the networking method is described that is traffic adaptive.

On the diameter of sensor networks

In space exploration, cooperative modulation techniques have been proposed for prolonging the life-time of sensor nodes within a multihop network. The desire to efficiently reduce the overall energy-per-bit of a node motivated this study on the hop diameter (synonymous to the number of hops in a path) of sensor networks. In this study, we analysed and found that when the number of transmissions are bounded by constants /spl les/ 20, the likelihood of successful broadcast is small. Using simulations, we observed that the diameter decreases very fast as the transmission radius increases. Another observation is that the largest connected component emerges when the transmission radius reaches 0.3/spl radic/A, where A is the area containing the nodes. This may be used to determine the ideal amplification, although further simulations on larger networks could be helpful. We also found a large gap between the number of nodes required to populate the area, when all the nodes must be connected, or when only 90% of the nodes are connected.

Performance evaluation modeling of networked sensors

Substantial benefits are promised by operating many spatially separated sensors collectively. Such systems are envisioned to consist of sensor nodes that are connected by a communications network. A simulation tool is being developed to evaluate the performance of networked sensor systems, incorporating such metrics as target detection probabilities, false alarms rates, and classification confusion probabilities. The tool will be used to determine configuration impacts associated with such aspects as spatial laydown, and mixture of different types of sensors (acoustic, seismic, imaging, magnetic, RF, etc.), and fusion architecture. The QualNet discrete-event simulation environment serves as the underlying basis for model development and execution. This platform is recognized for its capabilities in efficiently simulating networking among mobile entities that communicate via wireless media. We are extending QualNets communications modeling constructs to capture the sensing aspects of multi-target sensing (analogous to multiple access communications), unimodal multi-sensing (broadcast), and multi-modal sensing (multiple channels and correlated transmissions). Methods are also being developed for modeling the sensor signal sources (transmitters), signal propagation through the media, and sensors (receivers) that are consistent with the discrete event paradigm needed for performance determination of sensor network systems. This work is supported under the Microsensors Technical Area of the Army Research Laboratory (ARL) Advanced Sensors Collaborative Technology Alliance.

Evaluating graph theoretic clustering algorithms for reliable multicasting

In reliable multicast protocols, each data packet being sent must be acknowledged. Collecting the acknowledgments centrally at the sources can cause ACK-implosion and can result in poor scalability. To overcome this, clustering algorithms which use virtual structures to gather acknowledgments were proposed. In this work, we analyze the complexities of three such clustering algorithms: Lorax, k-degree, and Self-adjust. We compare the quality of the virtual structures produced by these! algorithms, focusing on the number of clusters, cluster size, cluster radius, and the optimal positioning of cluster leaders. Our simulation showed that the virtual structure produced by Self-adjust is better in terms of cluster radius and the location of cluster leaders. However, due to the self-adjusting nature of the algorithm, it might take longer time to compute than the other two algorithms.

A clustering structure for reliable multicasting

In reliable multicast, the multicast packets must be acknowledged. We propose a clustering structure which can be used by most of the existing reliable multicast protocols for collecting acknowledgements and for making local retransmissions. Given a network N and a multicast routing tree (or a set of trees) T, we consider a subgraph G of N induced by the members of a multicast group. We then form disjoint clusters (local groups) of multicast receivers such that the receivers within a cluster are densely connected in G. The goal is to obtain a balanced clustering structure (dependent on the topology of G) such that the number of clusters is constant and the cluster size is kept low. This structure enables different clusters to process acknowledgments concurrently. It is also used to localize retransmissions. That is, when a packet is missed at a node, we will obtain the lost packet from another node which resides in the same cluster or at a nearby cluster whenever possible.

The impact of traffic prioritization on Deep Space Network mission traffic

A select number of missions supported by NASAs Deep Space Network (DSN) are demanding very high data rates. For example, the Kepler Mission was launched March 7, 2009 and at that time required the highest data rate of any NASA mission, with maximum rates of 4.33 Mb/s being provided via Ka band downlinks. The James Webb Space Telescope will require a maximum 28 Mb/s science downlink data rate also using Ka band links; as of this writing the launch is scheduled for a June 2014 launch. The Lunar Reconnaissance Orbiter, launched June 18, 2009, has demonstrated data rates at 100 Mb/s at lunar-Earth distances using NASAs Near Earth Network (NEN) and K-band. As further advances are made in high data rate space telecommunications, particularly with emerging optical systems, it is expected that large surges in demand on the supporting ground systems will ensue. A performance analysis of the impact of high variance in demand has been conducted using our Multi-mission Advanced Communications Hybrid Environment for Test and Evaluation (MACHETE) simulation tool. A comparison is made regarding the incorporation of Quality of Service (QoS) mechanisms and the resulting ground-to-ground Wide Area Network (WAN) bandwidth necessary to meet latency requirements across different user missions. It is shown that substantial reduction in WAN bandwidth may be realized through QoS techniques when low data rate users with low-latency needs are mixed with high data rate users having delay-tolerant traffic.1 2