Nathaniel M. Jones

Optimal routing and scheduling for a simple network coding scheme

We consider jointly optimal routing, scheduling, and network coding strategies to maximize throughput in wireless networks. While routing and scheduling techniques for wireless networks have been studied for decades, network coding is a relatively new technique that allows for an increase in throughput under certain topological and routing conditions. In this work we introduce k-tuple coding, a generalization of pairwise coding with next-hop decodability, and fully characterize the region of arrival rates for which the network queues can be stabilized under this coding strategy. We propose a dynamic control policy for routing, scheduling, and k-tuple coding, and prove that our policy is throughput optimal subject to the k-tuple coding constraint. We provide analytical bounds on the coding gain of our policy, and present numerical results to support our analytical findings. We show that most of the gains are achieved with pairwise coding, and that the coding gain is greater under 2-hop than 1-hop interference. Simulations show that under 2-hop interference our policy yields median throughput gains of 31% beyond optimal scheduling and routing on random topologies with 16 nodes.

Rate Control for Network-Coded Multipath Relaying with Time-Varying Connectivity

This paper presents techniques for achieving high throughput in delay-constrained, multihop wireless communication networks with time-varying link connectivity. We develop a rate-controlled, multipath strategy using network coding, and compare its performance with that of multipath flooding and with the performance of traditional single-path strategies. These performance comparisons include both theoretical benchmarks and simulation results from cooperative relay scenarios, which incorporate different sets of link connectivity statistics that are drawn from field tests of mobile satellite communication terminals. The results indicate that with appropriate rate-control, network coding can provide throughput performance comparable to multipath flooding of the network while utilizing bandwidth nearly as efficiently as single-path routing.

An overlay architecture for throughput optimal multipath routing

Legacy networks are often designed to operate with simple single-path routing, like shortest-path, which is known to be throughput suboptimal. On the other hand, previously proposed throughput optimal policies (i.e., backpressure) require every device in the network to make dynamic routing decisions. In this work, we study an overlay architecture for dynamic routing such that only a subset of devices (overlay nodes) need to make dynamic routing decisions. We determine the essential collection of nodes that must bifurcate traffic for achieving the maximum multicommodity network throughput. We apply our optimal node placement algorithm to several graphs and the results show that a small fraction of overlay nodes is sufficient for achieving maximum throughput. Finally, we propose a heuristic policy (OBP), which dynamically controls traffic bifurcations at overlay nodes. In all studied simulation scenarios, OBP not only achieves full throughput, but also reduces delay in comparison to the throughput optimal backpressure routing.

Uncoordinated MAC for Adaptive Multi-Beam Directional Networks: Analysis and Evaluation

In this paper, we consider medium access control (MAC) policies for emerging systems that are equipped with fully digital antenna arrays which are capable of adaptive multi-beam directional communications. With this technology, a user can form multiple simultaneous transmit or receive beams, allowing for greater spatial reuse and higher network throughput. The enabling technology that we consider is the ability to use digital post-processing to form multiple receive beams in real-time without a priori knowledge of the time and angle-of-arrival of the transmission. We present a novel unslotted, uncoordinated ALOHA-like random access MAC policy for multi-beam directional systems that asymptotically achieves the capacity of the network. Such an approach is particularly useful for systems where propagation delay makes the overhead associated with any sort of coordination prohibitive. We also consider the impact of numerous practical considerations including power constraints, latency, and beamwidth on the performance of our MAC policy.

MQCC: Maximum Queue Congestion Control for Multipath Networks with Blockage

This paper presents a transport layer protocol for multi-path networks with blockage. Using urban SATCOM as an example, we see from data taken from a 2006 measurement campaign that these blockages are generally on the order of 1 -- 5seconds in length and the links are blocked approximately 33%of the time. To compensate for this type of impairment, we have developed a multipath IP overlay routing algorithm, a random linear coding reliability scheme, and a maximum-queue-based (MQCC) congestion control algorithm. MQCC uses average buffer occupancy as a measure of the congestion in a network (as opposed to packet loss or round trip time (RTT)) and updates the transmission rate of each source to avoid network congestion. This allows us to design a congestion control algorithm that is independent of the channel conditions and can be made resilient to channel losses. The reliability scheme uses selective negative acknowledgments (Snacks) to guarantee packet delivery to the destination. We show through simulation that we can approach the optimal benchmark in realistic loss blockage channels.

Simulation and modeling of a new medium access control scheme for multi-beam directional networking

In this paper, we analyze a new medium access control (MAC) protocol for a multi-beam directional network via high-fidelity simulation using a real-time emulator. Multi-beam directional systems are a novel approach to networking which leverage recent advances in physical layer technology, allowing formation of multiple simultaneous beams in both transmit and receive. These multiple beams significantly reduce the burden some coordination between the transmitter and receiver and enable an uncoordinated, distributed random access scheme that offers high throughput. This paper is the first to characterize the performance of such systems using real-time simulation tools. In addition to implementing the random access scheme, several MAC features are developed that allow for robust communication, such as location tracking and extrapolating neighbor transmit or receive state. For this paper, we implement our protocol in both simulation and a new Extendable Mobile Ad-hoc Network Emulator (EMANE) model that allows for real-time, high fidelity performance evaluation. Using EMANE allows us to better understand the performance of the newly developed protocols by running real-time applications through the network. We show that our results from the EMANE model and simulator coincide with the theoretical network throughput, which allows for empirical characterization of scenarios in which theoretical results have not been derived. Furthermore, through our work, we stress test the EMANE model and quantify the maximum number of nodes that we can operate in real-time. Through this testing, two bottlenecks are identified: 1) infrastructure issues, where the amount of data passed between the servers is too high, and 2) computation issues, where calculating the interference on the packets becomes too time consuming.

Distributed CSMA with pairwise coding

We consider distributed strategies for joint routing, scheduling, and network coding to maximize throughput in wireless networks. Network coding allows for an increase in network throughput under certain routing conditions. We previously developed a centralized control policy to jointly optimize for routing and scheduling combined with a simple network coding strategy using max-weight scheduling (MWS) [9]. In this work we focus on pairwise network coding and develop a distributed carrier sense multiple access (CSMA) policy that supports all arrival rates allowed by the network subject to the pairwise coding constraint. We extend our scheme to optimize for packet overhearing to increase the number of beneficial coding opportunities. Simulation results show that the CSMA strategy yields the same throughput as the optimal centralized policy of [9], but at the cost of increased delay. Moreover, overhearing provides up to an additional 25% increase in throughput on random topologies.

Request Protocol Performance Impact for Mobile SATCOM with Dynamic Resource Allocation

Future communication schemes for transponding satellites will use dynamic resource allocation for efficient transmission of multi-media packet traffic. In typical centrally controlled schemes, a controller terminal is designated to collect resource requests from the other terminals and allocate slots. The delay between request generation and the related allocations reduces performance. For communications with terminals on-the-move, occasional blockage of the link is an additional impairment to the request-allocation protocol. This results infrequent lost request and allocation messages which may impair the ability of the terminal to communicate even when it is not blocked. We consider how the request-allocation protocol interacts with the on-the-move SATCOM channel for several allocation algorithms in the context of user-relevant performance metrics

Performance characterization of dynamic allocation schemes for multi-frequency TDMA SATCOM-on-the-move

Future military networks will make significant use of transponding satellites to carry converged Internet protocol (IP) traffic to on-the-move communication nodes, which suffer frequent, short signal blockage events. Multi-frequency time-division multiple access (MF-TDMA) with dynamic resource allocation is a leading approach to efficiently use satellite resources under these conditions. The dynamic resource allocation protocol interacts with the dynamics of the blockage channel, the network traffic, and other protocols. We compare a range of protocols from fixed allocation to genie-aided approaches where optimistic assumptions about the information available to the algorithm provide an intuitive upper bound on performance. Specific quantitative metrics for user traffic quality are defined and compared for various algorithms and traffic loads

An Overlay Architecture for Throughput Optimal Multipath Routing

Legacy networks are often designed to operate with simple single-path routing, like the shortest path, which is known to be throughput suboptimal. On the other hand, previously proposed throughput optimal policies (i.e., backpressure) require every device in the network to make dynamic routing decisions. In this paper, we study an overlay architecture for dynamic routing, such that only a subset of devices (overlay nodes) need to make the dynamic routing decisions. We determine the essential collection of nodes that must bifurcate traffic for achieving the maximum multi-commodity network throughput. We apply our optimal node placement algorithm to several graphs and the results show that a small fraction of overlay nodes is sufficient for achieving maximum throughput. Finally, we propose a threshold-based policy (BP-T) and a heuristic policy (OBP), which dynamically control traffic bifurcations at overlay nodes. Policy BP-T is proved to maximize throughput for the case when underlay paths do no overlap. In all studied simulation scenarios, OBP not only achieves full throughput but also reduces delay in comparison to the throughput optimal backpressure routing.