Pablo Soldati

A generalized Markov chain model for effective analysis of slotted IEEE 802.15.4

A generalized analysis of the IEEE 802.15.4 medium access control (MAC) protocol in terms of reliability, delay and energy consumption is presented. The IEEE 802.15.4 exponential backoff process is modeled through a Markov chain taking into account retry limits, acknowledgements, and unsaturated traffic. Simple and effective approximations of the reliability, delay and energy consumption under low traffic regime are proposed. It is demonstrated that the delay distribution of IEEE 802.15.4 depends mainly on MAC parameters and collision probability. In addition, the impact of MAC parameters on the performance metrics is analyzed. The analysis is more general and gives more accurate results than existing methods in the literature. Monte Carlo simulations confirm that the proposed approximations offer a satisfactory accuracy.

Mathematical decomposition techniques for distributed cross-layer optimization of data networks

Network performance can be increased if the traditionally separated network layers are jointly optimized. Recently, network utility maximization has emerged as a powerful framework for studying such cross-layer issues. In this paper, we review and explain three distinct techniques that can be used to engineer utility-maximizing protocols: primal, dual, and cross decomposition. The techniques suggest layered, but loosely coupled, network architectures and protocols where different resource allocation updates should be run at different time-scales. The decomposition methods are applied to the design of fully distributed protocols for two wireless network technologies: networks with orthogonal channels and network-wide resource constraints, as well as wireless networks where the physical layer uses spatial-reuse time-division multiple access. Numerical examples are included to demonstrate the power of the approach

Optimal link scheduling and channel assignment for convergecast in linear WirelessHART networks

Convergecast, in which data from a set of sources is routed toward one data sink, is a critical functionality for wireless networks deployed for industrial monitoring and control. We address the joint link scheduling and channel assignment problem for convergecast in networks operating according to the recent WirelessHART standard. For a linear network with N single-buffer devices, we demonstrate that the minimum time to complete convergecast is 2N-1 time-slots, and that the minimum number of channels required for this operation is ⌈N/2⌉. When the devices are allowed to buffer multiple packets, we prove that the optimal convergecast time remains the same while the number of required channels can be reduced to . For both cases, we present jointly time- and channel-optimal scheduling policies with complexity O(N2). Numerical results demonstrate that our schemes are also efficient in terms of memory utilization.

Opportunistic Routing in Low Duty-Cycle Wireless Sensor Networks

Opportunistic routing is widely known to have substantially better performance than unicast routing in wireless networks with lossy links. However, wireless sensor networks are usually duty cycled, that is, they frequently enter sleep states to ensure long network lifetime. This renders existing opportunistic routing schemes impractical, as they assume that nodes are always awake and can overhear other transmissions. In this article we introduce ORW, a practical opportunistic routing scheme for wireless sensor networks. ORW uses a novel opportunistic routing metric, EDC, that reflects the expected number of duty-cycled wakeups that are required to successfully deliver a packet from source to destination. We devise distributed algorithms that find the EDC-optimal forwarding and demonstrate using analytical performance models and simulations that EDC-based opportunistic routing results in significantly reduced delay and improved energy efficiency compared to traditional unicast routing. In addition, we evaluate the performance of ORW in both simulations and testbed-based experiments. Our results show that ORW reduces radio duty cycles on average by 50% (up to 90% on individual nodes) and delays by 30% to 90% when compared to the state-of-the-art.

Proportionally fair allocation of end-to-end bandwidth in STDMA wireless networks

We consider the problem of designing distributed mechanisms for joint congestion control and resource allocation in spatial-reuse TDMA wireless networks. The design problem is posed as a utility maximization subject to link rate constraints that involve both power allocation and transmission scheduling over multiple time-slots. Starting from the performance limits of a centralized optimization based on global network information,we proceed systematically in the development of distributed and transparent protocols. In the process,we introduce a novel decomposition method for convex optimization,establish its convergence for the utility maximization problem and demonstrate how it suggests a distributed solution based on flow control optimization and incremental updates of the transmission schedule.We develop a two-step procedure for finding the schedule updates and suggest two schemes for distributed channel reservation and power control under realistic interference models. Although the final protocols are suboptimal,we isolate and quantify the performance losses incurred by each simplification and demonstrate strong performance in examples.

Optimal Routing and Scheduling of Deadline-Constrained Traffic over Lossy Networks

The traditionally wired automation infrastructure is quickly migrating to more flexible and scalable wireless solutions. To cope with the stringent requirements of process automation in terms of latency and reliability, the network resources must be optimized to ensure timely and reliable communication. This paper considers the joint routing and transmission scheduling problem for reliable real- time communication over lossy networks. Specifically, we impose a strict latency bound for packet delivery from source to destination, and devise optimal transmission scheduling policies that maximize the success probability of delivering the packet within the specified deadline. A solution to this problem allows to characterize the set of achievable latencies and packet reliability for a given network. We offer a complete understanding of the problem when erasure events on links are independent and follow a Bernoulli process. We consider both static and dynamic resource allocation policies, and compare them in numerical examples.

Performance Bounds and Latency-Optimal Scheduling for Convergecast in WirelessHART Networks

Convergecast, in which data from a set of source devices is delivered to a single data sink, is a critical functionality in networks deployed for industrial monitoring and control. We address the latency-optimal link scheduling problem for convergecast in networks operating according to the recent WirelessHART standard. When there is no restriction on the number of channels, we present a latency-optimal scheduling policy in which each routing node is required to buffer at most one packet at any point in time. For networks with a limited number of channels, we first establish a lower bound on the number of channels for latency-optimal convergecast and a lower bound on latency for convergecast using a fixed number of channels, and then present a heuristic scheme for channel-constrained latency-optimal convergecast scheduling. Simulation results confirm that, at much modest computational cost, our heuristic scheme can construct convergecast schedules with latency close to that of the optimal schedules.

Modular co-design of controllers and transmission schedules in WirelessHART

We consider the joint design of transmission schedules and controllers for networked control loops that use WirelessHART communication for sensor and actuator data. By parameterizing the design problem in terms of the sampling rate of the control loop, the co-design problem separates into two well-defined subproblems which admit optimal solutions: transmission scheduling should be done to maximize the delay-constrained reliability while the control design should optimize closed-loop performance under packet loss. We illustrate how these problems can be solved and demonstrate our co-design framework for the case of linear-quadratic control.

Rapid Convergecast on Commodity Hardware: Performance Limits and Optimal Policies

The increased industrial interest in wireless sensor networks demands a shift from optimizing protocols for energy-efficient reporting of sporadic events to developing solutions for high-rate real-time data collection and dissemination. We study time-optimal convergecast under the communication constraints of commodity sensor network platforms, and propose a novel convergecast model in which packet copying between the microcontroller and the radio transceiver is separated from packet transmission, thereby improving channel utilization and system throughput. Based on this model, we establish tight lower bound on the number of time slots for convergecast in networks with tree routing topology, and present both centralized and distributed algorithms for generating time-optimal convergecast schedules. Our scheme is also memory-efficient as each node needs to buffer at most one packet at any time. We evaluate our scheme in simulation and on real hardware, and show that our scheme can achieve a throughput of 203 kbit/s (86.4% of the theoretical upper bound) and up to 86.24% improvement compared with traditional TDMA-based convergecast. With optimal routing tree and maximum MAC layer payload, convergecast in a network with 20 sensor nodes can be completed in only 100 ms.

A metric for opportunistic routing in duty cycled wireless sensor networks

Opportunistic routing is widely known to have substantially better performance than traditional unicast routing in wireless networks with lossy links. However, wireless sensor networks are heavily duty-cycled, i.e. they frequently enter deep sleep states to ensure long network life-time. This renders existing opportunistic routing schemes impractical, as they assume that nodes are always awake and can overhear other transmissions. In this paper, we introduce a novel opportunistic routing metric that takes duty cycling into account. By analytical performance modeling and simulations, we show that our routing scheme results in significantly reduced delay and improved energy efficiency compared to traditional unicast routing. The method is based on a new metric, EDC, that reflects the expected number of duty cycled wakeups that are required to successfully deliver a packet from source to destination. We devise distributed algorithms that find the EDC-optimal forwarding, i.e. the optimal subset of neighbors that each node should permit to forward its packets. We compare the performance of the new routing with ETX-optimal single path routing in both simulations and testbed-based experiments.