Sensor networks can be considered distributed computing platforms with many severe constraints including limited CPU speed, memory size, power, and bandwidth. Individual nodes in sensor networks are typically unreliable and the network topology dynamically changes, possibly frequently. Sensor networks can also be considered a form of ad hoc network. However, here also many constraints in sensor networks are different or more severe. Sensor networks also differ because of their tight interaction with the physical environment via sensors and actuators. Due to all of these differences many solutions developed for general distributed computing platforms and for ad hoc networks cannot be applied to sensor networks. Many new and exciting research challenges exist. This paper discusses the state of the art and presents the key research challenges to be solved, some with initial solutions or approaches.

Abstract—In this paper, we present a novel packet delivery mechanism called Multi-Path and Multi-SPEED Routing Protocol (MMSPEED) for probabilistic QoS guarantee in wireless sensor networks. The QoS provisioning is performed in two quality domains, namely, timeliness and reliability. Multiple QoS levels are provided in the timeliness domain by guaranteeing multiple packet delivery speed options. In the reliability domain, various reliability requirements are supported by probabilistic multipath forwarding. These mechanisms for QoS provisioning are realized in a localized way without global network information by employing localized geographic packet forwarding augmented with dynamic compensation, which compensates for local decision inaccuracies as a packet travels towards its destination. This way, MMSPEED can guarantee end-to-end requirements in a localized way, which is desirable for scalability and adaptability to large scale dynamic sensor networks. Simulation results show that MMSPEED provides QoS differentiation in both reliability and timeliness domains and, as a result, significantly improves the effective capacity of a sensor network in terms of number of flows that meet both reliability and timeliness requirements up to 50 percent (12 flows versus 18 flows).

Abstract: Real-time wireless sensor networks are becoming more and more important by the requirement of message delivery timeliness in emerging new applications. Supporting real-time QoS in sensor networks faces severe challenges due to the wireless nature, limited resource, low node reliability, distributed architecture and dynamic network topology. There are tradeoffs between different application requirements including energy efficiency and delay performance. This paper studies the state of the art of current real-time solutions including MAC protocols, routing protocols, data processing strategies and cross-layer designs. Some research challenges and future design favors are also identified and discussed. Keywords: Real-time, wireless sensor networks, MAC, routing, cross-layer. 1.

Abstract — In this paper, we present a novel packet delivery

Multihop wireless sensor networks have recently emerged as an important embedded computing platform. This paper defines a quantitative notion of real-time capacity of a wireless network. Real-time capacity describes how much real-time data the network can transfer by their deadlines. A capacity bound is derived that can be used as a sufficient schedulability condition for a class of fixedpriority packet scheduling algorithms. Using this bound, a designer can perform capacity planning prior to network deployment to ensure satisfaction of applications ’ real-time requirements. 1

This paper presents a time division multiple access medium access control protocol for wireless sensor / actuator networks implemented with a contention-free message scheduler. A message scheduler is used to determine which message has access to the medium at any time. A set of messages is contention-free if only one message is ready at a time. Otherwise, some other criterion such as priority must be used to resolve contention. In this case the message scheduler at each node in the network must schedule all messages in order to resolve contention. If the schedule is contention-free however, then each node only schedules the messages that interest it. The key contribution of our contention-free scheduler protocol is scalability. Large wireless sensor / actuator networks may contain hundreds of nodes exchanging thousands of messages. Due to resource constraints it is infeasible that each node schedule every message. Our protocol scales by optimizing each node to schedule only the messages it is interested in. 1.

The design and deployment of wireless sensor applications has received increased research attention in recent years. In this work, we consider a class of wireless sensor applications—such as mobile robotics—that impose timeliness constraints. We assume that these applications are built using commodity 802.11 wireless networks and focus on the problem of providing qualitatively-better QoS during network transmission of sensor data. Our techniques are designed to explicitly avoid network collisions and minimize the completion time to transmit a set of sensor messages. We argue that this problem is NP-complete and present several heuristics, based on edge coloring, to achieve these goals. We present detailed simulation results to evaluate our heuristics and to compare them to the optimal solution.

Consider a team of robots equipped with sensors that collaborate with one another to achieve a common goal. Sensors on robots produce periodic updates that must be transmitted to other robots and processed in real-time to enable such collaboration. Since the robots communicate with one another over an ad-hoc wireless network, we consider the problem of providing timeliness guarantees for multihop message transmissions in such a network. We derive the effective deadline and the latest start time for per-hop message transmissions from the validity intervals of the sensor data and the constraints imposed by the consuming task at the destination. Our technique schedules messages by carefully exploiting spatial channel reuse for each per-hop transmission to avoid MAC layer collisions, so that deadline misses are minimized. Extensive simulations show the effectiveness of our channel reuse-based SLF (smallest lateststart-time first) technique when compared to a simple perhop SLF technique, especially at moderate to high channel utilization or when the probability of collisions is high. 1

Many new routing and MAC layer protocols have been proposed for wireless sensor networks tackling the issues raised by the resource constrained unattended sensor nodes in large-scale deployments. The majority of these protocols considered energy efficiency as the main objective and assumed data traffic with unconstrained delivery requirements. However, the growing interest in applications that demand certain end-toend performance guarantees and the introduction of imaging and video sensors have posed additional challenges. Transmission of data in such cases requires both energy and QoS aware network management in order to ensure efficient usage of the sensor resources and effective access to the gathered measurements. In this paper, we highlight the architectural and operational challenges of handling of QoS traffic in sensor networks. We report on progress make to-date and outline open research problems.

Abstract — Recent years have seen the emergence of wireless cyber-physical systems that must support real-time queries of physical environments through wireless sensor networks. This paper proposes Real-Time Query Scheduling (RTQS), a novel approach to conflict-free transmission scheduling for real-time queries in wireless sensor networks. First, we show that there is an inherent trade-off between latency and real-time capacity in query scheduling. We then present three new real-time schedulers. The non-preemptive query scheduler supports high real-time capacity but cannot provide low response times to high priority queries due to priority inversions. The preemptive query scheduler eliminates priority inversions at the cost of reduced capacity. The slack stealing query scheduler combines the benefits of the preemptive and non-preemptive schedulers to improve the capacity while meeting query deadlines. We provide schedulability analysis for each scheduler. The analysis and advantages of our approach are validated through NS2 simulations. Index Terms — Query scheduling, schedulability analysis, sensor networks