A Survey on Real-Time Communications in Wireless Sensor Networks
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paper_title: REAL-TIME QOS SUPPORT IN WIRELESS SENSOR NETWORKS : A SURVEY

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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 identi¯ed and discussed.
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paper_title: Real-time communication over unreliable wireless links: a theory and its applications

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Wireless networks are increasingly used to serve real-time flows. We provide an overview of an emerging theory on real-time wireless communications and some of its main results. This theory is based on a model that jointly considers the delay bounds and throughput requirements of clients, as well as the unreliable and heterogeneous nature of wireless links. This model can be further extended in several aspects. It provides solutions to three important problems, namely, admission control, packet scheduling, and utility maximization. The theory can also be extended to consider broadcast of real-time flows, and to incorporate network coding.
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paper_title: Contention-free MAC protocols for asynchronous wireless sensor networks

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A MAC protocol specifies how nodes in a sensor network access a shared communication channel. Desired properties of a MAC protocol are: it should be contention-free (avoid collisions); it should be distributed and self-stabilize to topological changes in the network; topological changes should be contained, namely, affect only the nodes in the vicinity of the change; it should not assume that nodes have a global time reference, that is, nodes may not be time-synchronized. We give a set of TDMA-based MAC protocols for asynchronous wireless sensor networks satisfying all of these requirements. The communication complexity, number and size of messages, for the protocols to stabilize is small, poly-logarithmic in the network size.
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paper_title: REAL -TIME ROUTING PROTOCOLS FOR WIRELESS SENSOR NETWORKS : A SURVEY

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Wireless sensor networks can be termed as a new generation of distributed embedded systems that has a capability of meeting broad range of real-time applications. Examples include radiation monitoring, fire monitoring, border surveillance, and medical care to name but a few. Wireless sensor networks that are deployed in time/mission-critical applications with highly dynamic environments have to interact with the physical phenomenon under stringent timing constraints and severe resource limitations. For such real-time wireless sensor networks, designing and developing a real-time routing protocol that meets the required real-time guarantee of data packets communication is a stimulating field of study that raised many challenges and research issues. In this paper, we present a comprehensive survey of real-time routing protocols in WSN, by discussing each protocol with its key features. Finally, we concluded this paper with open research issues and challenges of real-time routing in WSN.
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paper_title: A Survey on Real-Time MAC Protocols in Wireless Sensor Networks

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As wireless sensor network becomes pervasive, new requirements have been continuously emerged. How-ever, the most of research efforts in wireless sensor network are focused on energy problem since the nodes are usually battery-powered. Among these requirements, real-time communication is one of the big research challenges in wireless sensor networks because most of query messages carry time information. To meet this requirement, recently several real-time medium access control protocols have been proposed for wireless sensor networks in the literature because waiting time to share medium on each node is one of main source for end-to-end delay. In this paper, we first introduce the specific requirement of wireless sensor real-time MAC protocol. Then, a collection of recent wireless sensor real-time MAC protocols are surveyed, classified, and described emphasizing their advantages and disadvantages whenever possible. Finally we present a dis-cussion about the challenges of current wireless sensor real-time MAC protocols in the literature, and show the conclusion in the end.
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paper_title: Timeliness in Wireless Sensor Networks: Common Misconceptions

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Existing real-time methods enable wireless sensor networks (WSN) to achieve –in principle– different levels of timeliness guarantees. However, the design and evaluation processes of these methods are often grounded on naive assumptions that constrain their usability in real-world
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paper_title: Real-Time QoS Routing Protocols in Wireless Multimedia Sensor Networks: Study and Analysis

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Many routing protocols have been proposed for wireless sensor networks. These routing protocols are almost always based on energy efficiency. However, recent advances in complementary metal-oxide semiconductor (CMOS) cameras and small microphones have led to the development of Wireless Multimedia Sensor Networks (WMSN) as a class of wireless sensor networks which pose additional challenges. The transmission of imaging and video data needs routing protocols with both energy efficiency and Quality of Service (QoS) characteristics in order to guarantee the efficient use of the sensor nodes and effective access to the collected data. Also, with integration of real time applications in Wireless Senor Networks (WSNs), the use of QoS routing protocols is not only becoming a significant topic, but is also gaining the attention of researchers. In designing an efficient QoS routing protocol, the reliability and guarantee of end-to-end delay are critical events while conserving energy. Thus, considerable research has been focused on designing energy efficient and robust QoS routing protocols. In this paper, we present a state of the art research work based on real-time QoS routing protocols for WMSNs that have already been proposed. This paper categorizes the real-time QoS routing protocols into probabilistic and deterministic protocols. In addition, both categories are classified into soft and hard real time protocols by highlighting the QoS issues including the limitations and features of each protocol. Furthermore, we have compared the performance of mobility-aware query based real-time QoS routing protocols from each category using Network Simulator-2 (NS2). This paper also focuses on the design challenges and future research directions as well as highlights the characteristics of each QoS routing protocol.
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paper_title: A Feedback-Based Timeout Packets Dropping Strategy in Real-Time Wireless Sensor Networks

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Resource limited real-time wireless sensor network is a new type of application of sensor network. In this kind of networks, there are numerous invalid real-time packets, which have already expired or are destined to expire. In this paper, a feedback-based timeout packet dropping strategy (FBTPD) is proposed. With this strategy, timestamps are piggybacked in real-time packets. So that sink node can gather the historical information and estimate the future transmitting time of some sensor node. Then the estimated transmitting time is fed back to the sensor node in a feedback packet. The sensor node hereby drops the invalid real-time packets which will expire later. Simulation experiments reveal that: compared with other dropping policies, FBTPD can drop more invalid real-time packets, and can significantly improve the throughput of the non real-time packets. Thus FBTPD can efficiently boost the resources utilization in wireless sensor networks, especially in hybrid real-time wireless sensor networks.
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paper_title: Wireless Sensor Network System for the Real-Time Health Monitoring

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Based on the conventional physical parameters measurement of human body as an example, this paper has put forward a scheme of real-time health monitoring on wireless sensor networks. The scheme has been integrated into most detection sensors of human physiological parameters, And human physiological parameters for data acquisition, and the measured data is transmitted to the monitoring center through wireless sensor network the monitoring center, and these physiological parameters processed, the human body health evaluated, and information immediate feedbacked for prevention and treatmen.
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paper_title: Real-Time Health Monitoring System on Wireless Sensor Network

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In this work, a portable real-time wireless health monitoring system is developed. The system is used for remote monitoring of patients’ heart rate and oxygen saturation in blood. The system was designed and implemented using ZigBee wireless technologies. All pulse oximetry data are transferred within a group of wireless personal area network (WPAN) to database computer server. The sensor modules were designed for low power operation with a program that can adjust power management depending on scenarios of power source and current power operation. The sensor unit consists of (1) two types of LEDs and photodiode packed in Velcro strip that is facing to a patient’s fingertip; (2) Microcontroller unit for interfacing with ZigBee module, processing pulse oximetry data and storing some data before sending to base PC; (3) ZigBee module for communicating the data of pulse oximetry, ZigBee module gets all commands from microcontroller unit and it has a complete ZigBee stack inside and (4) Base node for receiving and storing the data before sending to PC.
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paper_title: Achieving Real-Time Target Tracking UsingWireless Sensor Networks

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Target tracking systems, consisting of thousands of low-cost sensor nodes, have been used in many application domains such as battlefield surveillance, wildlife monitoring and border security. These applications need to meet certain real-time constraints in response to transient events, such as fast-moving targets. While the real-time performance is a major concern in these applications, it should be compatible with other important system properties such as energy consumption and accuracy. Hence, it is desirable to have the ability to exploit the tradeoffs among them. This work presents the real-time design and analysis of VigilNet, a large-scale sensor network system which tracks, detects and classifies targets in a timely and energy efficient manner. Based on a deadline partition method and theoretical derivations of each sub-deadline, we are able to make guided engineering decisions to meet the end-to-end tracking deadline. To confirm our design and obtain an empirical understanding of these tradeoffs, we invest significant efforts to perform large-scale simulations with 10,000 nodes as well as a field test with 200 XSM motes, running VigilNet. The results from both analysis and evaluation can serve as general design guidelines to build similar real-time systems.
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paper_title: AirCloud: a cloud-based air-quality monitoring system for everyone

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We present the design, implementation, and evaluation of AirCloud -- a novel client-cloud system for pervasive and personal air-quality monitoring at low cost. At the frontend, we create two types of Internet-connected particulate matter (PM2:5) monitors -- AQM and miniAQM, with carefully designed mechanical structures for optimal air-flow. On the cloud-side, we create an air-quality analytics engine that learn and create models of air-quality based on a fusion of sensor data. This engine is used to calibrate AQMs and mini-AQMs in real-time, and infer PM2:5 concentrations. We evaluate AirCloud using 5 months of data and 2 month of continuous deployment, and show that AirCloud is able to achieve good accuracies at much lower cost than previous solutions. We also show three real applications built on top of AirCloud by 3rd party developers to further demonstrate the value of our system.
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paper_title: In-Motes EYE: A Real Time Application for Automobiles in Wireless Sensor Networks

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Wireless sensor networks have been identified as one of the key technologies for the 21st century. In order to overcome their limitations such as fault tolerance and conservation of energy, we propose a middleware solution, In-Motes. In-Motes stands as a fault tolerant platform for deploying and monitoring applications in real time offers a number of possibilities for the end user giving him in parallel the freedom to experiment with various parameters, in an effort the deployed applications to run in an energy efficient manner inside the network. The proposed scheme is evaluated through the In-Motes EYE application, aiming to test its merits under real time conditions. In-Motes EYE application which is an agent based real time In-Motes application developed for sensing acceleration variations in an environment. The application was tested in a prototype area, road alike, for a period of four months.
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paper_title: Field calibrations of a low-cost aerosol sensor at a regulatory monitoring site in California

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Abstract. Health effects attributed to ambient fine particulate matter (PM2.5) now rank it among the risk factors with the highest health burdens in the world, but existing monitoring infrastructure cannot adequately characterize spatial and temporal variability in urban PM2.5 concentrations, nor in human population exposures. The development and evaluation of more portable and affordable monitoring instruments based on low-cost sensors may offer a means to supplement and extend existing infrastructure, increasing the density and coverage of empirical measurements and thereby improving exposure science and control. Here, we report on field calibrations of a custom-built, battery-operated aerosol monitoring instrument we developed using low-cost, off-the-shelf optical aerosol sensors. We calibrated our instruments using 1 h and 24 h PM2.5 data from a class III US EPA Federal Equivalent Method (FEM) PM2.5 β-attenuation monitor in continuous operation at a regulatory monitoring site in Oakland, California. We observed negligible associations with ambient humidity and temperature; linear corrections were sufficient to explain 60% of the variance in 1 h reference PM2.5 data and 72% of the variance in 24 h data. Performance at 1 h integration times was comparable to commercially available optical instruments costing considerably more. These findings warrant further exploration of the circumstances under which this class of aerosol sensors may profitably be deployed to generate improved PM2.5 data sets.
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paper_title: A portable Wireless Sensor Network system for real-time environmental monitoring

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Environmental contaminations such as fine particulate matter (PM2.5) and ultraviolet (UV) are expanding public health concerns globally. Existing centralized monitoring stations, however, are unable to properly estimate human exposure due to the low resolution spatiotemporal data. Wireless Sensor Network (WSN) uses real-time capable instrumentation could not only provide fine-grained readings of multiple environmental factors, but also create real-time mapping of the contaminants. In this paper we present a WSN system which is capable of sensing multiple environmental factors, collecting data from multiple dispersed sensor nodes and displaying the aggregated data in real-time. Each individual sensor node is capable of probing multiple factors, including temperature, humidity, atmospheric pressure, PM2.5, UV radiation, and geographical location. Sensor data are transmitted to a server, which then be stored into a database through Wi-Fi networks. Each sensor node is portable enough to be carried for personal use, enabling broad potential application of our system.
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paper_title: Heterogeneous Wireless Sensor Network for Real Time Remote Monitoring of Sand Dynamics on Coastal Dunes

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In this paper, the architecture of a heterogeneous Wireless Sensor Network (WSN) to be deployed on coastal sand dunes is described, the aim of which is to provide real time measurements of physical parameters to better define the sediment transport in connection with Aeolian processes. The WSN integrates different typologies of sensors and is provided with both local and remote connection. In particular, three different typologies of sensors are integrated in the network: a multilayer anemometric station, a sensor developed ad-hoc to measure the sand dune level and a sand collector capable of measuring the weight of trapped sand and its quantity. Each sensor node is made up at least of a ZigBee radio module that is able to transmit the data collected by the sensor at a distance of about 100 meters. While the sand level sensor and the sand collector are provided only with this transmission module, the anemometric station also integrates a microprocessor board in charge of data processing. A Gateway node provided with a GSM connection for remote data transmission and a Zigbee radio module for Local Area communication has also been developed. This node is in charge of collecting all the data packets sent by the Sensor Nodes and transmit them to a remote server through GPRS connection. A Web server has been set up to collect these packets and store them in a database. The proposed WSN can provide both a static and a dynamic framework of sand transport processes acting on coastal dunes.
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paper_title: A-LNT: a wireless sensor network platform for low-power real-time voice communications

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Combining wireless sensor networks and voice communication for multidata hybrid wireless network suggests possible applications in numerous fields. However, voice communication and sensor data transmissions have significant differences, Meanwhile, high-speed massive real-time voice data processing poses challenges for hardware design, protocol design, and especially power management. In this paper, we present a wireless audio sensor network platform A-LNT and study and discuss key elements for systematic design and implementation: node hardware design, low-power voice codec and processing, wireless network topology, hybrid MAC protocol design based on superframe, radio channel allocation, and clock synchronization. Furthermore, we discuss energy management methods such as address filtering and efficient power management in detail. The experimental and simulation results show that A-LNT is a lightweight, low-power, low-speed, and high-performance wireless sensor network platform for multichannel real-time voice communications.
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paper_title: PAVENET OS: A Compact Hard Real-Time Operating System for Precise Sampling in Wireless Sensor Networks

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The paper shows a compact hard real-time operating system for wireless sensor nodes called PAVENET OS. PAVENET OS provides hybrid multithreading: preemptive multithreading and cooperative multithreading. Both of the multithreading are optimized for two kinds of tasks on wireless sensor networks, and those are real-time tasks and best-effort ones. PAVENET OS can efficiently perform hard real-time tasks that cannot be performed by TinyOS. The paper demonstrates the hybrid multithreading realizes compactness and low overheads, which are comparable to those of TinyOS, through quantitative evaluation. The evaluation results show PAVENET OS performs 100 Hz sensor sampling with 0.01% jitter while performing wireless communication tasks, whereas optimized TinyOS has 0.62% jitter. In addition, PAVENET OS has a small footprint and low overheads (minimum RAM size: 29 bytes, minimum ROM size: 490 bytes, minimum task switch time: 23 cycles).
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paper_title: FireFly Mosaic: A Vision-Enabled Wireless Sensor Networking System

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With the advent of CMOS cameras, it is now possible to make compact, cheap and low-power image sensors capable of on-board image processing. These embedded vision sensors provide a rich new sensing modality enabling new classes of wireless sensor networking applications. In order to build these applications, system designers need to overcome challenges associated with limited bandwidth, limited power, group coordination and fusing of multiple camera views with various other sensory inputs. Real-time properties must be upheld if multiple vision sensors are to process data, communicate with each other and make a group decision before the measured environmental feature changes. In this paper, we present FireFly Mosaic, a wireless sensor network image processing framework with operating system, networking and image processing primitives that assist in the development of distributed vision-sensing tasks. Each FireFly Mosaic wireless camera consists of a FireFly (Rowe et al., 2006) node coupled with a CMUcam3 (Rowe et al., 2007) embedded vision processor. The FireFly nodes run the nano-RK (Eswaran et al., 2005) real-time operating system and communicate using the RT-link (Rowe et al., 2006) collision-free TDMA link protocol. Using FireFly Mosaic, we demonstrate an assisted living application capable of fusing multiple cameras with overlapping views to discover and monitor daily activities in a home. Using this application, we show how an integrated platform with support for time synchronization, a collision-free TDMA link layer, an underlying RTOS and an interface to an embedded vision sensor provides a stable framework for distributed real-time vision processing. To the best of our knowledge, this is the first wireless sensor networking system to integrate multiple coordinating cameras performing local processing.
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paper_title: Timeliness in Wireless Sensor Networks: Common Misconceptions

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Existing real-time methods enable wireless sensor networks (WSN) to achieve –in principle– different levels of timeliness guarantees. However, the design and evaluation processes of these methods are often grounded on naive assumptions that constrain their usability in real-world
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paper_title: Hard Real-Time wireless communication in the northern Pierre Auger Observatory

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The Pierre Auger Cosmic Ray Observatory employs a large array of Surface Detector stations to detect the secondary particle showers generated by the arrivals of Ultra High Energy Cosmic Rays. The operational Auger South site uses a tower-based wireless network for communication between the stations and observatory campus. Plans for a larger Auger North array call for a similar system. However, a variety of factors have rendered direct station-to-tower routing infeasible in Auger North. Thus, it will employ a new paradigm, the Wireless Architecture for Hard Real-Time Embedded Networks (WAHREN) designed specifically for highly reliable message delivery over a fixed network, under hard real-time deadlines. This paper describes the WAHREN topology and protocols, as well as real-time performance evaluation, formal verification, testbed operation, and Markov reliability modeling. The status of system hardware development and an on-site Research and Development Array are also discussed.
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paper_title: The capacity of implicit EDF in wireless sensor networks

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Distributed networks of wireless sensors/actuators will enable the reliable monitoring and intelligent control of the physical environment accomplishing different tasks ranging from space monitoring and surveillance to homeland security without human intervention. Motivated by the observation that many of these applications are safety critical and have hard real-time requirements, this paper focuses on the problem of providing delay and throughput guarantee to real-time messages in wireless sensor networks. It extends the cellular structure described by M. Caccamo et al. (2002) and analyzes the sensor network capacity when messages are scheduled with the implicit-EDF (earliest deadline first) algorithm.
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paper_title: WICKPro: A Hard Real-Time protocol for Wireless Mesh Networks with chain topologies

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Wireless Mesh Networks (WMNs) with chain topologies are very useful in road and railroad transportation or in tunnel and mine applications. The proposed protocols for WMNs usually support best-effort traffic or some kind of Quality of Service (QoS). However, some applications such as remote-controlled machines in industrial control networks have Hard Real-Time (HRT) requirements, i.e., strict deadlines. Moreover, voice, video and data communications can be added to HRT traffic in order to provide them with reserved bandwidth and bounded latency. For this reason, this paper presents WICKPro (WIreless Chain networK Protocol), a HRT protocol for WMNs with chain topologies. WICKPro is a token-passing approach and is based on the ideas of the Timed-token protocol and the cyclic executive. We have designed this protocol to work on top of IEEE 802.11, made a testbed using commercial 802.11 wireless cards and compared the protocol's performance with the 802.11 protocol and three specific protocols for WMNs with chain topologies.
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paper_title: Priority based bandwidth allocation in wireless sensor networks

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Most of the sensor network applications need real time communication and the need for deadline aware real time communication is becoming eminent in these applications. These applications have different dead line requirements also. The real time applications of wireless sensor networks are bandwidth sensitive and need higher share of bandwidth for higher priority data to meet the dead line requirements. In this paper we focus on the MAC layer modifications to meet the real time requirements of different priority data. Bandwidth partitioning among different priority transmissions is implemented through MAC layer modifications. The MAC layer implements a queuing model that supports lower transfer rate for lower priority packets and higher transfer rate for real time packets with higher priority, minimizing the end to end delay. The performance of the algorithm is evaluated with varying node distribution.
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paper_title: EchoRing: A Low-Latency, Reliable Token-Passing MAC Protocol for Wireless Industrial Networks

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Spurred by recent industrial trends, such as factory automation or phase synchronization in the smart grid, there is a significant interest for wireless industrial networks lately. In contrast to t ...
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paper_title: ( m , k )-firm pattern spinning to improve the GTS allocation of periodic messages in IEEE 802 . 15 . 4 networks

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The IEEE 802.15.4 standard provides a flexible communication support for low-rate wireless personal area networks) applications. When active, the beacon-enabled mode provides a real-time communication to the supported application by adopting a guaranteed time slot (GTS) mechanism. However, this mechanism permits only up to seven real-time communicating devices. One way to deal with this limitation is to share the communication opportunities among the periodic tasks, by skipping some of the task activations in a controlled way. One of the widely accepted periodic task models that allows skips in periodic activations is the (m,k)-firm model. Motivated by this problem, this paper proposes the use of a dynamic GTS scheduling approach based on the (m,k)-firm task model, to deal with the GTS starvation problem. The proposed scheduling approach is based on pre-defined spins of the originally defined (m,k)-firm pattern. The use of an exact schedulability analysis test ensures that for each admitted message stream, at least m messages will be transmitted within each window of k consecutive deadlines. The schedulability analysis may be executed in polynomial time and therefore can be used as an online admission test for GTS requests. The effectiveness of the approach has been assessed both by a set of simulations and an experimental evaluation.
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paper_title: IEEE 802.15.4e: A survey

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Abstract Several studies have highlighted that the IEEE 802.15.4 standard presents a number of limitations such as low reliability, unbounded packet delays and no protection against interference/fading, that prevent its adoption in applications with stringent requirements in terms of reliability and latency. Recently, the IEEE has released the 802.15.4e amendment that introduces a number of enhancements/modifications to the MAC layer of the original standard in order to overcome such limitations. In this paper we provide a clear and structured overview of all the new 802.15.4e mechanisms. After a general introduction to the 802.15.4e standard, we describe the details of the main 802.15.4e MAC behavior modes, namely Time Slotted Channel Hopping (TSCH), Deterministic and Synchronous Multi-channel Extension (DSME), and Low Latency Deterministic Network (LLDN). For each of them, we provide a detailed description and highlight the main features and possible application domains. Also, we survey the current literature and summarize open research issues.
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paper_title: SAS-TDMA: a source aware scheduling algorithm for real-time communication in industrial wireless sensor networks

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Scheduling algorithms play an important role for TDMA-based wireless sensor networks. Existing TDMA scheduling algorithms address a multitude of objectives. However, their adaptation to the dynamics of a realistic wireless sensor network has not been investigated in a satisfactory manner. This is a key issue considering the challenges within industrial applications for wireless sensor networks, given the time-constraints and harsh environments. In response to those challenges, we present SAS-TDMA, a source-aware scheduling algorithm. It is a cross-layer solution which adapts itself to network dynamics. It realizes a trade-off between scheduling length and its configurational overhead incurred by rapid responses to routes changes. We implemented a TDMA stack instead of the default CSMA stack and introduced a cross-layer for scheduling in TOSSIM, the TinyOS simulator. Numerical results show that SAS-TDMA improves the quality of service for the entire network. It achieves significant improvements for realistic dynamic wireless sensor networks when compared to existing scheduling algorithms with the aim to minimize latency for real-time communication.
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paper_title: Distributed DBP: A (m,k)-firm based distributed approach for QoS provision in IEEE 802.15.4 networks

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IEEE 802.15.4/Zigbee is one of the most widespread Wireless Sensor Network technologies. It may work either in beacon-enabled mode or in beaconless mode. When operating in beacon-enabled mode, it divides the superframe in sixteen equally-sized time slots, where the request for slot allocation is achieved through a CSMA/CA approach. This allocation method does not allow the provision of any service differentiation scheme, which is required for the transfer of time constrained messages. This paper proposes a QoS provision approach with minimal changes to existing protocols. Our approach improves the CSMA/CA algorithm by assigning decentralized priorities based on the (m,k)-firm task model. In this paper, a set of experimental results is presented and compared with the traditional approach currently offered by the standard. This set of experiments highlight the promising behavior of the proposed approach, when dealing with high network load scenarios.
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paper_title: Real-Time Wireless Sensor-Actuator Networks for Industrial Cyber-Physical Systems

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With recent adoption of wireless sensor-actuator networks (WSANs) in industrial automation, industrial wireless control systems have emerged as a frontier of cyber-physical systems. Despite their success in industrial monitoring applications, existing WSAN technologies face significant challenges in supporting control systems due to their lack of real-time performance and dynamic wireless conditions in industrial plants. This article reviews a series of recent advances in real-time WSANs for industrial control systems: 1) real-time scheduling algorithms and analyses for WSANs; 2) implementation and experimentation of industrial WSAN protocols; 3) cyber-physical codesign of wireless control systems that integrate wireless and control designs; and 4) a wireless cyber-physical simulator for codesign and evaluation of wireless control systems. This article concludes by highlighting research directions in industrial cyber-physical systems.
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paper_title: A novel scheduling for (m, k)-firm streams in wireless sensor networks

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Even though many time-sensitive applications have been emerged, it is not easy to guarantee real-time delivery within deadline in wireless sensor networks due to several constraints such as short ranged wireless communication and low computing capacity on a node. In addition, it is very hard to develop real-time communication scheme without any specific traffic model because one scheme cannot meet various requirements. In this paper, we propose a novel scheduling algorithm for (m, k)-firm real-time streams in wireless sensor networks because queuing delay on a node is the main source of end-to-end delay. The priority of each packet is dynamically adjusted by current DBP (Distance Based Priority) state, slack time, and current position relative to the sink. Simulation studies reveal that dynamic failure probability in (m, k)-firm stream is reduced rather than general priority scheme.
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paper_title: Dual-mode real-time MAC protocol for wireless sensor networks: a validation/simulation approach

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Several wireless sensor network applications are currently appearing in various domains. Their goal is often to monitor a geographical area. When a sensor detects a monitored event, it informs a sink node using alarm messages. The area surveillance application needs to react to such an event with a finite, bounded and known delay: these are real-time constraints. This work proposes a real-time MAC protocol with realistic assumptions for a random linear network, where sensors are deployed randomly along a line. We present a formal validation of this protocol both for initialization and run-time, and present simulation results on a realistic scenario.
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paper_title: MMSPEED: multipath Multi-SPEED protocol for QoS guarantee of reliability and. Timeliness in wireless sensor networks

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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).
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paper_title: Enhancing Real-Time Delivery of Gradient Routing for Industrial Wireless Sensor Networks

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This paper proposes gradient routing with two-hop information for industrial wireless sensor networks to enhance real-time performance with energy efficiency. Two-hop information routing is adopted from the two-hop velocity-based routing, and the proposed routing algorithm is based on the number of hops to the sink instead of distance. Additionally, an acknowledgment control scheme reduces energy consumption and computational complexity. The simulation results show a reduction in end-to-end delay and enhanced energy efficiency.
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paper_title: A two-hop based adaptive routing protocol for real-time wireless sensor networks

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One of the most important and challenging issues in wireless sensor networks (WSNs) is to optimally manage the limited energy of nodes without degrading the routing efficiency. In this paper, we propose an energy-efficient adaptive routing mechanism for WSNs, which saves energy of nodes by removing the much delayed packets without degrading the real-time performance of the used routing protocol. It uses the adaptive transmission power algorithm which is based on the attenuation of the wireless link to improve the energy efficiency. The proposed routing mechanism can be associated with any geographic routing protocol and its performance is evaluated by integrating with the well known two-hop based real-time routing protocol, PATH and the resulting protocol is energy-efficient adaptive routing protocol (EE-ARP). The EE-ARP performs well in terms of energy consumption, deadline miss ratio, packet drop and end-to-end delay.
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paper_title: Energy efficient routing with delay guarantee for sensor networks

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The paper presents a routing algorithm that maximizes the lifetime of a sensor network in which all data packets are destined for a single collection node. Lifetime is maximized by adjusting the number of packets traversing each node. The adjustment is carried out by transmitting over alternative routes. The first part of the paper assumes that the worst case delay resulting from energy efficient routing is less than the maximum tolerable value. Ignoring the delay constraint of the network, the routes are selected as the solution to a linear programming (LP) problem in which the objective is to maximize the minimum lifetime of each node. The solution is implemented in a centralized algorithm, and then approximated by an iterative algorithm based on least cost path routing, in which each step is implemented efficiently in a distributed manner. The second part of the paper incorporates delay guarantee into energy efficient routing by constraining the length of the routing paths from each sensor node to the collection node. Simulations reveal that the lifetime of the network increases significantly by optimal routing, and including delay constraint in energy efficient routing improves the network performance since the delay of the network keeps increasing as the delay constraint is relaxed beyond the value at which the optimal lifetime is achieved.
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paper_title: Energy-Efficient SPEED Routing Protocol for Wireless Sensor Networks

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In this paper, we propose an approach for routing in SPEED protocol considering residual energy in routing decisions. Due to the limited energy of a sensor node, energy-efficient routing is a very important issue in sensor networks. This approach finds energy-efficient paths for delay-constrained data in real-time traffic. The SPEED protocol does not consider any energy metric in its routing. In our approach, routing is based on a weight function, which is a combination of the three factors: Delay, Energy & Speed. Here, the node with the greatest value in the weight function is to be selected as the next hop forwarding. We increase the network lifetime by considering energy metric in routing decisions. This method aims to construct a nearly stateless routing protocol, which can be used to route data based on the nodes’ residual energy. Simulation results demonstrate that the new scheme improves network lifetime about 15% longer than the traditional SPEED protocol.
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paper_title: Improved Multi-Path and Multi-Speed Routing Protocol in Wireless Sensor Networks

paper_content:

In our presented paper by proposing a optimum routing protocol, in some of Quality of Service achieved improvements in the field of reliability in data sending to destination and load balancing in wireless sensor network. In our proposed protocol, to ensure that a data packet correctly send to the destination, it used of an improved hybrid method based on multipath data sending. The routing decisions in this method are by considering the remaining energy of nodes that are in neighbors of sender nodes. Simulation results shows that release rate of data packets in this method is reduced and reliability in data sending to destination is increased. Also, the energy efficiency of sensor nodes effectively improved and thus increase the overall lifetime of wireless sensor networks.
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paper_title: POWER-SPEED: A Power-Controlled Real-Time Data Transport Protocol for Wireless Sensor-Actuator Networks

paper_content:

This paper investigates the data transport problem for reporting delay-sensitive events in wireless sensor-actuator networks (WSANs). We specifically tailor the protocol design according to the features of WSANs and propose POWER-SPEED, a real-time data transport protocol for WSANs to achieve energy-efficient data transport for delay-sensitive event reporting. In POWER-SPEED, sensor nodes select the next-hop neighbor to actuators according to the spatio-temporal historic data of the upstream QoS condition, which completely avoids control packets. With an adaptive transmitter power control scheme, POWER-SPEED conveys packets in an energy-efficient manner while maintaining soft real-time packet transport. It thus reduces the energy consumption of data transport while ensuring the QoS requirement in timeliness domain. We demonstrate the effectiveness of POWER-SPEED through simulations with NS2.
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paper_title: SPEED: a stateless protocol for real-time communication in sensor networks

paper_content:

In this paper, we present a real-time communication protocol for sensor networks, called SPEED. The protocol provides three types of real-time communication services, namely, real-time unicast, real-time area-multicast and real-time area-anycast. SPEED is specifically tailored to be a stateless, localized algorithm with minimal control overhead End-to-end soft real-time communication is achieved by maintaining a desired delivery speed across the sensor network through a novel combination of feedback control and non-deterministic geographic forwarding. SPEED is a highly efficient and scalable protocol for sensor networks where the resources of each node are scarce. Theoretical analysis, simulation experiments and a real implementation on Berkeley motes are provided to validate our claims.
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paper_title: FT-SPEED: A Fault-Tolerant, Real-Time Routing Protocol for Wireless Sensor Networks

paper_content:

Due to the high sensor failure probability in WSN, there will be some "holes" in the sensor network which block the routing. The void problem hasn't been handled well in current real-time routing protocols. In this paper, we present a novel fault-tolerant real-time routing protocol FT-SPEED. In FT- SPEED, void announces scheme is proposed to prevent the packets reaching the void through other routing path. To route the packets around two sides of the void to guarantee the packets be delivered rather than just being dropped, we introduce the void bypass scheme in FT-SPEED. Simulation results show that, compared with SPEED and MMSPEED, FT-SPEED could handle the void problem much more effectively.
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paper_title: Real-time routing in wireless sensor networks: A potential field approach

paper_content:

Wireless Sensor Networks (WSNs) are embracing an increasing number of real-time applications subject to strict delay constraints. Utilizing the methodology of potential field in physics, in this article we effectively address the challenges of real-time routing in WSNs. In particular, based on a virtual composite potential field, we propose the Potential-based Real-Time Routing (PRTR) protocol that supports real-time routing using multipath transmission. PRTR minimizes delay for real-time traffic and alleviates possible congestions simultaneously. Since the delay bounds of real-time flows are extremely important, the end-to-end delay bound for a single flow is derived based on the Network Calculus theory. The simulation results show that PRTR minimizes the end-to-end delay for real-time routing, and also guarantees a tight bound on the delay.
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paper_title: EARQ: Energy Aware Routing for Real-Time and Reliable Communication in Wireless Industrial Sensor Networks

paper_content:

Wireless industrial sensor networks are wireless sensor networks which have been adapted to industrial applications. Most techniques for wireless sensor networks can be applied to wireless industrial sensor networks. However, for industrial applications of wireless industrial sensor networks, new requirements such as real-time, reliable delivery need to be considered. In this paper, we propose EARQ, which is a novel routing protocol for wireless industrial sensor networks. It provides real-time, reliable delivery of a packet, while considering energy awareness. In EARQ, a node estimates the energy cost, delay and reliability of a path to the sink node, based only on information from neighboring nodes. Then, it calculates the probability of selecting a path, using the estimates. When packet forwarding is required, it randomly selects the next node. A path with lower energy cost is likely to be selected, because the probability is inversely proportional to the energy cost to the sink node. To achieve real-time delivery, only paths that may deliver a packet in time are selected. To achieve reliability, it may send a redundant packet via an alternate path, but only if it is a source of a packet. Experimental results show that EARQ is suitable for industrial applications, due to its capability for energy efficient, real-time, reliable communications.
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paper_title: Comparative Study of Speed Protocols in Wireless Sensor Network

paper_content:

Wireless sensor network (WSN) [1] is based on distributed event-based system that is different from the traditional communication network. The main goal of the network is to deliver a data packet with the highest efficiency and quality of service. In WSN, Quality of service (QoS) [2] is an important performance indicator, therefore the QoS routing protocols are designed to provide more efficiency and quality of service. Here they concentrate on SPEED [3] protocol (Stateless Protocol for End to End Delay) is a QoS based protocol and which provides efficiency, softness-real time communication, desired delivery speed and a low overhead over the network. MMSPEED [4] is another multi-path multi SPEED protocol, which provides more efficiency and minimal control overhead over the network. This paper represents SPEED protocol and a comparative study on several versions of SPEED protocols.
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paper_title: An enhanced real-time routing protocol with load distribution for mobile wireless sensor networks

paper_content:

Mobile wireless sensor network (MWSN) is a wireless ad hoc network that consists of a very large number of tiny sensor nodes communicating with each other in which sensor nodes are either equipped with motors for active mobility or attached to mobile objects for passive mobility. A real-time routing protocol for MWSN is an exciting area of research because messages in the network are delivered according to their end-to-end deadlines (packet lifetime) while sensor nodes are mobile. This paper proposes an enhanced real-time with load distribution (ERTLD) routing protocol for MWSN which is based on our previous routing protocol RTLD. ERTLD utilized corona mechanism and optimal forwarding metrics to forward the data packet in MWSN. It computes the optimal forwarding node based on RSSI, remaining battery level of sensor nodes and packet delay over one-hop. ERTLD ensures high packet delivery ratio and experiences minimum end-to-end delay in WSN and MWSN compared to baseline routing protocol. In this paper we consider a highly dynamic wireless sensor network system in which the sensor nodes and the base station (sink) are mobile. ERTLD has been successfully studied and verified through simulation experiment.
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paper_title: EA-SPEED: energy-aware real-time routing protocol for wireless sensor networks

paper_content:

We propose two simple mechanisms that aim at improving the energy efficiency of SPEED; the well-known real-time routing protocol dedicated for wireless sensor networks WSNs. The first mechanism calculates the expected end-to-end delay on the basis of previous hops' delay and drops a packet if such expected delay is greater than the packet deadline. By dropping the useless delayed packets, the mechanism increases fluidity of links and economises energy of nodes. The second mechanism extends the component stateless non-deterministic geographic forwarding SNGF of the SPEED protocol. While the original SNGF performs energy balancing by randomly selecting the next hop from the forwarding candidate neighbours set FS, the proposed mechanism makes use of a decision parameter which takes into account both the relay speed and the residual energy of the candidates. Thus, it prolong the network lifetime without degrading the real-time packet delivery performance. Also, it uses a low-cost algorithm updating continually the residual energy of neighbours in a node by exploiting both the overhearing mechanism and the location beaconing which are implemented in the SPEED protocol. Associated with SPEED, the proposed mechanisms achieved good performance in terms of node energy balancing and network energy consumption without degrading the packet delivery ratio.
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paper_title: Routing in WSN network using neural Network (NN) and SPEED protocol

paper_content:

Routing stands for the sending the required data to the destination in such a manner that it reaches efficiently with high throughput and accuracy. The data is transferring over the network each sensor use some energy in receiving data, sending data. The life of the network depend how much energy used up in each transmission. The problem occurs when the transmission path meets with some sort of failure like path failure or node goes to sleep mode. The focus, however, has been given to the routing protocols which might differ depending on the application and network architecture. In this paper, we have proposed the state-of-the-art routing technique using SPEED protocol and NN technique to choose an alternative path in WSNs.
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paper_title: QoS and energy aware routing for real-time traffic in wireless sensor networks

paper_content:

Wireless sensor networks are being built to facilitate automated information gathering in military, industrial, environmental and surveillance applications. Many such applications of Sensor Networks require improved QoS (packet delivery within a defined deadline) guarantees as well as high reliability. These applications demand high packet delivery ratio and are extremely delay-sensitive. However, certain factors limit the ability of the multihop sensor network to achieve the desired goals. These factors include the delay caused by network congestion, hot regions in the network, limited energy of the sensor nodes, packet loss due to collisions and link failure. In this paper, we propose an energy aware dual-path routing scheme for real-time traffic, which balances node energy utilization to increase the network lifetime, takes network congestion into account to reduce the routing delay across the network and increases the reliability of the packets reaching the destination by introducing minimal data redundancy. This paper also introduces an adaptive prioritized Medium Access Layer (MAC) to provide a differentiated service model for real-time packets. Our claims are well supported by simulation results.
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paper_title: RAP: a real-time communication architecture for large-scale wireless sensor networks

paper_content:

Large-scale wireless sensor networks represent a new generation of real-time embedded systems with significantly different communication constraints from traditional networked systems. This paper presents RAP, a new real-time communication architecture for large-scale sensor networks. RAP provides convenient, high-level query and event services for distributed micro-sensing applications. Novel location-addressed communication models are supported by a scalable and light-weight network stack. We present and evaluate a new packet scheduling policy called velocity monotonic scheduling that inherently accounts for both time and distance constraints. We show that this policy is particularly suitable for communication scheduling in sensor networks in which a large number of wireless devices are seamlessly integrated into a physical space to perform real-time monitoring and control. Detailed simulations of representative sensor network environments demonstrate that RAP significantly reduces the end-to-end deadline miss ratio in the sensor network.
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paper_title: RTH-MAC: A real time hybrid MAC protocol for WSN

paper_content:

Recently, Wireless Sensor Networks (WSNs) grow to be one of the dominant technology trends; new needs are continuously emerging and demanding more complex constraints, such as the real time communication. The MAC layer plays a crucial role in these networks; it controls the communication module and manages the medium sharing. In this work, we describe the Real Time Hybrid MAC (RTH-MAC) protocol for WSNs. It combines the advantages of both TDMA and FDMA in order to offer a soft real time communication to a converge-cast WSN randomly deployed. It schedules nodes communication in a central manner that eliminates collisions, minimizes interferences, and ensures a small bounded end-to-end delay. Moreover, this protocol offers a probabilistic end-to-end reliability guarantee using an acknowledgment mechanism, and offers to the end user the possibility of adapting the duty cycling ratio. Simulation results show that RTH-MAC presents good performances in term of the end-to-end delay, reliability and scalability.
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paper_title: Adaptive TDMA scheduling for real-time flows in cluster-based wireless sensor networks

paper_content:

To prevent scalability problem caused by increasing traffic of real-time ::: applications, cluster architecture with Time Division Multiple Access (TDMA) ::: scheduling scheme is mostly deployed in wireless sensor networks. However, ::: even though it have proven good scalability and suitability for real-time ::: communications, but static scheduling lacks of adaptability in several ::: situations by not admitting some real-time flows where slots remain ::: available. To solve mentioned problem, in this paper, we propose an adaptive ::: cluster-based scheduling scheme for real-time flows by utilizing the time ::: slots for flows according to type of flows such as inside or outside ::: cluster. So, the proposed scheme can achieve better utilization of channel ::: by minimizing the number of unused channels for real-time flows than static ::: scheme through new adaptive TDMA scheduling scheme . Simulation results show ::: that more flows are admitted and delivered within the deadline in the ::: proposed scheme by utilizing unused time slots accordingly.
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paper_title: RTXP : A Localized Real-Time Mac-Routing Protocol for Wireless Sensor Networks

paper_content:

Abstract Protocols developed during the last years for Wireless Sensor Networks (WSNs) are mainly focused on energy efficiency and autonomous mechanisms (e.g. self-organization, self-configuration, etc.). Nevertheless, with new WSN applications, new QoS requirements appear, such as time constraints. Real-time applications require the packets to be delivered before a known time bound which depends on the application requirements. We particularly focus on applications which consist in alarms sent to the sink node. We propose Real-Time X-layer Protocol (RTXP), a real-time communication protocol. RTXP is a MAC and routing real-time communication protocol that is not centralized, but instead relies only on local information. To the best of our knowledge, it is the first real-time protocol for WSNs using an opportunistic routing scheme in order to increase the packet delivery ratio. In this paper we describe the protocol mechanisms. We give theoretical bounds on the end-to-end delay and the capacity of the protocol. Intensive simulation results confirm the theoretical predictions and allow to compare RTXP with a real-time scheduled solution. RTXP is also simulated under harsh radio channel, in this case, the radio link introduces probabilistic behavior. Nevertheless, we show that RTXP performs better than a non-deterministic solution. It thus advocates for the usefulness of designing real-time (deterministic) protocols even for highly unreliable networks such as WSNs.
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paper_title: Low Power, Accurate Time Synchronization MAC Protocol for Real-Time Wireless Data Acquisition

paper_content:

The issues of real-time wireless data acquisition networks, i.e., the design and management of large scale sensor networks distributed over a vast geographical area, have received more attention during the last few years. This paper proposes a real-time wireless data acquisition media access control (MAC) protocol fulfilling the requirements of high throughput, low end-to-end latency, low energy consumption and accurate time synchronization. A hybrid approach, combining the advantages of Time Division Multiple Access (TDMA) and Frequency Hopping Spread Spectrum (FHSS), is adopted for anti-jamming and collision prevention. The packets of commands and data are delivered in a “bucket brigade”-like manner for optimum bandwidth utilization and low end-to-end latency. Correspondingly, we propose a novel time synchronization approach which is tightly coupled with TDMA scheme. As the non-determinism of timestamp is removed from the transmission path, the frequency difference between local clock and parent's clock is accurately estimated by periodically receiving beacons. The results show that our algorithm provides a hop-to-hop latency of 14 milliseconds and synchronization accuracy within 0.6 microseconds under an acceptable power consumption of 34.61 mW.
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paper_title: Empirical testing of prototype real-time multi-hop MAC for Wireless Sensor Networks

paper_content:

Internet of Things for the Smart Home Environment has been realized through the use of Wireless Sensor Network devices. Bandwidth hungry data such as real-time audio and video are becoming more important for monitoring and security in Smart Homes. TSMAC, a practical implementation of a multi-hop mesh topology real-time IEEE 802.15.4-based MAC protocol for Contiki OS was prototyped and studied using the TI CC2530 platform. Results indicate that TSMAC is able to support higher continuous bandwidth data streams of 16.6 kbps compared to the existing CSMA MAC protocol which manages only 3.2 kbps.
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paper_title: SDRCS: A service-differentiated real-time communication scheme for event sensing in wireless sensor networks

paper_content:

Real-time communication is crucial for wireless sensor networks (WSNs) to accomplish collaborative event sensing tasks with specific timing constraints. In this work, a service-differentiated real-time communication scheme (SDRCS) is developed to provide soft real-time guarantees for event-based traffic in WSNs. SDRCS features a cross-layer packet forwarding design to integrate the real-time routing functionality with a novel prioritized medium access control scheme. Based on this design, SDRCS performs distributed packet traversal speed estimation for traffic classification and admission control. SDRCS also performs prioritized packet forwarding so that the routing decisions are locally performed for maximized packet traversal speed. SDRCS requires no extra hardware for localization, transmission power adaptation or multi-channel transmission. It also adapts well to network dynamics, such as channel quality and communication voids. Performance evaluations show that SDRCS significantly improves the on-time delivery ratio and service-differentiation granularity for mixed priority traffic flows in unsynchronized WSNs, compared with currently used communication schemes. SDRCS also provides higher end-to-end throughput in terms of supporting higher source data rates with tight end-to-end latency requirements.
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paper_title: An architecture for (m, k)-firm real-time streams in wireless sensor networks

paper_content:

Despite increasing demand of application specific requirements in wireless sensor networks, lots of research is still going on under assumption of general requirement without considering specific traffic model as well as several constraints. To address this issue, we have already introduced a new real-time application with (m, k)-firm model and developed communication protocols for multimedia sensor networks. But, scalability problem and lack of architecture for (m, k)-firm stream are major research challenges that remain unsolved. Based on corresponding demands, in this paper, we propose a new integrated architecture for (m, k)-firm stream in wireless sensor networks with following functions, that is, flow aggregation based on compositional hierarchical model, velocity based routing protocol, hybrid medium access control protocol, and congestion control scheme. Simulation results are given to prove the suitability of the architecture and performance improvement under several scenarios.
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paper_title: A revised SPEED protocol for (m, k)-firm streams in wireless sensor networks

paper_content:

The SPEED routing protocol was proposed to minimizing the packet deadline miss ratio in wireless sensor networks. To achieve this goal, velocity calculation is used to determine the forwarding node. However, due to general traffic model, it lacks of adaptability so we extend the SPEED protocol to support (m, k)-firm streams in wireless sensor networks. Simulations and their analysis are followed to validate the suitability of reduced end-to-end delay and enhanced reliability.
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paper_title: Research on Wireless Sensor Networks Routing Protocol for Wetland Water Environment Monitoring

paper_content:

A Link Quality Estimation based Routing protocol (LQER) is proposed to meet the high reliability of transmitting data in water environment monitoring in wetlands based on (m,k)-firm link quality estimating. It considers both energy efficiency and link qualities when the route is selected, which makes routing data more reliable and decreases the probability of retransmission, thus saves the energy and prolongs the lifetime of the whole network. Simulation results show that LQER can meet the requirements of energy efficiency, reliability and scalability for water environment monitoring in wetlands.
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paper_title: Load Balancing Routing Protocol for considering Energy Efficiency in Wireless Sensor Network

paper_content:

A WSN is composed of a large number of sensor nodes, which are the finite energy resources. Therefore, extending the life of the sensor nodes and minimization in overall power consumption is very important. In this paper, we propose the LBRP that supports to balance the traffic throughout the WSN. In LBPR, the traffics passed by each node are distributed through multiple paths instead of a single path. It allows significant energy saving and makes long-lived sensor nodes. Simulation results show that significant energy savings can be achieved compared to the multi-hop routing protocols.
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paper_title: Evaluating an (m, k)-firm Deadline Real-time Stream Based on a Reliable Transport Protocol in Wireless Sensor Networks

paper_content:

As application-specific requirements for wireless sensor networks emerge, both real-time and reliable communications become major research challenges in wireless sensor networks due to the many constraints on nodes and wireless links. To support these services, several protocols have been proposed. However, since most of them were designed as well as developed for general purpose applications, it is not recommended that they be directly adapted to applications with special requirements. In this paper, we propose a way to extend the current reliable transport protocol to cover a special real-time service, the (m,k)-firm deadline stream, in wireless sensor networks. While the proposed scheme is basically built on the PSFQ protocol for reliability, some features have been newly developed to support the (m,k)-firm stream efficiently. Finally, simulation results are given to demonstrate the feasibility of the proposed scheme in high traffic and with failed links.
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paper_title: Modeling and routing scheme for m, k-firm streams in wireless multimedia sensor networks

paper_content:

In this paper, we propose a novel scheme of how to efficiently and reliably deliver multimedia real-time streams in wireless sensor networks. To specify the property of streams, a multimedia stream is modeled as an m,k-firm stream that is known to have the characteristics of weakly hard real-time system. In addition, a distributed, measurement-based routing scheme for m,k-firm streams is newly proposed while considering several constraints on node as well as on communication medium. Simulations and their analysis are provided to validate the suitability of traffic modeling, reduced dynamic failure probability, and extended network lifetime when the proposed routing protocol is employed. Copyright © 2013 John Wiley & Sons, Ltd.
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paper_title: Cross-Layered Approach for (m, k)-Firm Stream in Wireless Sensor Networks

paper_content:

As most sensor nodes are deployed to detect and transmit interesting events or phenomena around them, the real-time delivery of sensed information within a predetermined deadline becomes one of the biggest challenges in wireless sensor networks. Even though some of the recent research works have been published in specific field, they are designed to cover general purpose applications. Therefore, most of applications cannot be supported by existing schemes especially when the requirement is application-specific real-time. In order to solve this problem, we model the target application as a specific traffic, (m, k)-firm stream, which is known as hard real-time requirement, and then propose a scheme to guarantee real-time delivery. To develop these, we introduce a cross-layer design to bring collaboration between layers under the basic principle that real-time delivery will hardly achieve by independent scheme in each layer. Thus, the requirements of application are passed to lower layers and used to adjust transmission power, prioritize packets, and find the adequate path in order to meet (m, k)-firm constraints for any stream dynamically. Finally, simulation results reveal that low failure probability for real-time requirements on (m, k)-firm stream is achieved by the proposed scheme.
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paper_title: Clustering Scheme for (m,k)-Firm Streams in Wireless Sensor Networks

paper_content:

As good example of potential application-specific requirement, (m,k)-firm real-time streams have been recently introduced to deliver multimedia data efficiently in wireless sensor networks. In addition to stream model, communication protocols to meet specific (m,k)-firm real-time streams have been newly developed or extended from existing protocols. However, since the existing schemes for an (m,k)-firm stream have been proposed under typical flat architecture, the scalability problem remains unsolved when the number of real-time flows increases in the networks. To solve this problem, in this paper, we propose a new clustering scheme for an (m,k)-firm stream. The two different clustering algorithms are performed according to either the (m,k)-firm requirement or the deadline. Simulation results are presented to demonstrate the suitability of the proposed scheme under hierarchical architecture by showing that its performance is acceptable irrespective of the increase in the number of flows.
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paper_title: A real-time routing protocol for (m,k)-firm streams in wireless sensor networks

paper_content:

As a novel real-time application in wireless sensor networks (WSNs), multimedia transmission has posed a new challenge that both reliability and timeliness must be satisfied at the same time to support an acceptable quality of service (QoS). However, the inherent resource constraint of sensor nodes and instability of wireless communication make it not practical for existing routing mechanisms to meet the requirements of this QoS aware application. To address the problem, we propose an innovative protocol in this paper, for energy and QoS aware routing of (m,k)-firm based real-time applications over WSNs. The routing depends on optimal forwarding decision which takes into account of packet end-to-end deadline, node condition and remaining energy of next hop. A local status indicator (LSI), which was specially devised for (m,k)-firm stream, is used in the routing scheme for each node to monitor and evaluate its local condition. The proposed protocol has been well studied and verified through simulations. The results have proved the efficiency of the proposed routing protocol in terms of higher successful transmission ratio and smaller end-to-end delay.
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paper_title: Real-time routing protocols for (m,k)-firm streams based on multi-criteria in wireless sensor networks

paper_content:

Even though some velocity based routing protocols for (m,k)-firm stream have been recently proposed in multimedia wireless sensor networks, there are still many perspective parameters to be considered for forwarding procedure. Moreover, since they are not correlated with each other, multi-criteria system for forwarding is desirable to select next hop. However, current existing protocols apply these parameters sequentially without any prioritization. To address this issue, in this paper, we propose two (m,k)-firm specific routing protocols based on fuzzy interference system and analytical hierarchical process in conjunction with the gray relational analysis. In each protocol, delivery ratio, energy, speed, (m,k)-firm stream requirement as well as current stream status are used to select the best appropriate next hop while considering given node's constraints. Through the simulation results, we demonstrate that the proposed scheme gracefully maintains (m,k)-firm requirement while extending the network lifetime. Finally, superiority of the proposed approach to existing velocity based routing protocols is also proven through diverse simulation scenarios.
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paper_title: An ( m , k ) -Firm Real-Time Aware Fault-Tolerant Mechanism in Wireless Sensor Networks

paper_content:

Many real-time routing mechanisms have been proposed to support the newly developed wireless sensor networks (WSNs) applications such as the transmission and retrieval of multimedia traffic. However, the inherent source constraints of sensor network and instability of wireless communication set quite a problem for the existing routing mechanisms to meet the quality of service (QoS) requirements of some specific QoS-aware applications. Hence, real-time fault-tolerant schemes are highly desired for WSNs to address these challenges. In this paper, we propose an -firm-based real-time fault-tolerant mechanism, which helps routing mechanisms to achieve specific QoS requirement by employing a local status indicator (LSI) at each sensor node to monitor and evaluate the local conditions of node and network. Therefore, specific fault recovery mechanisms could be implemented for ensuring an acceptable QoS performance, according to the evaluated LSI values. By using this fault-tolerant scheme, each node dynamically adjusts its transmission capability to mitigate the performance degradation of real-time service caused by network faults and to maintain the desired reliability and timeliness. Simulation result shows that LSI cannot only help to reduce the effects of congestion, link failure, and void, but also reach higher successful transmission ratio and smaller transmission delay.
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paper_title: Accurate Timeliness Simulations for Real-Time Wireless Sensor Networks

paper_content:

The use of wireless sensor networks is rapidly growing in various types of applications that benefit from spatially distributed data collection. Some of these applications, such as industrial automation, fire detection or health monitoring, have strong timeliness constraints. Since field deployments are difficult to monitor and debug, the development of real-time communication protocols for wireless sensor networks necessitates accurate simulation models. This paper presents open source Omnet++ simulation models based on the IEEE 802.15.4 standard. Four evaluation scenarios are used to compare simulation timeliness and packet error rate results with experimental measurements. The small scale of the scenarios allows to isolate the effect of each system component. The comparison validates the models for timeliness estimates in sensor networks and pinpoints the variability of software implementations in embedded systems as a major cause of differences between simulated and measured results.
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paper_title: A Hybrid Framework for Soft Real-Time WSN Simulation

paper_content:

The design of a wireless sensor network is a challenging task due to its intrinsically application-specific nature.Although a typical choice for testing such kind of networks requires devising ad-hoc testbeds, this is often impractical as it depends on expensive, and hard to maintain deployment of nodes. On the other hand, simulation is a valuable option, as long as the actual functioning conditions are reliably modeled, and carefully replicated.The present work describes a framework for supporting the user in early design and testing of a wireless sensor network with an augmented version of TOSSIM, the de-facto standard for simulators, that allows merging actual and virtual nodes seamlessly interacting with each other; the proposed tool does not require any special modification to the original simulation code, but it allows contemporary execution of code in actual, and virtual nodes, as well as simulation of nodes executing different application logics. The reported experimental results will also show how soft-real time constraints are guaranteed for the augmented simulation.
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paper_title: Superframe Duration Allocation Schemes to Improve the Throughput of Cluster-Tree Wireless Sensor Networks

paper_content:

The use of Wireless Sensor Network (WSN) technologies is an attractive option to support wide-scale monitoring applications, such as the ones that can be found in precision agriculture, environmental monitoring and industrial automation. The IEEE 802.15.4/ZigBee cluster-tree topology is a suitable topology to build wide-scale WSNs. Despite some of its known advantages, including timing synchronisation and duty-cycle operation, cluster-tree networks may suffer from severe network congestion problems due to the convergecast pattern of its communication traffic. Therefore, the careful adjustment of transmission opportunities (superframe durations) allocated to the cluster-heads is an important research issue. This paper proposes a set of proportional Superframe Duration Allocation (SDA) schemes, based on well-defined protocol and timing models, and on the message load imposed by child nodes (Load-SDA scheme), or by number of descendant nodes (Nodes-SDA scheme) of each cluster-head. The underlying reasoning is to adequately allocate transmission opportunities (superframe durations) and parametrize buffer sizes, in order to improve the network throughput and avoid typical problems, such as: network congestion, high end-to-end communication delays and discarded messages due to buffer overflows. Simulation assessments show how proposed allocation schemes may clearly improve the operation of wide-scale cluster-tree networks.
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paper_title: Programming wireless sensor networks: Fundamental concepts and state of the art

paper_content:

Wireless sensor networks (WSNs) are attracting great interest in a number of application domains concerned with monitoring and control of physical phenomena, as they enable dense and untethered deployments at low cost and with unprecedented flexibility. However, application development is still one of the main hurdles to a wide adoption of WSN technology. In current real-world WSN deployments, programming is typically carried out very close to the operating system, therefore requiring the programmer to focus on low-level system issues. This not only distracts the programmer from the application logic, but also requires a technical background rarely found among application domain experts. The need for appropriate high-level programming abstractions, capable of simplifying the programming chore without sacrificing efficiency, has long been recognized, and several solutions have hitherto been proposed, which differ along many dimensions. In this article, we survey the state of the art in programming approaches for WSNs. We begin by presenting a taxonomy of WSN applications, to identify the fundamental requirements programming platforms must deal with. Then, we introduce a taxonomy of WSN programming approaches that captures the fundamental differences among existing solutions, and constitutes the core contribution of this article. Our presentation style relies on concrete examples and code snippets taken from programming platforms representative of the taxonomy dimensions being discussed. We use the taxonomy to provide an exhaustive classification of existing approaches. Moreover, we also map existing approaches back to the application requirements, therefore providing not only a complete view of the state of the art, but also useful insights for selecting the programming abstraction most appropriate to the application at hand.
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paper_title: SPEED: a stateless protocol for real-time communication in sensor networks

paper_content:

In this paper, we present a real-time communication protocol for sensor networks, called SPEED. The protocol provides three types of real-time communication services, namely, real-time unicast, real-time area-multicast and real-time area-anycast. SPEED is specifically tailored to be a stateless, localized algorithm with minimal control overhead End-to-end soft real-time communication is achieved by maintaining a desired delivery speed across the sensor network through a novel combination of feedback control and non-deterministic geographic forwarding. SPEED is a highly efficient and scalable protocol for sensor networks where the resources of each node are scarce. Theoretical analysis, simulation experiments and a real implementation on Berkeley motes are provided to validate our claims.
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paper_title: Real-Time Communications for Wireless Sensor Networks: A Two-Tiered Architecture

paper_content:

While many approaches have been proposed to deal with energy/latency trade-offs, they are likely to be insufficient for the applications where reduced delay guarantee is the main concern. In this article, we investigated the potential application of a decentralized two-tiered network architecture, in large-scale wireless sensor networks, where an upper layer Wireless Local Area Network (WLAN), offering more powerful capabilities, serves as a backbone to an adaptively-clustered Low-Energy Adaptive Clustering Hierarch (LEACH)-based wireless sensor network. The WLAN layer will be involved in the communication between the sensor network and the control station, mitigating the impact of the limited capacities of the sensor nodes. With this two-tiered architecture we target to provide more reliable data delivery with reduced delay bounds, and lower energy consumption in the underlying sensor network, thereby increasing its lifetime. Simulation results show that the two-tiered network architecture achieved a rel...
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Title: A Survey on Real-Time Communications in Wireless Sensor Networks
Section 1: Introduction
Description 1: Write about the increasing importance of wireless sensor networks (WSNs) and the challenge of real-time communication in these networks.

Section 2: Real-Time Applications and Platform
Description 2: Discuss representative applications requiring real-time delivery and describe several sensor node platforms.

Section 3: Real-Time Applications
Description 3: Describe potential and practical real-time applications such as health monitoring, target tracking, and environmental tracking.

Section 4: Platform for Real-Time Communications
Description 4: Detail various platforms supporting real-time communication in WSNs, mentioning examples like FireFly, PAVENET OS, A-LNT, and A-Stack.

Section 5: Real-Time Communications in WSN
Description 5: Analyze the general concepts of real-time communications in WSNs, including hard, firm, and soft real-time classifications.

Section 6: Hard Real-Time Communications in WSN
Description 6: Explore hard real-time communication methods, emphasizing MAC and scheduling protocols as well as routing challenges and approaches.

Section 7: MAC and Scheduling
Description 7: Break down the specific MAC and scheduling algorithms used for hard real-time models in WSNs.

Section 8: Routing
Description 8: Discuss the routing protocols addressing hard, firm, and soft real-time communications in WSNs, including specific routing techniques.

Section 9: Soft Real-Time Communications in WSN
Description 9: Explore MAC and routing protocols tailored for soft real-time requirements in WSNs.

Section 10: Firm Real-Time Communications in WSN
Description 10: Detail the research and protocols focusing on (m, k)-firm stream requirements for real-time communication in WSNs.

Section 11: Multicast and Broadcast
Description 11: Explain the challenges and approaches for real-time multicast and broadcast in WSNs.

Section 12: Energy Efficiency
Description 12: Cover the importance of energy efficiency in real-time communications and methods to achieve it in WSNs.

Section 13: Simulation Model
Description 13: Describe the simulation platforms and approaches for validating real-time communication performance in WSNs.

Section 14: Network Architecture
Description 14: Discuss the architectural considerations for real-time communication in WSNs, including clustering and cross-layered architecture.

Section 15: Programming Models and Tools
Description 15: Introduce the programming models and tools needed for real-time communication in WSNs.

Section 16: Applicability to Ad Hoc Networks
Description 16: Explain the applicability of WSN real-time communication protocols to general ad hoc networks.

Section 17: Deployment and Applications
Description 17: Provide an overview of real-world deployments and applications of real-time communication systems in WSNs.

Section 18: Conclusion
Description 18: Summarize the survey findings and discuss future research challenges and open issues in real-time communication in WSNs.