The impact of wakeup schedule distribution in synchronous power save protocols on the performance of multihop wireless networks

By definition, the operation of an asynchronous power save protocol permits an arbitrary distribution of nodes' wakeup schedules. This wakeup schedule distribution creates an uncoordinated pattern of times at which nodes will attempt to transmit. Intuitively, we would expect that some patterns will be more (or less) favorable than others for a given traffic pattern. We investigate the impact of this wakeup pattern on network capacity and present simulation data showing that the capacity associated with the best wakeup patterns is significantly larger than that of the worst. This result not only gives insight to the behavior of such protocols, but also acts as a feasibility study showing the potential benefit of mechanisms by which nodes adapt their wakeup schedules to obtain improved performance

Timing is everything: the impact of wakeup schedule distribution on asynchronous power save protocols

Asynchronous power save protocols have been proposed for use in ad hoc networks. In many protocols, nodes independently follow a common periodic wakeup schedule, each with some unknown offset relative to its neighbors. The schedule is defined to ensure deterministic intervals of overlap between nodes, regardless of the distribution of the nodes' wakeup schedules. This paper studies the sensitivity of a simple asynchronous power save protocol to the actual distribution of the nodes' wakeup schedules. In practical terms: For given topology and traffic load, are there particularly "good" or "bad" distributions? We define a simplified model of network operation that allows us to study this question in simulation. The results show that the performance variation has a narrow probability distribution, but with long tails. The variation is shown to derive largely from timing dependencies rather than overall capacity of the system. The result suggests the feasibility of manipulating the wakeup schedule distribution to improve performance. Although the best wakeup distributions often mitigate the performance penalty imposed by the power save protocol, their relative rarity implies that randomized strategies will not be sufficient to obtain maximum advantage

Energy Efficient Protocols for Delay Tolerant Networks

The delay tolerant networks (DTNs) is characterized by frequent disconnections and long delays of links among devices due to mobility, sparse deployment of devices, attacks, and noise, etc. Considerable research efforts have been devoted recently to DTNs enabling communications between network entities with intermittent connectivity. Unfortunately, mobile devices have limited energy capacity, and the fundamental problem is that traditional power-saving mechanisms are designed assuming well connected networks. Due to much larger inter-contact durations than contact durations, devices spend most of their life time in the neighbor discovery, and centralized power-saving strategies are difficult. Consequently, mobile devices consume a significant amount of energy in the neighbor discovery, rather than in infrequent data transfers. Therefore, distributed energy efficient neighbor discovery protocols for DTNs are essential to minimize the degradation of network connectivity and maximize the benefits from mobility. In this thesis, we develop sleep scheduling protocols in the medium access control (MAC) layer that are adaptive and distributed under different clock synchronization conditions: synchronous, asynchronous, and semi-asynchronous. In addition, we propose a distributed clock synchronization protocol to mitigate the clock synchronization problem in DTNs. Our research accomplishments are briefly outlined as follows: Firstly, we design an adaptive exponential beacon (AEB) protocol. By exploiting the trend of contact availability, beacon periods are independently adjusted by each device and optimized using the distribution of contact durations. The AEB protocol significantly reduces energy consumption while maintaining comparable packet delivery delay and delivery ratio. Secondly, we design two asynchronous clock based sleep scheduling (ACDS) protocols. Based on the fact that global clock synchronization is difficult to achieve in general, predetermined patterns of sleep schedules are constructed using hierarchical arrangements of cyclic difference sets such that devices independently selecting different duty cycle lengths are still guaranteed to have overlapping awake intervals with other devices within the communication range. Thirdly, we design a distributed semi-asynchronous sleep scheduling (DSA) protocol. Although the synchronization error is unavoidable, some level of clock accuracy may be possible for many practical scenarios. The sleep schedules are constructed to guarantee contacts among devices having loosely synchronized clocks, and parameters are optimized using the distribution of synchronization error. We also define conditions for which the proposed semi-asynchronous protocol outperforms existing asynchronous sleep scheduling protocols. Lastly, we design a distributed clock synchronization (DCS) protocol. The proposed protocol considers asynchronous and long delayed connections when exchanging relative clock information among nodes. As a result, smaller synchronization error achieved by the proposed protocol allows more accurate timing information and renders neighbor discovery more energy efficient. The designed protocols improve the lifetime of mobile devices in DTNs by means of energy efficient neighbor discoveries that reduce the energy waste caused by idle listening problems

Efficient duty cycle MAC protocols for dynamic traffic loads in wireless sensor networks

Idle listening is one of the most significant causes of energy consumption in wireless sensor networks (WSNs), and many protocols have been proposed based on duty cycling to reduce this cost. These protocols, either synchronous or asynchronous, are mainly optimized for light traffic loads. A WSN, however, could often experience bursty and high traffic loads, as may happen for example with broadcast or convergecast traffic. In this thesis, I design and evaluate a new synchronous protocol, DW-MAC (Demand Wakeup MAC), and a new asynchronous protocol, RI-MAC (Receiver Initiated MAC), that are both efficient under dynamic traffic loads, including light or heavy loads. I also design and evaluate ADB (Asynchronous Duty-cycle Broadcasting), a new protocol for efficient multihop broadcasting in WSNs using asynchronous duty cycling. DW-MAC introduces a new low-overhead scheduling algorithm that allows nodes to wake up on demand during the Sleep period of an operational cycle and ensures that data transmissions do not collide at their intended receivers; this demand wakeup adaptively increases effective channel capacity as traffic load increases. RI-MAC, instead, uses receiver-initiated transmissions, in which each transmitter passively waits until its intended receiver wakes up and transmits a beacon frame; this technique minimizes the time a sender and its intended receiver occupy the wireless medium to find a rendezvous time for exchanging data. ADB is integrated with RI-MAC to exploit information only available at this layer; rather than treating the data transmission from a node to all of its neighbors as the basic unit of progress for the multihop broadcast. ADB dynamically optimizes the broadcast at the level of transmission to each individual neighbor of a node as the neighbors asynchronously wakeup, avoiding redundant transmissions and transmissions over poor links, and allowing a transmitter to go to sleep as early as possible. In detailed simulation of all three protocols using ns-2, they each substantially outperform earlier competing protocols in terms of reduced energy and latency and increased packet delivery ratio. I also implemented RI-MAC and ADB in a testbed of MICAz motes using TinyOS and further demonstrate the significant performance improvements made over prior protocols

On the Medium Access Control Protocols Suitable for Wireless Sensor Networks – A Survey

A MAC (Medium Access Control) protocol has direct impact on the energy efficiency and traffic characteristics of any Wireless Sensor Network (WSN). Due to the inherent differences in WSN’s requirements and application scenarios, different kinds of MAC protocols have so far been designed especially targeted to WSNs, though the primary mode of communications is wireless like any other wireless network. This is the subject topic of this survey work to analyze various aspects of the MAC protocols proposed for WSNs. To avoid collision and ensure reliability, before any data transmission between neighboring nodes in MAC layer, sensor nodes may need sampling channel and synchronizing. Based on these needs, we categorize the major MAC protocols into three classes, analyze each protocol’s relative advantages and disadvantages, and finally present a comparative summary which could give a snapshot of the state-of-the-art to guide other researchers find appropriate areas to work on. In spite of various existing survey works, we have tried to cover all necessary aspects with the latest advancements considering the major works in this area

Physical Implementation of Synchronous Duty-Cycling MAC Protocols: Experiences and Evaluation

Energy consumption and network latency are important issues in wireless sensor networks. The mechanism duty cycling is generally used in wireless sensor networks for avoiding energy consumption due to idle listening. Duty cycling, however, also introduces additional latency in communication among sensors. Some protocols have been proposed to work in wireless sensor networks with duty cycling, such as S-MAC and DW-MAC. Those protocols also tried to make efficient channel utilization and to mitigate the chance of packet collision and the network latency increase resulting from collision. DW-MAC was also designed to tolerate bursty and high traffic loads without increasing energy consumption, by spreading packet transmission and node wakeup times during a cycle. Some performance comparison between S-MAC and DW-MAC has been done in previous work; however, this comparison was performed in the ns-2 simulator only. In the real world, there are further issues not considered or discussed in the simulation, and some of those issues contribute significant influences to the MAC protocol performance. In this work, I implemented both S-MAC and DW-MAC physically on MICAz sensor motes and compared their performance through experiments. Through my implementation, experiments, and performance evaluation, hardware properties and issues that were not addressed in the previous work are presented, and their impacts on the performance are shown and discussed. I also simulated S-MAC and DW-MAC on ns-2 to give a mutual validation of the experimental results and my interpretation of the results. The experiences of physical implementations presented in this work can contribute new information and insights for helping in future MAC protocol design and implementation in wireless sensor networks

Support for a long lifetime and short end-to-end delays with TDMA protocols in sensor networks

This work addresses a tough challenge of achieving two opposing goals: ensuring long lifetimes and supporting short end-to-end delays in sensor networks. Obviously, sensor nodes must wake up often to support short delays in multi-hop networks. As event occurs seldom in common applications, most wake-up are useless: nodes waste energy due to idle listening. We introduce a set of solutions, referred to as LETED (limiting end-to-end delays), which shorten the wake-up periods, reduce idle listening, and save energy. We exploit hardware features of available transceivers that allow early detection of idle wake-up periods. This feature is introduced on top of our approach to reduce idle listening stemming from clock drift owing to the estimation of run-time drift. To evaluate LETED and other MAC protocols that support short end-to-end delays we present an analytical model, which considers almost 30 hardware and software parameters. Our evaluation revealed that LETED reduces idle listening by 15x and more against similar solutions. Also, LETED outperforms other protocols and provides significant longer lifetimes. For example, nodes with LETED work 8x longer than those with a common TDMA and 2x-3x longer than with protocols based on preamble sampling, like B-MAC

Energy-efficient wireless sensor networks via scheduling algorithm and radio Wake-up technology

One of the most important requirements for wireless sensor networks (WSNs) is the energy efficiency, since sensors are usually fed by a battery that cannot be replaced or recharged. Radio wake-up - the technology that lets a sensor completely turn off and be reactivated by converting the electromagnetic field of radio waves into energy - is now one of the most emergent strategies in the design of wireless sensor networks. This work presents Scheduled on Demand Radio WakeUp (SORW), a flexible scheduler designed for a wireless sensor network where duty cycling strategy and radio wake-up technology are combined in order to optimize the network lifetime. In particular, it tries to keep sensors sleeping as much as possible, still guaranteeing a minimum number of detections per unit of time. Performances of SORW are provided through the use of OMNet++ simulator and compared to results obtained by other basic approaches. Results show that with SORW it is possible to reach a theoretical lifetime of several years, compared to simpler schedulers that only reach days of activity of the network

RTXP : A Localized Real-Time Mac-Routing Protocol for Wireless Sensor Networks

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, appear new QoS requirements 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. To the best of our knowledge, RTXP is the first MAC and routing real-time communication protocol that is not centralized, but instead relies only on local information. The solution is cross-layer (X-layer) because it allows to control the delays due to MAC and Routing layers interactions. RTXP uses a suited hop-count-based Virtual Coordinate System which allows deterministic medium access and forwarder selection. In this paper we describe the protocol mechanisms. We give theoretical bound on the end-to-end delay and the capacity of the protocol. Intensive simulation results confirm the theoretical predictions and allow to compare with a real-time centralized solution. RTXP is also simulated under harsh radio channel, in this case the radio link introduces probabilistic behavior. Nevertheless, we show that RTXP it 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

Performance evaluation of wake-up radio based wireless body area network

Abstract. The last decade has been really ambitious in new research and development techniques to reduce energy consumption especially in wireless sensor networks (WSNs). Sensor nodes are usually battery-powered and thus have very limited lifetime. Energy efficiency has been the most important aspect to discuss when talking about wireless body area network (WBAN) in particular, since it is the bottleneck of these networks. Medium access control (MAC) protocols hold the vital position to determine the energy efficiency of a WBAN, which is a key design issue for battery operated sensor nodes. The wake-up radio (WUR) based MAC and physical layer (PHY) have been evaluated in this research work in order to contribute to the energy efficient solutions development. WUR is an on-demand approach in which the node is woken up by the wake-up signal (WUS). A WUS switches a node from sleep mode to wake up mode to start signal transmission and reception. The WUS is transmitted or received by a secondary radio transceiver, which operates on very low power. The energy benefit of using WUR is compared with conventional duty-cycling approach. As the protocol defines the nodes in WUR based network do not waste energy on idle listening and are only awakened when there is a request for communication, therefore, energy consumption is extremely low. The performance of WUR based MAC protocol has been evaluated for both physical layer (PHY) and MAC for transmission of WUS and data. The probabilities of miss detection, false alarm and detection error rates are calculated for PHY and the probabilities of collision and successful data transmission for channel access method Aloha is evaluated. The results are obtained to compute and compare the total energy consumption of WUR based network with duty cycling. The results prove that the WUR based networks have significant potential to improve energy efficiency, in comparison to conventional duty cycling approach especially, in the case of low data-reporting rate applications. The duty cycle approach is better than WUR approach when sufficiently low duty cycle is combined with highly frequent communication between the network nodes