Many studies have tried to evaluate wireless networks and especially the IEEE 802.15.4 standard. Hence, several papers have aimed to describe the functionalities of the physical (PHY) and medium access control (MAC) layers. They have highlighted some characteristics with experimental results and or have attempted to reproduce them using theoretical models. In this paper, we use the first way to better understand IEEE 802.15.4 standard. Indeed, we provide a comprehensive model, able more faithfully to mimic the functionalities of this standard at the PHY and MAC layers. We propose a combination of two relevant models for the two layers. The PHY layer behavior is reproduced by a mathematical framework, which is based on radio and channel models, in order to quantify link reliability. On the other hand, the MAC layer is mimed by an enhanced Markov chain. The results show the pertinence of our approach compared to the model based on a Markov chain for IEEE 802.15.4 MAC layer. This contribution allows us fully and more precisely to estimate the network performance with different network sizes, as well as different metrics such as node reliability and delay. Our contribution enables us to catch possible failures at both layers.
  The wireless sensor networks community, has now an increased understanding of the need for realistic link layer models. Recent experimental studies have shown that real deployments have a "transitional region" with highly unreliable links, and that therefore the idealized perfect-reception-within-range models used in common network simulation tools can be very misleading. In this paper, we use mathematical techniques from communication theory to model and analyze the low power wireless links. The primary contribution of this work is the identification of the causes of the transitional region, and a quantification of their influence. Specifically, we derive expressions for the packet reception rate as a function of distance, and for the width of the transitional region. These expressions incorporate important channel and radio parameters such as the path loss exponent and shadowing variance of the channel; and the modulation and encoding of the radio. A key finding is that for radios using narrow-band modulation, the transitional region is not an artifact of the radio non-ideality, as it would exist even with perfect-threshold receivers because of multi-path fading. However, we hypothesize that radios with mechanisms to combat multi-path effects, such as spread-spectrum and diversity techniques, can reduce the transitional region. Experimental studies have demonstrated that the behavior of real links in low-power wireless networks (such as wireless sensor networks) deviates to a large extent from the ideal binary model used in several simulation studies. In particular, there is a large transitional region in wireless link quality that is characterized by significant levels of unreliability and asymmetry, significantly impacting the performance of higher-layer protocols. We provide a comprehensive analysis of the root causes of unreliability and asymmetry. In particular, we derive expressions for the distribution, expectation, and variance of the packet reception rate as a function of distance, as well as for the location and extent of the transitional region. These expressions incorporate important environmental and radio parameters such as the path loss exponent and shadowing variance of the channel, and the modulation, encoding, and hardware variance of the radios. A surface maintenance machine that includes a powered vehicle having a rotary tool in a tool enclosure to direct debris from the surface through an enclosure outlet, a debris hopper having a debris inlet locatable adjacent the enclosure outlet, and a discharge opening remote from the hopper inlet, a hingedly mounted door for closing the discharge opening, lift arms mounted on the vehicle for mounting the hopper for pivotable movement relative thereto, supporting the hopper in a debris collecting position, and selectively elevating the hopper from a street level position to a high dump position, control arms and piston cylinder combinations to control the pivoting of the hopper as it is moved between its street level and high dump positions, the piston cylinder combinations being operated from a retracted condition to an extended condition for pivoting the hopper to a dumping position in either the street level position or the high dump position, latches mounted on the hopper that are operated through the hopper pivoting to a dumping position to release their latching engagement with the door, and a door holder assembly which upon the door swinging open as the hopper is moved to a dumping position is resiliently operated to maintain a sufficient minimum spacing of the door from the discharge opening and that upon the piston cylinder combinations being substantially fully retracted is operated to release the door and permit the door swinging under gravity to a closed position with sufficient force to insure the door is latched in a closed position, the door holder assembly being operated to permit the door swinging closed by a cam surface on the adjacent control arm. A new class of networked systems is emerging that involve very large numbers of small, low-power, wireless devices. We present findings from a large scale empirical study involving over 150 such nodes operated at various transmission power settings. The instrumentation in our experiments permits us to separate effects at the various layers of the protocol stack. At the link layer, we present statistics on packet reception, effective communication range and link asymmetry; at the MAC layer, we measure contention, collision and latency; and at the application layer, we analyze the structure of trees constructed using flooding. The study reveals that even a simple protocol, flooding, can exhibit surprising complexity at scale. The data and analysis lay a foundation for a much wider set of algorithmic studies in this space.