Study of Fast Multi-hop ALOHA with Instant Forwarding

The wireless channel-oriented ad-hoc multi-hop broadband (W-CHAMB) is a new link layer protocol with the aim of being able to support quality of service (QoS) in multi-hop operation in fully distributed ad hoc networks. It is based on time division multiple access (TDMA) technology, operating on a single frequency channel. In this paper, we improve the prioritized access mechanism by inserting the contention phase (CP) into the access slot. Then we propose 3 new algorithms to enhance the traffic performance in the multi-hop environment: on demand time division duplex (on-demand-TDD) for achieving high channel utilization; fairness algorithm (FA) for acquiring the fairness even in a highly interfered multi-hop network; setting valid transmission time (VTT) for each traffic channel to prevent an excessive use of the traffic channel. The extensive simulations are performed by evaluating transmission control protocol (TCP) performance over W-CHAMB multi-hop networks under heavy load situations. The achievable TCP throughput, packet delay, fairness and stability have been investigated. Results exhibit the outstanding multi-hop performance of the W-CHAMB protocol

Rapid development of industrial applications imposes more challenges on traditional sensors that only provide raw signal. As a trend, smart sensor and smart sensor network sensors are getting more attention in industrial areas for the values they can bring into the system. One way or another, sensor data will ultimately be used for control purpose. Smart sensor and smart sensor networks will go beyond raw signal and/or data, and provide certain level of information and/or knowledge, which can significantly improve the overall control system performance and reduce reliability. This paper gives a brief introduction on smart sensor and smart sensor networks and their progresses. Some issues observed are also highlighted. By raising people's attention and interests, authors believe that researchers/developers from both academic and industrial societies can and should work together to address those issues and promote the development of smart sensor and smart sensor networks.

Several analytical models of different wireless networking schemes such as wireless LANs and meshes have been reported in the literature. To the best of our knowledge, all these models fail to address the accurate end-to-end delay analysis of multi-hop wireless networks under unsaturated traffic condition considering the hidden and exposed terminal situation. In an effort to gain deep understanding of delay, this paper firstly proposes a new analytical model to predict accurate media access delay by obtaining its distribution function in a single wireless node. The interesting point of having the media access delay distribution is its generality that not only enables us to derive the average delay which has been reported in almost most of the previous studies as a special case but also facilitates obtaining higher moments of delay such as variance and skewness to capture the QoS parameters such as jitters in recently popular multimedia applications. Secondly, using the obtained single node media access delay distribution, we extend our modeling approach to investigate the delay in multi-hop networks. Moreover, probabilities of collisions in both hidden and exposed terminal conditions have been calculated. The validity of the model is demonstrated by comparing results predicted by the analytical model against those obtained through simulation experiments.

Supporting QoS over multihop wireless mesh networks is difficult because end-to-end delay increases quickly with the increasing number of hops. This paper introduces a novel multichannel time-division multiple-access media access control (McTMAC) protocol that can help to efficiently reduce delay over multihop networks. Performance evaluation results demonstrate that McTMAC outperforms existing alternative protocols. The max-delay can be reduced by as much as 60% by using McTMAC.

Due to a poor understanding of the interactions among transmitters, wireless multihop networks have commonly been stigmatized as unpredictable in nature. Even elementary questions regarding the throughput limitations of these networks cannot be answered in general. In this paper we investigate the behavior of wireless multihop networks using carrier sense multiple access with collision avoidance (CSMA/CA). Our goal is to understand how the transmissions of a particular node affect the medium access, and ultimately the throughput, of other nodes in the network. We introduce a theory which accurately models the behavior of these networks and show that, contrary to popular belief, their performance is easily predictable and can be described by a system of equations. Using the proposed theory, we provide the analytical expressions necessary to fully characterize the capacity region of any wireless CSMA/CA multihop network. We show that this region is nonconvex in general and entirely agnostic to the probability distributions of all network parameters, depending only on their expected values.

Delay issue is a challenge in multi-hop cognitive radio networking because of the dynamic change of channels available to the cognitive radio nodes. To address this challenge, we propose a channel allocation and rate allocation scheme with low complexity in this paper. Our scheme is flexible so that it can react to the dynamic change of channel availability, and it can minimize the delay by considering the primary user activities. First, we model the primary users' activity, channel availability and the interference among the cognitive radio nodes in a cognitive radio network environment, and show how to implement a channel allocation algorithm and a rate allocation scheme in multi-hop cognitive radio networks. Second, we formulate an optimization problem to minimize the end-to-end delay of the network. We consider the channel availability constraint and the primary user activities jointly, and analyze the end-to-end delay in multi-hop cognitive radio networks. In order to reduce the computing complexity, we propose a graph coloring based channel allocation and rate selection algorithm using gradient descent method. Furthermore, we show the performance of our schemes and compare them with existing schemes for different scenarios of channel availability and number of channels through simulations. Our work brings insights on how to make rate selection and channel allocation in multi-hop cognitive radio networks.

Network coding is an effective idea to boost the capacity of wireless networks, and a variety of studies have explored its advantages in different scenarios. However, there is not much analytical study on throughput and end-to-end delay of network coding in multi-hop wireless networks considering the specifications of IEEE 802.11 Distributed Coordination Function. In this paper, we utilize queuing theory to propose an analytical framework for bidirectional unicast flows in multi-hop wireless mesh networks. We study the throughput and end-to-end delay of inter-flow network coding under the IEEE 802.11 standard with CSMA/CA random access and exponential back-off time considering clock freezing and virtual carrier sensing, and formulate several parameters such as the probability of successful transmission in terms of bit error rate and collision probability, waiting time of packets at nodes, and retransmission mechanism. Our model uses a multi-class queuing network with stable queues, where coded packets have a non-preemptive higher priority over native packets, and forwarding of native packets is not delayed if no coding opportunities are available. Finally, we use computer simulations to verify the accuracy of our analytical model.

The recent advent of ambient intelligence is enabled by parallel technological advancements in sensing, context recognition, embedded systems and communications. This paper focuses on the communication issues of embedded systems, particularly the latency Quality of Service (QoS) metric and the multi-hop communications with Bluetooth standard, to examine the viability of communications of embedded systems in AmI environments and applications. Bluetooth is a worldwide radio license-free technology that enables the creation of low-power multi-hop networks interconnecting multiple devices. Bluetooth sets the procedure to establish piconets (point-to-point and point-to-multipoint links) and scatternets (multi-hop communications), and hence, Bluetooth nodes can be interconnected to form wireless networks. This paper presents research on multi-hop latency that was conducted using a custom-built test platform. Moreover, an empirical model is derived to calculate the latency over asynchronous links when links in scatternets are always active or in sniff mode. The designers of ambient intelligent devices and networks can take advantage of the model and the estimation of the delay in Bluetooth multi-hop networks presented in this paper.

In order to reduce the costs of implementation and maintenance, communication networks are used in networked Discrete-Event Systems (NDES), providing an efficient way to establish communication between several devices in the plant. In many cases, multi-hop mesh networks are used, as on wireless sensor networks, with re-transmissions due to degradation, leading to delays in communication and, consequently, the observation of events in a different order from their occurrence in the system. In this paper, we address the problem of fault diagnosis of NDES, resilient to observation delays in the communication network. We consider that each event is communicated together with its sequence number, and we show how the sequence number can be used to improve the fault diagnosis capability in the presence of observation delays, without the need to reorder the sequence of events according to the sequence number.

IEEE 802.15.4 which is used as the Low Rate(LR)-Wireless Personal Area Networks (WPAN) standard supports Guaranteed Time Slot(GTS) allocation scheme to ensure the Quality of Service(QoS) in the Contention-Free Period(CFP). However, it must be competitive in the same way as other devices via Carrier Sense Multiple Access(CSMA)-Collision Avoidance(CA) in order to request the assignment of the GTS and possible assignment of up to seven GTS. In addition, multi hop devices from the PAN coordinator are not easy to be guaranteed QoS through the GTS. In this paper, we propose not only the scheduling algorithm of GTS allocation but also MAC frame format for resolving transmission problems in multi-hop wireless sensor networks. With proposed schemes we can offer the more efficient use of GTS than the previous one and guarantee the QoS without the waste of symbols.

The IEEE 802.11 Distributed Coordination Function (DCF) Medium Access Control (MAC) protocol is designed to efficiently facilitate the limited communication bandwidth of wireless channel. This protocol uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism. This mechanism continues suffer from throughput degradation when directly applied in multihop Wireless Mesh Network (WMN). The major reason for this poor performance is due to larger signaling overheads (RTS and CTS signaling packet) introduced in order to forward a single data packet in multihop WMN. This inefficient forwarding operation caused the throughput degradation signifficantly. Therefore, an efficient forwarding operation is proposed in this paper to reduce the amount of signaling overheads which are needed to forward a single packet in multihop WMN. The proposed protocol uses the capability of overhearing in order to forward the data packet from one hop to another hop. This process will continue until the data packet reaches the respective destination. As a result, the enhanced protocol reduces the latency caused by signaling thus improve its throughput in multihop WMN. The multihop network performances are evaluated analytically in terms of throughput and delay. Through the simulation, it is proven that the proposed protocol provides significant improvement in throughput and delay. The results show that the proposed protocol outperforms the existing IEEE DCF MAC protocol when it is evaluated in multihop WMN.

The IEEE 802.15.4 standard provides Guaranteed Time Slot (GTS) mechanism for reliable transmission. GTS mechanism is suitable for transmitting time-sensitive data because it allocates time slots to a specific node. However, the GTS mechanism in the standard can be used only for one-hop communication. This paper proposes and implements a multihop GTS mechanism for reliable transmission in multihop networks. Simulation results using NS-2 show that low delay and high delivery ratio can be achieved using the proposed mechanism.

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.

In this paper, we consider CSMA policies for scheduling packet transmissions in multihop wireless networks with one-hop traffic. The main contribution of the paper is to propose a novel CSMA policy, called Unlocking CSMA (U-CSMA), that enables to obtain both high throughput and low packet delays in large wireless networks. More precisely, we show that for torus interference graph topologies with one-hop traffic, U-CSMA is throughput optimal and achieves order-optimal delay. For one-hop traffic, the delay performance is defined to be order-optimal if the delay stays bounded as the network-size increases. Simulations that we conducted suggest that (a) U-CSMA is throughput-optimal and achieves order-optimal delay for general geometric interference graphs and (b) that U-CSMA can be combined with congestion control algorithms to maximize the network-wide utility and obtain order-optimal delay. To the best of our knowledge, this is the first time that a simple distributed scheduling policy has been proposed that is both throughput/utility optimal and achieves order-optimal delay.

Forecasting skills of the complex weather and climate models have been improved by tuning the sensitive parameters that exert the greatest impact on simulated results based on more effective optimization methods. However, whether the optimal parameter values are still work when the model simulation conditions vary, which is a scientific problem deserving of study. In this study, a highly-effective optimization method, adaptive surrogate model-based optimization (ASMO), was firstly used to tune nine sensitive parameters from four physical parameterization schemes of the Weather Research and Forecasting (WRF) model to obtain better summer precipitation forecasting over the Greater Beijing Area in China. Then, to assess the applicability of the optimal parameter values, simulation results from the WRF model with default and optimal parameter values were compared across precipitation events, boundary conditions, spatial scales, and physical processes in the Greater Beijing Area. The summer precipitation events from 6 years were used to calibrate and evaluate the optimal parameter values of WRF model. Three boundary data and two spatial resolutions were adopted to evaluate the superiority of the calibrated optimal parameters to default parameters under the WRF simulations with different boundary conditions and spatial resolutions, respectively. Physical interpretations of the optimal parameters indicating how to improve precipitation simulation results were also examined. All the results showed that the optimal parameters obtained by ASMO are superior to the default parameters for WRF simulations for predicting summer precipitation in the Greater Beijing Area because the optimal parameters are not constrained by specific precipitation events, boundary conditions, and spatial resolutions. The optimal values of the nine parameters were determined from 127 parameter samples using the ASMO method, which showed that the ASMO method is very highly-efficient for optimizing WRF model parameters.