Mobility impact in IEEE 802.11p infrastructureless vehicular networks

Vehicular ad hoc networks (VANETs) are an extreme case of mobile ad hoc networks (MANETs). High speed and frequent network topology changes are the main characteristics of vehicular networks. These characteristics lead to special issues and challenges in the network design, especially at the medium access control (MAC) layer. In this paper, we provide a comprehensive evaluation of mobility impact on the IEEE 802.11p MAC performance. The study evaluates basic performance metrics such as packet delivery ratio, throughput, and delay. An unfairness problem due to the relative speed is identified for both broadcast and unicast scenarios. We propose two dynamic contention window mechanisms to alleviate network performance degradation due to high mobility. The first scheme provides dynamic level of service priority via adaptation to the number of neighboring nodes, while the second scheme provides service priority based on node relative speed. Extensive simulation results demonstrate a significant impact of mobility on the IEEE 802.11p MAC performance, the unfairness problem in the vehicle-to-vehicle (V2V) communications, and the effectiveness of the proposed MAC schemes.     

Assessing the IEEE 802.11e QoS effectiveness in multi-hop indoor scenarios

The IEEE 802.11e technology is receiving much interest due to the enhancements offered to wireless local area networks in terms of QoS. Other application fields for this technology are wireless ad hoc networks, wireless mesh networks, and vehicular ad hoc networks. In the literature, most of the research works available focusing on the IEEE 802.11e technology offer simulation results alone, being hard to find empirical results of implementations that prove its effectiveness in realistic scenarios. Additionally, we consider that studies of IEEE 802.11e based on simulation platforms have not been thoroughly validated using real-life results. In this work we analyze the performance of the IEEE 802.11e technology in real multi-hop ad hoc networks. With this purpose we first we devise a set of experiments where we compare the results obtained on a small testbed to those from the ns-2 simulation platform. A significant consistency in terms of overall trends is found, although remarkable differences can be appreciated in terms of both delay and throughput results. Afterward we proceed with a full deployment of IEEE 802.11e enabled stations throughout the floor of an university building, performing several experiments using both static and dynamic routing. Experimental results show that QoS can be reasonably sustained for both voice and video traffic in multi-hop ad hoc networks, although dynamic routing protocols can hinder performance by provoking frequent on-off connectivity problems.     

Performance Evaluation of Routing Protocols for Ad Hoc Wireless Networks

A mobile ad hoc network is a collection of autonomous mobile nodes that communicate with each other over wireless links. Such networks are expected to play an increasingly important role in future civilian and military settings, being useful for providing communication support where no fixed infrastructure exists or the deployment of a fixed infrastructure is not economically profitable and movement of communicating parties is possible. However, since there is no stationary infrastructure such as base stations, mobile hosts need to operate as routers in order to maintain the information about the network connectivity. Therefore, a number of routing protocols have been proposed for ad hoc wireless networks. In this paper, we study and compare the performance of the following routing protocols AODV, PAODV (preemptive AODV), CBRP, DSR, and DSDV. A variety of workload and scenarios, as characterized by mobility, load and size of the ad hoc network were simulated. Our results indicate that despite its improvement in reducing route request packets, CBRP has a higher overhead than DSR because of its periodic hello messages while AODV's end-to-end packet delay is the shortest when compared to DSR and CBRP. PAODV has shown little improvements over AODV.     

Ad Hoc网络中两种按需路由协议的比较

Ad hoc网络的特征是多跳的无线连接，网络拓扑的频繁变化，以及需要有效的动态路由协议。重点比较Ad hoc网络中两种主要的按需路由协议：动态源路由（DSR）协议和Ad hoc按需距离矢量路由（AODV）协议。尽管DSR和AODV都是按需路由协议，但不同的协议机制导致了网络性能的显著差异。在OPNET仿真平台下，对采用不用路由协议的网络时延和吞吐量进行比较，从而分析不同的路由机制引起的网络性能差异。     

High throughput reactive routing in multi-rate ad hoc networks

A reactive routing algorithm for multi-rate ad hoc wireless networks is proposed. It enhances the AODV protocol and achieves higher throughput by utilising the multi-rate capability of the underlying wireless channel via the MAC delay routing metric. Simulations show that the protocol leads to a significant throughput increase over traditional ad hoc routing protocols.     

Vehicle density based multihop broadcast protocol in VANET

In vehicular ad hoc networks (VANET), the medium access control (MAC) protocol was of crucial importance to provide time-critical multihop broadcast. Contemporary multihop broadcast protocols in VANET usually choose the farthest node in broadcast range as the forwarder to reduce the number of forwarding hops. However, it was demonstrated that the farthest forwarder may experience long contention delay in case of high vehicle density. An IEEE 802.11p-based multihop broadcast protocol vehicle density based forwarding (VDF) was proposed, which adaptively chose the forwarder according to the vehicle density. The evaluation results in safety warning and online game applications show that, VDF could shorten broadcast delay by achieving the proper trade-off between the contention delay and coverage speed.     

Cluster based emergency message broadcasting technique for vehicular ad hoc network

Broadcasting is one of the major emergency services of Vehicular ad hoc network, since the number of vehicles increases every day. Because of the high density of the vehicles, it is necessary to broadcast emergency messages to all the vehicles to avoid traffic jam and vehicle accidents. Reliable broadcasting of messages in self-organizing Ad hoc networks is a promising research field. In this paper a cluster based emergency message broadcasting algorithm is proposed. In this regard we have proposed cluster based architecture for emergency message dissemination and collision avoidance in VANET. In this paper, first the formation of cluster is done in such a way that it avoids any kind of collision. Once cluster head is selected it takes the responsibility of intra cluster management to avoid interference between the clusters. To increase the reliability during emergency message dissemination two MAC layer broadcasts protocol is used. This assures that message is delivered on time without any hazards. The proposed model is simulated for DSDV, AODV, and DSR protocols with the standards 802.11 and 802.11p in SHWM, Manhattan and freeway mobility models.     

无线Mesh网中实时数据业务仿真与分析

无线Mesh网络是一种架构式多跳无线网络,具有结构灵活、快速部署、自组织和自愈合等优点,具有广泛的应用前景。通过采用ns-2仿真软件对基于IEEE 802.11 MAC协议和AODV路由协议的无线Mesh网中实时数据业务的数据传输进行建模与仿真,分析了Mesh网络中数据流经过的传输跳数与吞吐率的关系,以及MAC层RTS/CTS、重传次数以及MAC层和路由层相互作用对无线Mesh网中实时数据业务性能的影响,指出了提高无线Mesh网中实时数据业务性能的所需要的改进方向。     

ACAR: Adaptive Connectivity Aware Routing for Vehicular Ad Hoc Networks in City Scenarios

Multi-hop vehicle-to-vehicle communication is useful for supporting many vehicular applications that provide drivers with safety and convenience. Developing multi-hop communication in vehicular ad hoc networks (VANET) is a challenging problem due to the rapidly changing topology and frequent network disconnections, which cause failure or inefficiency in traditional ad hoc routing protocols. We propose an adaptive connectivity aware routing (ACAR) protocol that addresses these problems by adaptively selecting an optimal route with the best network transmission quality based on statistical and real-time density data that are gathered through an on-the-fly density collection process. The protocol consists of two parts: 1) select an optimal route, consisting of road segments, with the best estimated transmission quality, and 2) in each road segment of the chosen route, select the most efficient multi-hop path that will improve the delivery ratio and throughput. The optimal route is selected using our transmission quality model that takes into account vehicle densities and traffic light periods to estimate the probability of network connectivity and data delivery ratio for transmitting packets. Our simulation results show that the proposed ACAR protocol outperforms existing VANET routing protocols in terms of data delivery ratio, throughput and data packet delay. Since the proposed model is not constrained by network densities, the ACAR protocol is suitable for both daytime and nighttime city VANET scenarios.     

Routing in Sparse Vehicular Ad Hoc Wireless Networks

A vehicular ad hoc network (VANET) may exhibit a bipolar behavior, i.e., the network can either be fully connected or sparsely connected depending on the time of day or on the market penetration rate of the wireless communication devices. In this paper, we use empirical vehicle traffic data measured on 1-80 freeway in California to develop a comprehensive analytical framework to study the disconnected network phenomenon and its network characteristics. These characteristics shed light on the key routing performance metrics of interest in disconnected VANETs, such as the average time taken to propagate a packet to disconnected nodes (i.e., the re-healing time). Our results show that, depending on the sparsity of vehicles or the market penetration rate of cars using Dedicated Short Range Communication (DSRC) technology, the network re-healing time can vary from a few seconds to several minutes. This suggests that, for vehicular safety applications, a new ad hoc routing protocol will be needed as the conventional ad hoc routing protocols such as Dynamic Source Routing (DSR) and Ad Hoc On-Demand Distance Vector Routing (AODV) will not work with such long re-healing times. In addition, the developed analytical framework and its predictions provide valuable insights into the VANET routing performance in the disconnected network regime.     

基于NS2的无线自组织网络路由协议仿真

在无线自组织网络中,由于节点移动,网络拓扑结构变化频繁,所以路由协议的选择一直都是关键问题。该文采用NS2软件对两种主要的协议：动态源路由协议（DSR）和自组网按需距离矢量路由协议（AODV）进行了仿真。并且通过端到端延时、路由开销和分组投递率三种参数在不同条件下的数据对两种协议进行了评估。实验结果表明没有一种协议能够完全适用于自组网,对于特殊环境选择不同协议以满足需要。     

SDR: A Stable Direction-Based Routing for Vehicular Ad Hoc Networks

A stable and reliable routing mechanism for vehicular ad hoc networks (VANETs) is an important step toward the provision of long data transmission applications, such as file sharing and music download. Traditional mobile ad hoc network (MANET) routing protocols are not suitable for VANET because the mobility model and environment of VANET are different from those of traditional MANET. To solve this problem, we proposed a new stable routing algorithm, called stable directional forward routing. The novelty of the proposed routing protocol is its combining direction broadcast and path duration prediction into ad hoc on-demand distance vector routing protocols, which including: (1) Nodes in VANET are grouped based on the position, only nodes in a given direction range participating in the route discovery process to reduce the frequency of flood requests, (2) Route selection is based on the link duration while not the hops or other metrics to increase the path duration, (3) Route discovery is executed before the path expiration in order to decrease the end to end delay. The performance of the new scheme is evaluated through extensive simulations with Qualnet. Simulation results indicate the benefits of the proposed routing strategy in terms of decreasing routing control packet, reducing the number of link-breakage events, improving the packet delivery ratio and decreasing the end-to-end delay.     

A Scalable CSMA and Self-Organizing TDMA MAC for IEEE 802.11 p/1609.x in VANETs

vehicular ad hoc networks (VANETs) have been a key topic for research community and industry alike. The wireless access in vehicular environment standard employs the IEEE 802.11p/1609.4 for the Medium Access Control (MAC) layer implementation for VANETs. However, the carrier sense multiple access (CSMA) based mechanism cannot provide reliable broadcast services, and the multi-channel operation defined in IEEE 1609.4 divides the available access time into fixed alternating control channel intervals (CCH) and service channel (SCH) intervals, which may lead to the low utilization of the scarce resources. In this paper, a novel multichannel MAC protocol called CS-TDMA considering the channel access scheduling and channel switching concurrently is proposed. The protocol combines CSMA with the time division multiple access (TDMA) to improve the broadcast performance in VANETs. Meanwhile, the dwelling ratio between CCH and SCH changes dynamically according to the traffic density, resulting in the improvement of resource utilization efficiency. Simulation results are presented to verify the effectiveness of our mechanism and comparisons are made with three existing MAC protocols, IEEE MAC, SOFT MAC and VeMAC. The simulation results demonstrate the superiority of CS-TDMA in the reduction of transmission delay and packet collision rate and improvement of network throughput.     

Increasing fairness and efficiency using the MadMac protocol in ad hoc networks

The IEEE 802.11 MAC layer is known for its unfairness behavior in ad hoc networks. Introducing fairness in the 802.11 MAC protocol may lead to a global throughput decrease. It is still a real challenge to design a fair MAC protocol for ad hoc networks that is distributed, topology independent, that relies on no explicit information exchanges and that is efficient, i.e. that achieves a good aggregate throughput. The MadMac protocol deals with fairness and throughput by maximizing aggregate throughput when unfairness is solved. Fairness provided by MadMac is only based on information provided by the 802.11 MAC layer. MadMac has been tested in many configurations that are known to be unfair and compared with three protocols (IEEE 802.11 and two fair MAC protocols). In these configurations, MadMac provides a good aggregate throughput while solving the fairness issues.     

Cross-Layer Interaction of TCP and Ad Hoc Routing Protocols in Multihop IEEE 802.11 Networks

In this research, we first investigate the cross-layer interaction between TCP and routing protocols in the IEEE 802.11 ad hoc network. On-demand ad hoc routing protocols respond to network events such as channel noise, mobility, and congestion in the same manner, which, in association with TCP, deteriorates the quality of an existing end-to-end connection. The poor end-to-end connectivity deteriorates TCP's performance in turn. Based on the well-known TCP-friendly equation, we conduct a quantitative study on the TCP operation range using static routing and long-lived TCP flows and show that the additive-increase, multiplicative-decrease (AIMD) behavior of the TCP window mechanism is aggressive for a typical multihop IEEE 802.11 network with a low-bandwidth-delay product. Then, to address these problems, we propose two complementary mechanisms, that is, the TCP fractional window increment (FeW) scheme and the Route-failure notification using BUIk-losS Trigger (ROBUST) policy. The TCP FeW scheme is a preventive solution used to reduce the congestion-driven wireless link loss. The ROBUST policy is a corrective solution that enables on-demand routing protocols to suppress overreactions induced by the aggressive TCP behavior. It is shown by computer simulation that these two mechanisms result in a significant improvement of TCP throughput without modifying the basic TCP window or the wireless MAC mechanisms.     

Study of MANET routing protocols by GloMoSim simulator

This paper compares ad hoc on‐demand distance vector (AODV), dynamic source routing (DSR) and wireless routing protocol (WRP) for MANETs to distance vector protocol to better understand the major characteristics of the three routing protocols, using a parallel discrete event‐driven simulator, GloMoSim. MANET (mobile ad hoc network) is a multi‐hop wireless network without a fixed infrastructure. Following are some of our key findings: (1) AODV is most sensitive to changes in traffic load in the messaging overhead for routing. The number of control packets generated by AODV became 36 times larger when the traffic load was increased. For distance vector, WRP and DSR, their increase was approximately 1.3 times, 1.1 times and 7.6 times, respectively. (2) Two advantages common in the three MANET routing protocols compared to classical distance vector protocol were identified to be scalability for node mobility in end‐to‐end delay and scalability for node density in messaging overhead. (3) WRP resulted in the shortest delay and highest packet delivery rate, implying that WRP will be the best for real‐time applications in the four protocols compared. WRP demonstrated the best traffic scalability; control overhead will not increase much when traffic load increases. Copyright © 2005 John Wiley & Sons, Ltd.     

Non-IP Multi-protocol Stack for Vehicular Communications

Wireless vehicular communications are gaining momentum, mainly to improve road safety through the use of cooperative systems. Such visionary cooperative applications are based on recently established families of standards, notably the IEEE Wireless Access for Vehicular Environment (WAVE) and the ETSI ITS G5. Despite some relevant differences, both family of standards share the same physical and MAC layers, described in the IEEE 802.11:2012 - amendment 6, also know as IEEE 802.11p. Due to the characteristics of the wireless communications in vehicular environment, e.g., high-speed mobility causing unpredictable time-varying changes in connectivity, IP protocols are not suitable for safety communications as they require channel scanning, authentication and association under strict time limits. Safety vehicular communications rely, instead on non-IP protocols, either the WAVE Short Message Protocol (WSMP) or the FAST Network and Transport Protocol (FNTP). In this paper, we explore some of the challenges of implementing such protocols, and designed an architecture for a stack capable of handling both standards. The proposed architecture, including the communication and transport layers of the stack, was implemented in a custom ETSI ITS G5 compliant platform (IT2S), and its performance was assessed using a prototype.     

Mitigation and Performance Analysis of Routing Protocols Under Black-Hole Attack in Vehicular Ad-hoc Network (VANET)

Vehicular Ad-hoc network (VANET) is a self-organized ad hoc network. VANET becomes a most challenging research area as it has several issues related to routing protocols, quality of service, security, etc. Vehicular communication is critically unsafe to several kinds of active and passive routing attacks. This paper analyzes the impact of a compromised node (vehicle) on zone routing protocol and ad-hoc on-demand distance vector, and recommends a suitable solution called secure vehicular on demand routing to find out and mitigate the black hole attack. The given study analyses the effect of vehicle density on the average throughput, packet delivery ratio, end-to-end delay, normalized routing load and average path length.     

IEEE 802.11n: enhancements for higher throughput in wireless LANs

This article introduces a new standardization effort, IEEE 802.11n, an amendment to IEEE 802.11 standards that is capable of much higher throughputs, with a maximum throughput of at least 100 Mb/s, as measured at the medium access control data services access point. The IEEE 802.11n will provide both physical layer and MAC enhancements. In this article we introduce some PHY proposals and study the fundamental issue of MAC inefficiency. We propose several MAC enhancements via various frame aggregation mechanisms that overcome the theoretical throughput limit and reach higher throughput. We classify frame aggregation mechanisms into many different and orthogonal aspects, such as distributed vs. centrally controlled, ad hoc vs. infrastructure, uplink vs. downlink, single-destination vs. multi-destination, PHY-level vs. MAC-level, single-rate vs. multirate, immediate ACK vs. delayed ACK, and no spacing vs. SIFS spacing.     

多跳无线Ad hoc网络路由协议的模拟和性能比较

文中采用随机场景的模拟方法对3种多跳无线Ad hoc网络路由协议的性能做了定量性的分析.对网络模拟器NS-2做了更进一步的拓展:增加SASR路由协议、增加AODV的MAC层虚拟HELLO机制,在此基础上对路由协议AODV,DSR和SASR进行了模拟.模拟结果显示:3种路由协议都能适应网络的拓扑结构变化,但是SASR和AODV路由协议在多跳无线Ad Hoc网络仍然具有相对的优越性.