Throughput and PER estimates harnessing link-layer measurements for indoor 802.11n WLAN

The research work reported in this paper investigates if a Markov chain can model the throughput and packet error rate (PER) performance of off-the-shelf IEEE 802.11n wireless LAN network interface cards (NICs). We draw together uplink -downlink information from the NIC with a Markov chain to examine the performance of 802.11n within an indoor environment. Site measurements and point-estimates are taken and compared with the model predictions. Errors of less than 4% were recorded for the Markov model estimates while point-estimates recorded average errors of 9% both compared to site-measured throughput.     

Performance of IP-forwarding of Linux hosts with multiple network interfaces

This paper evaluates and compares the performance of IP-packet forwarding of a Linux host equipped with multiple network interface cards (NICs), namely two receiving NICs and one transmitting NIC. We consider a Linux host with SMP (Symmetric Multiprocessing) or multicore multiprocessor (MCMP) architecture. We measure IP forwarding by subjecting an MCMP Linux host to different traffic load conditions of up to 1 Gbps. We used the IXIA hardware traffic generator to generate traffic with fixed- and variable-size packets. At the Linux host, generated packets are forwarded/routed from the two receiving NICs to the transmitting NIC. We consider two NIC affinity modes: (I) both receiving NICs are affinitized (or bound) to two cores of the same processor while the transmitting NIC is affinitized to a core on a separate processor, and (II) the transmitting NIC and one receiving NIC are affinitized to two cores of the same processor while the second receving NIC is affinitized to a core on a separate processor. For each affinity mode, we measure the performance for three packet reception mechanisms: NAPI (New API) with a default budget of 300, NAPI with a budget of 2, and Disable and Enable interrupt handling. The performance is measured and compared in terms of various key performance metrics which include throughput, packet loss, round-trip delay, interrupt rates, and CPU availability.     

AODV算法在视频监控系统中的应用与研究

在Linux嵌入式系统平台上,针对按需距离矢量路由协议AODV算法在无线Mesh网络的应用开发问题,本文采用比较有代表性的UU实现方法,提出了基于Netfilter功能框架的AODV协议在Linux平台上实现的具体方法,在真实的无线网络环境下实现AODV路由协议。将AODV应用开发到嵌入式Linux平台上,并在此基础上用IEEE802.11b标准的网卡组建一个无线自组织网络,从而扩展IEEE802.11的组网方式。最后,在Linux操作系统上实现了AODV路由协议,并通过视频监控系统验证了AODV协议实现的功能与相应的协议标准中所定义的基本功能的一致性。     

802.11协议中RTS/CTS机制的安全漏洞分析

802.11协议中的RTS/CTS机制可以解决隐藏节点问题,通过分析RTS/CTS机制中的安全漏洞,发现由于RTS/CTS握手过程中没有认证机制,攻击者可以通过连续发送设定足够大网络分配矢量值的伪造RTS/CTS帧,以达到拒绝服务攻击的目的。在理论分析的基础上,使用aircrack-ng的软件开发包,在真实的无线网络环境中进行了实验,实验结果显示,RTS/CTS机制确实存在着被拒绝服务攻击的安全漏洞,可随时被攻击者利用,需引起充分重视。     

Providing MAC QoS for multimedia traffic in 802.11e based multi-hop ad hoc wireless networks

Ad hoc wireless networks with their widespread deployment, now need to support applications that generate multimedia and real-time traffic. Video, audio, real-time voice over IP, and other multimedia applications require the network to provide guarantees on the Quality of Service (QoS) of the connection. The 802.11e Medium Access Control (MAC) protocol was proposed with the aim of providing QoS support at the MAC layer. The 802.11e performs well in wireless LANs due to the presence of Access Points (APs), but in ad hoc networks, especially multi-hop ones, it is still incapable of supporting multimedia traffic.One of the most important QoS parameters for multimedia and real-time traffic is delay. Our primary goal is to reduce the end-to-end delay, thereby improving the Packet Delivery Ratio of multimedia traffic, that is, the proportion of packets that reach the destination within the deadline, in 802.11e based multi-hop ad hoc wireless networks.Our contribution is threefold: first we propose dynamic ReAllocative Priority (ReAP) scheme, wherein the priorities of packets in the MAC queues are not fixed, but keep changing dynamically. We use the laxity and the hop length information to decide the priority of the packet. ReAP improves the PDR by over 28% in comparison with 802.11e, especially under heavy loads. Second, we introduce Adaptive-TXOP (A-TXOP), where transmission opportunity (TXOP) is the time interval during which a node has the right to initiate transmissions. This scheme reduces the delay of video traffic by reducing the number of channel accesses required to transmit large video frames. It involves modifying the TXOP interval dynamically based on the packets in the queue, so that fragments of the same packet are sent in the same TXOP interval. A-TXOP is implemented over ReAP to further improve the performance of video traffic. ReAP with A-TXOP helps in reducing the delay of video traffic by over 27% and further improves the quality of video in comparison with ReAP without A-TXOP. Finally, we have TXOP-sharing, which is aimed at reducing the delay of voice traffic. It involves using the TXOP to transmit to multiple receivers, in order to utilize the TXOP interval fully. It reduces the number of contentions to the channel and thereby reduces the delay of voice traffic by over 14%. A-TXOP is implemented over ReAP to further improve the performance of voice traffic. The three schemes (ReAP, A-TXOP, and TXOP-sharing) work together to improve the performance of multimedia traffic in 802.11e based multi-hop ad hoc wireless networks.     

Evaluating transport protocol performance over a wireless mesh backbone

IEEE 802.11n wireless physical layer technology increases the deployment of high throughput wireless indoor mesh backbones for ubiquitous Internet connectivity at the urban and metropolitan areas. Most of the network traffic flows in today’s Internet use ‘Transmission Control Protocol’ (TCP) as the transport layer protocol. There has been extensive works that deal with TCP issues over wireless mesh networks as well as noisy wireless channels. Further, IEEE 802.11n is well known for its susceptibility to increased channel losses during high data rate communication. This paper investigates the dynamics of an end-to-end transport layer protocol like TCP in the presence of burst and correlated losses during IEEE 802.11n high data rate communication, while maintaining fairness among all the end-to-end flows. For this purpose, we evaluate four TCP variants-Loss Tolerant TCP (LT-TCP), Network Coded TCP (TCP/NC), TCP-Horizon and Wireless Control Protocol (WCP), where the first two protocols are known to perform very well in extreme lossy networks, and the last two are specifically designed for mesh networks. Our evaluation shows that WCP performs better in a IEEE 802.11n supported mesh networks compared to other three variants. However, WCP also results in negative impact at high data rates, where end-to-end goodput drops with the increase in physical data rate. The analysis of the results reveals that explicit loss notifications and flow balancing are not sufficient to improve transport protocol performance in an IEEE 802.11n supported mesh backbone, rather a specific mechanism is required to synchronize the transport queue management with lower layer scheduling that depends on IEEE 802.11n features, like channel bonding and frame aggregation. The findings of this paper give the direction to design a new transport protocol that can utilize the full capacity of IEEE 802.11n mesh backbone.     

Runtime optimization of IEEE 802.11 wireless LANs performance

IEEE 802.11 is the standard for wireless local area networks (WLANs) promoted by the Institute of Electrical and Electronics Engineers. Wireless technologies in the LAN environment are becoming increasingly important and the IEEE 802.11 is the most mature technology to date. Previous works have pointed out that the standard protocol can be very inefficient and that an appropriate tuning of its congestion control mechanism (i.e., the backoff algorithm) can drive the IEEE 802.11 protocol close to its optimal behavior. To perform this tuning, a station must have exact knowledge of the network contention level; unfortunately, in a real case, a station cannot have exact knowledge of the network contention level (i.e., number of active stations and length of the message transmitted on the channel), but it, at most, can estimate it. We present and evaluate a distributed mechanism for contention control in IEEE 802.11 wireless LANs. Our mechanism, named asymptotically optimal backoff (AOB), dynamically adapts the backoff window size to the current network contention level and guarantees that an IEEE 802.11 WLAN asymptotically achieves its optimal channel utilization. The AOB mechanism measures the network contention level by using two simple estimates: the slot utilization and the average size of transmitted frames. These estimates are simple and can be obtained by exploiting information that is already available in the standard protocol. AOB can be used to extend the standard 802.11 access mechanism without requiring any additional hardware. The performance of the IEEE 802.11 protocol, with and without the AOB mechanism, is investigated through simulation. Simulation results indicate that our mechanism is very effective, robust, and has traffic differentiation potentialities.     

Traffic sensing and characterization in multi-channel wireless networks for cognitive networking

Traffic sensing and characterization is an important building block of cognitive networking systems, however, it is very challenging to perform traffic characterization in multi-channel multi-radio wireless networks. Due to the presence of network traffic in multiple channels, the existing count-based packet sampling methods demand continuous capture on each channel to be effective; this requires a dedicated wireless interface per channel, and hence the existing sampling methods require a very expensive infrastructure and have poor scalability. Time-based sampling methods, on the other hand, offer a cost-effective and scalable solution by reducing the amount and cost of the resources necessary to monitor and characterize the wireless spectrum.The contributions of this paper include the following: (i) a discussion of packet sampling techniques for traffic sensing in multi-channel wireless networks, (ii) a comparison of various time-based sampling strategies using the Kullback–Leibler divergence (KLD) measure, (iii) a study on the effect of the sampling parameters on the accuracy of the sampling strategies, (iv) development of sampling accuracy graphs for easing the process of best sampling scheme selection in multi-channel wireless networks, (v) the proposal of a new metric (traffic intensity) which estimates the busyness of channels by taking into consideration not only the successfully received packets but also corrupt or broken packets, (vi) implementation of time-based sampling in a prototype traffic sensor device for multi-channel traffic sensing in IEEE 802.11 b/g networks, and (vii) characterization of a campus IEEE 802.11 network environment in a spatio-temporal–spectral fashion using sampled traffic traces collected by traffic sensors.     

Interworking wireless mesh networks: Problems,performance characterization,and perspectives

Wireless broadband networks based on the IEEE 802.11 technology are being increasingly deployed as mesh networks to provide users with extended coverage for wireless Internet access. These wireless mesh networks, however, may be deployed by different authorities without any coordination a priori, and hence it is possible that they overlap partially or even entirely in service area, resulting in contention of radio resources among them. In this paper, we investigate the artifacts that result from the uncoordinated deployment of wireless mesh networks. We use a network optimization approach to model the problem as resource sharing among nodes belonging to one or different networks. Based on the proposed LP formulation, we then conduct simulations to characterize the performance of overlaying wireless mesh networks, with the goal to provide perspectives for addressing the problems. We find that in a system with multiple overlaying wireless mesh networks, if no form of inter-domain coordination is present, individual mesh networks could suffer from capacity degradation due to increased network contention. One solution toward addressing the performance degradation is to “interwork” these wireless mesh networks by allowing inter-domain traffic relay through provisioning of “bridge” nodes. However, if such bridge nodes are chosen arbitrarily, the problems of throughput sub-optimality and unfairness may arise. We profile the impact of bridge node selection and show the importance in controlling network unfairness for wireless mesh network interworking. We conclude that mesh network interworking is a promising direction to address the artifacts due to uncoordinated deployment of wireless mesh networks if it is supplemented with appropriate mechanisms.     

一种基于方向天线和时间同步机制的多信道无线网状网MAC协议

在城市小区和广阔的农村建立低成本的无线网状网,为无线终端用户提供高速的接入服务,是一项极具现实意义的工作.设计了一种适合于此类网络的MAC层协议称为Mesh-MAC,该协议建立在已有的IEEE 802.11硬件产品基础上,每两个节点之间的通信是由一对方向天线完成,通过一种全新的时间同步方案,可以实现M ESH网内高效的数据转发.仿真结果表明,与IEEE 802.11及2P协议相比,Mesh-MAC协议在吞吐量和端到端传输延时等方面,都有明显的改善.     

存在隐藏终端的IEEE802.11e多跳无线网络模型分析

首次将802.11e的接入机制放入多跳环境中进行仿真分析和定量研究。提出一种新的多跳环境下802.11e网络模型,结合M/G/1/K排队模型,定量分析在隐藏终端影响下802.11e网络的MAC层吞吐率、MAC时延和帧丢失率,研究802.11e在多跳环境下性能表现的内在原因。经过仿真实验结果与数值分析结果的对比,验证了分析模型的准确性;通过有(无)隐藏终端影响下MAC层性能的对比分析,指出了802.11e在多跳无线网络中支持QoS的局限性:受隐藏终端的影响,802.11e对不同接入等级的业务提供QoS区分的性能明显降级。因而有必要研究隐藏终端问题的解决方案,提高802.11e在多跳环境下的性能。     

存在隐藏终端的IEEE 802.11e多跳无线网络模型分析*

首次将802.11e的接入机制放入多跳环境中进行仿真分析和定量研究。提出一种新的多跳环境下802.11e网络模型,结合M/G/1/K排队模型,定量分析在隐藏终端影响下802.11e网络的MAC层吞吐率、MAC时延和帧丢失率,研究802.11e在多跳环境下性能表现的内在原因。经过仿真实验结果与数值分析结果的对比,验证了分析模型的准确性;通过有（无）隐藏终端影响下MAC层性能的对比分析,指出了802.11e在多跳无线网络中支持QoS的局限性：受隐藏终端的影响,802.11e对不同接入等级的业务提供QoS区分的性能明显降级。因而有必要研究隐藏终端问题的解决方案,提高802.11e在多跳环境下的性能。     

Service level agreements (SLAs) parameter negotiation between heterogeneous 4G wireless network operators

We are currently witnessing a growing interest of network operators to migrate their existing 2G/3G networks to 4G technologies such as long-term evolution (LTE) to enhance the user experience and service opportunities in terms of providing multi-megabit bandwidth, more efficient use of radio networks, latency reduction, and improved mobility. Along with this, there is a strong deployment of packet data networks such as those based on IEEE 802.11 and 802.16 standards. Mobile devices are having increased capabilities to access many of these wireless networks types at the same time. Reinforcing quality of service (QoS) in 4G wireless networks will be a major challenge because of varying bit rates, channel characteristics, bandwidth allocation and global roaming support among heterogeneous wireless networks. As a mobile user moves across access networks, to the issue of mapping resource reservations between different networks to maintain QoS behavior becomes crucial. To support global roaming and interoperability across heterogeneous wireless networks, it is important for wireless network operators to negotiate service level agreement (SLA) contracts relevant to the QoS requirements. Wireless IP traffic modeling (in terms of providing assured QoS) is still immature because the majority of the existing work is merely based on the characterization of wireless IP traffic without investigating the behavior of queueing systems for such traffic. To overcome such limitations, we investigate SLA parameter negotiation among heterogeneous wireless network operators by focusing on traffic engineering and QoS together for 4G wireless networks. We present a novel mechanism that achieves service continuity through SLA parameter negotiation by using a translation matrix, which maps QoS parameters between different access networks. The SLA matrix composition is modeled analytically based on the G/M/1 queueing system. We evaluate the model using two different scheduling schemes and we derive closed form expressions for different QoS parameters for performance metrics such as packet delay and packet loss rate. We also develop a discrete event simulator and conduct a series of simulation experiments in order to understand the QoS behavior of corresponding traffic classes.     

Design and performance evaluation of throughput-aware rate adaptation protocols for IEEE 802.11 wireless networks

Experimental studies have shown that traditional rate adaptation schemes for 802.11 wireless networks suffer from significant throughput degradation in highly congested networks. To address this problem, this paper makes the following two main contributions. First, we design a method to accurately estimate the per-packet transmission times, and we use these measurements to provide a broad classification of the network state and to identify network congestion. Second, we design a new Throughput-Aware Rate Adaptation (TARA) scheme, which uses the congestion estimates to mitigate the negative impact of link-layer collisions on the operations of rate adaptation, without requiring changes in the 802.11 MAC specification. Another key feature of our solution is to minimize probing overheads and limit unnecessary rate decreases by predicting the throughput gain that could be brought about by a change in the transmission rate. Through experiments conducted across a variety of network scenarios and traffic patterns, we show that TARA achieves significantly higher throughput than the other rate adaptation algorithms implemented in the legacy Madwifi driver.     

无线网络中实时业务的信道预留

随着无线数据服务的流行和多媒体应用需求的增长,需要无线网络能够为不同种类的业务提供有区别的服务质量(QoS),因此如何在无线网络中提供有区分和有保证的服务质量,成为一个非常重要的问题。论文提出了一种在IEEE802.11无线网络中为实时业务提供服务质量保证的有效方法,即通过修改介质访问控制(MAC)层的分布式协调功能(DCF),为实时数据预留信道。当一个节点竞争到信道后,可以连续向同一个目标节点发送多个实时数据帧,称作传送突发(TB,TransmissionBurst),即在一个TB中第一个数据帧将为实时业务预留信道,直到这个TB结束,这样实时业务将比其他业务占有更多的信道资源。仿真结果表明,该方法能够明显改进实时业务的吞吐率和延迟性能,提高无线网络的信道利用率,而且,对DCF的修改没有增加任何控制开销。     

Pricing-based call admission control for wireless MPEG-4 traffic

Next generation wireless technologies will need to incorporate new sets of traffic control procedures in order to cope with the challenges related to supporting both the existing and the ever-increasing new multimedia services. The work presented in this paper focuses on traffic control for MPEG-4 videoconference over cellular networks, and is twofold. We first introduce a traffic model for medium quality MPEG-4 videoconference traffic. We then proceed to use this model, as well as a previously developed model for high quality MPEG-4 traffic, in the implementation of a new Call Admission Control (CAC) mechanism. Providers need to use efficient resource management mechanisms in order to keep existing clients satisfied and attract new customers, so that they can increase their revenue. Towards this goal, our CAC mechanism makes decisions on the acceptance/rejection of a new video call not only based on the predicted capacity that users will consume, but also on the possible revenue gained for the provider when degrading current users in order to accommodate new ones. Our mechanism is implemented with both a static and a dynamic pricing policy and is shown, via an extensive simulation study, to provide excellent Quality of Service (QoS) to wireless videoconference users.     

Nonsaturation throughput enhancement of IEEE 802.11b distributed coordination function for heterogeneous traffic under noisy environment

In this paper, we propose a mechanism named modified backoff (MB) mechanism to decrease the channel idle time in IEEE 802.11 distributed coordination function (DCF). In the noisy channel, when signal-to-noise ratio (SNR) is low, applying this mechanism in DCF greatly improves the throughput and lowers the channel idle time. This paper presents an analytical model for the performance study of IEEE 802.11 MB-DCF for nonsaturated heterogeneous traffic in the presence of transmission errors. First, we introduce the MB-DCF and compare its performance to IEEE 802.11 DCF with binary exponential backoff (BEB). The IEEE 802.11 DCF with BEB mechanism suffers from more channel idle time under low SNR. The MB-DCF ensures high throughput and low packet delay by reducing the channel idle time under the low traffic in the network. However, to the best of the authors' knowledge, there are no previous works that enhance the performance of the DCF under imperfect wireless channel. We show through analysis that the proposed mechanism greatly outperforms the original IEEE 802.11 DCF in the imperfect channel condition. The effectiveness of physical and link layer parameters on throughput performance is explored. We also present a throughput investigation of the heterogeneous traffic for different radio conditions.     

支持QoS的IEEE 802.11 EDCA性能研究

随着无线网络应用的不断发展,为了适应网络中不同类型业务的区分服务需要,IEEE 802.11工作组在IEEE 802.11 DCF(distributed coordination function)的基础上推出了支持QoS的IEEE 802.11 EDCA协议.针对EDCA(enhanced distributed channel access)协议的优先级区分信道接入特性,提出了一种基于Markov链的协议性能模型分析方法.与已有文献不同,该模型分析同时包含了EDCA主要的3个关键区分信道接入机制:Wmin/Wmax,AIFS(arbitration inter-frame space),TXOP(transmission opportunity).通过模型分析,获得了EDCA协议各优先级接入的传输吞吐量、信道接入延迟、数据丢失率等性能分析.不仅分析了饱和情形下的EDCA性能,而且还对非饱和情形下的EDCA性能进行了分析.仿真验证结果表明,模型分析结果和仿真结果取得了很好的一致性.利用给出的模型分析方法,提出了一种基于TXOP动态调整的D-TXOP(dynamic TXOP)接纳控制算法.算法根据网络中业务流的QoS要求,在动态调整各优先级的TXOP参数设置的基础上,对网络中新到业务流进行接纳控制,达到提高网络的业务承载能力的目的.     

无线局域网安全协议的分析和研究

文章阐述了无线局域网几类安全协议的技术标准、发展历程,介绍了无线局域网的安全标准WEP、802.11i和802.11w的安全保障机制,重点分析了WEP、802.11i和802.11w标准应用中存在的安全问题以及可能的典型攻击方式,最后提出了加强无线局域网安全的措施。     

A multirate MAC protocol for reliable multicast in multihop wireless networks

Many multicast applications, such as audio/video streaming, file sharing or emergency reporting, are becoming quite common in wireless mobile environment, through the widespread deployment of 802.11-based wireless networks. However, despite the growing interest in the above applications, the current IEEE 802.11 standard does not offer any medium access control (MAC) layer support to the efficient and reliable provision of multicast services. It does not provide any MAC-layer recovery mechanism for unsuccessful multicast transmissions. Consequently, lost frames cannot be detected, hence retransmitted, causing a significant quality of service degradation. In addition, 802.11 multicast traffic is sent at the basic data rate, often resulting in severe throughput reduction.In this work, we address these issues by presenting a reliable multicast MAC protocol for wireless multihop networks, which is coupled with a lightweight rate adaptation scheme. Simulation results show that our schemes provide high packet delivery ratio and when compared with other state-of-the-art solutions, they also provide reduced control overhead and data delivery delay.