基于虚拟接口的802.11网络切换技术

802.11网络现有的切换方案可以解决域内切换问题,域间切换涉及网络层切换操作,导致较大的网络延迟和大量的分组丢失。该文提出一种基于虚拟接口的802.11网络切换方法,通过对一个物理接口设备分时使用,使得无线终端在保证现有通信质量的情况下,平滑地完成与新无线访问点AP建立无线链路的过程。该方法具有实现简单、不用修改现有的Mobile IP协议等优点,在NS系统上的模拟表明,该方法能够满足实时交互式应用对移动切换过程中的网络质量需求。     

TEASE: A novel Tunnel-based sEcure Authentication SchemE to support smooth handoff in IEEE 802.11 wireless networks

With the growing popularity of WiFi-based devices, WiFi-based wireless networks have received a great deal of interest in the wireless networks community. However, due to the limited transmission range of WiFi-based networks, mobile users have to switch their associated access points constantly to maintain continuing communications during their movement. The process of switching access points is called handoff. Handoff management is a key service in mobile networks, because providing seamless roaming in wireless networks is mandatory for supporting real-time applications in a mobile environment, such as VoIP, online games, and eConference. Security is another important issue in network communications, and to prevent possible attacks, authentication is required during the handoff process to guarantee the reliability of mobile clients and access points. In this paper, we propose a novel authentication scheme to achieve a smooth handoff in WiFi-based networks, which we refer to as TEASE. A tunnel is introduced to forward data packets between the new access point and the original reliable access point. The processing of a complete secure authentication and the transmitting of data between mobile terminals and their correspondence nodes can go on simultaneously. The security of handoff is achieved without increasing overhead to authentication servers, and handoff latency can be minimized to support seamless roaming. Simulation results show that our proposed scheme reduces significantly the communication interruption time and generates low packet loss ratio, and our method is suitable to be used for secure handoff in real-time applications.     

一种基于LWAP的IEEE 802.11平滑切换机制

IEEE 802.11网络中的BSS切换不可避免地产生丢包问题,这会造成网络性能严重下降,进而限制了移动环境下WLAN的广泛应用。为了降低切换过程中的丢包率,本文提出了一种在数据链路层、由无线链路事件触发的用户空间平滑切换机制,并且在基于嵌入式Linux平台的无线AP上进行了实现。大量的实验结果表明,在无需对当前IEEE 802.11协议进行改动的前提下,新切换机制可以保证在允许的时延范围内,较大地降低丢包率,提高网络性能,并且可以支持平滑漫游。改进的平滑切换机制独立于驱动,具有良好的可移植性,可以在现有AP上增量添加,而无需更改STA,降低了升级成本。     

Revealing the problems with 802.11 medium access control protocol in multi-hop wireless ad hoc networks

The IEEE 802.11 medium access control (MAC) protocol is a standard for wireless LANs, it is also widely used in almost all test beds and simulations for the research in wireless mobile multi-hop ad hoc networks. However, this protocol was not designed for multi-hop networks. Although it can support some ad hoc network architecture, it is not intended to support the wireless mobile ad hoc network, in which multi-hop connectivity is one of the most prominent features. In this paper, we focus on the following question: can IEEE 802.11 MAC protocol function well in multi-hop networks? By presenting several serious problems encountered in transmission control protocol (TCP) connections in an IEEE 802.11 based multi-hop network, we show that the current TCP protocol does not work well above the current 802.11 MAC layer. The relevant problems include the TCP instability problem found in this kind of network, the severe unfairness problem, and the incompatibility problem. We illustrate that all these problems are rooted in the MAC layer. Furthermore, by revealing the in-depth cause of these problems, we conclude that the current version of this wireless LAN protocol does not function well in multi-hop ad hoc networks. We thus doubt whether the current WaveLAN based system is workable as a mobile multi-hop ad hoc test bed. All the results shown in this paper are based on NS2 simulations, and are compatible with the results from the OPNET simulations.     

Enhanced MAC protocol to support multimedia traffic in cognitive wireless mesh networks

With the advanced physical layer techniques such as multiple-input and multiple-output (MIMO) and orthogonal frequency-division multiplexing (OFDM), transmission real-time 2D/3D contents and applications becomes more and more necessary in wireless networks for the amazing growing in demand of customers. However, the low efficiency of medium access control (MAC) protocol degrades the performance of real-time traffic greatly in multihop, wireless and mobile environment. Focusing on supporting real-time multimedia traffic in cognitive wireless mesh networks (WMNs), an enhanced MAC protocol is proposed. And the contribution of this paper is twofold: (1) An efficient carrier sense multiple access with collision avoidance (CSMA/CA) compatible time division multiple access (TDMA)-like MAC protocol called T-MAC is proposed, which aims to improve the system performance by allocating more channel access time in centralized manner and decreasing overhead. (2) An optimal adaptive scheduling scheme is proposed to support real-time multimedia applications and guarantee QoS for different priority traffic, which aims to find the optimized schedule among all possible sequences of concurrent transmissions by minimizing the occupied resources. Detailed simulation results and comparison with IEEE 802.11e MAC scheme show that the proposed T-MAC can effectively improve quality of service (QoS) for multimedia traffic in terms of throughput, end-to-end delay and packet loss rate, which also manifests that T-MAC is an efficient multimedia applications transmission scheme for mobile terminals and MAPs in cognitive WMNs.     

Design and evaluation of a fast MAC layer handoff management scheme for WiFi-based multichannel Vehicular Mesh Networks

Vehicular Mesh Networks (VMNs) are important components of the wireless metropolitan networks. Compared with traditional wireless networks, they are more efficient in terms of deployment, configuration, and maintenance. However, connecting all mesh routers through wireless connection in VMNs results in the network backbone's remarkably lower bandwidth. Multichannel technology, where non-interfering channels are used to enable mesh routers to send and receive packets simultaneously, can be adopted to improve the throughput of VMNs. Due to the limited radio range of mesh routers, mobile clients have to switch among different mesh routers to maintain wireless connections. Therefore, providing smooth roaming in multichannel VMNs has become an important topic in VMN research. In this paper, a novel MAC layer handoff scheme is proposed as a means of minimizing handoff latency for WiFi-based multichannel VMNs to provide smooth communication for real-time applications. By designing a dynamic grouping algorithm for channel selection and allowing mesh routers to switch their channels for probe response messages, our scheme can shorten the waiting time for the detection of available access routers, decrease the loss ratio of data packets during the handoff, and consequently achieve smooth handoff in the MAC layer.     

A survey of energy efficient MAC protocols for IEEE 802.11 WLAN

In recent years, IEEE 802.11 wireless local area networks (WLANs) have been widely deployed, and more and more mobile devices have built-in WLAN interfaces. However, WLAN employs the carrier sense multiple access with collision avoidance (CSMA/CA) medium access control (MAC) protocol, which consumes a significant portion of the energy resources of a mobile device. Hence, minimizing the energy consumption of the WLAN interface in mobile devices has recently attracted considerable interest in both academia and industry. This article provides a survey and an experimental study of the energy consumption issues and energy-efficient technologies of the MAC protocol in IEEE 802.11 WLAN.     

A mobility management scheme for wireless mesh networks based on a hybrid routing protocol

Recent advances in wireless mesh networks (WMNs) have overcome the drawbacks of traditional wired networks and wireless ad hoc networks. WMNs will play a leading role in the next generation of networks, and the question of how to provide seamless mobility management for WMNs is the driving force behind the research. The inherent characteristics of WMNs, such as relatively static backbones and highly mobile clients, require new mobility management solutions to be designed and implemented.In this paper, a hybrid routing protocol for forwarding packets is proposed: this involves both link layer routing and network layer routing. Based on the hybrid routing protocol, a mobility management scheme for WMNs is presented. Both intra-domain and inter-domain mobility management have been designed to support seamless roaming in WiFi-based WMNs. During intra-domain handoff, gratuitous ARP messages are used to provide new routing information, thus avoiding re-routing and location update. For inter-domain handoff, redundant tunnels are removed in order to minimize forwarding latency. Comprehensive simulation results illustrate that our scheme has low packet latency, low packet loss ratio and short handoff latency. As a result, real-time applications over 802.11 WMNs such as VoIP can be supported.     

Differentiation, QoS guarantee, and optimization for real-time traffic over one-hop ad hoc networks

Nodes having a self-centrically broadcasting nature of communication form a wireless ad hoc network. Many issues are involved to provide quality of service (QoS) for ad hoc networks, including routing, medium access, resource reservation, mobility management, etc. Previous work mostly focuses on QoS routing with an assumption that the medium access control (MAC) layer can support QoS very well. However, contention-based MAC protocols are adopted in most ad hoc networks since there is no centralized control. QoS support in contention-based MAC layer is a very challenging issue. Carefully designed distributed medium access techniques must be used as foundations for most ad hoc networks. In this paper, we study and enhance distributed medium access techniques for real-time transmissions in the IEEE 802.11 single-hop ad hoc wireless networks. In the IEEE 802.11 MAC, error control adopts positive acknowledgement and retransmission to improve transmission reliability in the wireless medium (WM). However, for real-time multimedia traffic with sensitive delay requirements, retransmitted frames may be too late to be useful due to the fact that the delay of competing the WM is unpredictable. In this paper, we address several MAC issues and QoS issues for delay-sensitive real-time traffic. First, a priority scheme is proposed to differentiate the delay sensitive real-time traffic from the best-effort traffic. In the proposed priority scheme, retransmission is not used for the real-time traffic, and a smaller backoff window size is adopted. Second, we propose several schemes to guarantee QoS requirements. The first scheme is to guarantee frame-dropping probability for the real-time traffic. The second scheme is to guarantee throughput and delay. The last scheme is to guarantee throughput, delay, and frame-dropping probability simultaneously. Finally, we propose adaptive window backoff schemes to optimize throughput with and without QoS constraints.     

面向工业无线网络的动态TDMA系统设计与实现

随着工业4.0的发展, 不同种类的新型工业应用被部署到工厂中, 这对现有工业无线技术提出了实时性和高速率的要求. 为了同时满足这两种需求, 本文在支持高速率的IEEE802.11的基础上, 提出了基于软件定义的动态时分多址(Time division multiple access, TDMA)机制无线接入系统. 首先, 为了提供时延有界的传输服务, 设计并实现了基于MAC (Medium access control)层的动态TDMA接入机制. 然后, 为了满足工业无线网络中的动态变化的带宽需求, 考虑设备数据量的动态变化, 在SDN (Software defined network)控制器上通过基于最小二乘法的线性回归算法预测设备时隙需求, 再将动态时隙分配问题转化为优化问题以最大化网络中所有设备动态时隙需求. 最后, 通过仿真对比TDMA时隙分配算法的性能, 并在实际网络环境中开展系统部署与测试. 结果表明, 相对于其他TDMA接入机制, 动态TDMA机制在保障时延有界的同时能有效提升传输性能.     

Handoff with energy awareness for future pervasive environments

Handoff management is one of the main research challenges for the realization of the envisioned mobile and wireless Internet. This is mainly due to the latency delay and energy consumption introduced during handoff, which are of major concern for real-time applications and battery-constrained mobile terminals. In this paper, we explore whether handoff management can benefit from the pervasiveness of future communication networks. The key idea is to follow the ambient intelligence paradigm for the purpose of context-aware handoff. More precisely, we focus on the Radio Frequency Identification (RFID) and Wireless Sensor and Actuator Networks (WSANs) pervasive technologies, and propose two schemes for handoff prediction at the network layer or at both link and network layers, respectively. Analytical models for their time response and energy consumption are firstly derived. Then simulation-based results show that both of our schemes can achieve 38% to 93% reduction in time response while the second scheme provides 88% energy savings, validating their superiority over the standard solutions.     

一种基于IAPP协议的支持VoWLAN应用的快速IP切换方法

在多个WLAN之间进行实时通信时,网络层切换带来的过长时延成为保障实时通信质量的重要制约因素,如今提出的许多IP切换方法产生的时延都大到不能满足实时通信的要求。本文提出一种基于IEEE802.11的IAPP协议的快速IP切换方法。该方法切换时产生的时延小,能够较好地支持一些实时应用,如在WLAN上传输语音（VoWLAN）等应用。     

IP Mobility Issues for a Mobile Tele-Robotic System-NEPWAK

The network-centric applied research team (N-CART) is continuing its work on an ambitious project known as the network-enabled powered wheelchair adaptor kit (NEPWAK). It introduces techniques for modifying and using powered wheelchairs as mobile platforms enabling communication and remote control. The wheelchair is equipped with a laptop computer, a CCD camera and a wireless network interface card (NIC) for 802.11b Internet access. The laptop acts as a server allowing network clients to gain access through a custom control interface on the chair. The remote controlling client receives a video and audio feed from the chair and sends control signals for maneuvering. While traveling, the chair is able to change its network association from one access point (AP) to another within the same subnet-the process is known as handoff. However, there is no inter-network handoff mechanism presently available in IP networks. This restricts the mobility of the wheelchair to within the coverage area of the subnet APs. This paper shows that the Internet engineering task force's (IETF) network layer mobility protocol-Mobile IP suffers from large handoff latencies that can hinder communication between the client and the wheelchair during handoff. Mobile IP alone is not a sufficient solution for a mobile telebotic system such as NEPWAK. An interesting solution to the handoff latency problem comes from the Fast-handover protocol described in Section 4.4 with simulation results in Section 6.2.     

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.     

IEEE802.11网络中过后注册切换机制实现及性能分析

首先对移动切换过程和过后注册切换机制进行分析.在IEEE802.11网络上,对管理帧的信息单元进行扩展来构造二层触发消息,并在Linux上实现了过后注册切换机制.最后从丢包和切换时延两方面对过后注册切换机制进行性能分析.分析结果表明,过后注册切换机制明显降低了切换时延,减少了数据包的丢失.     

A Wireless LAN under High Load and in Noise: Throughput Evaluation

IEEE 802.11 specifies a network technology for wireless local area networks (LANs) and mobile user connections. The fundamental access mechanism in the IEEE 802.11 MAC protocol is the Distributed Coordination Function (DCF). The throughput of LANs with the DCF mechanism under high load and in noise is evaluated by an analytical method, which ensures high estimation accuracy for any values of protocol parameters and bit error rates.     

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。     

Wireless multimedia delivery over 802.11e with cross-layer optimization techniques

The use of wireless networks has spread further than simple data transfer to delay sensitive and loss tolerant multimedia applications. Over the past few years, wireless multimedia transmission across Wireless Local area Networks (WLANs) has gained a lot of attention because of the introduction of technologies such as Bluetooth, IEEE 802.11, 3G, and WiMAX. The IEEE 802.11 WLAN has become a dominating technology due to its low cost and ease of implementation. But, transmitting video over WLANs in real time remains a challenge because it imposes strong demands on video codec, the underlying network, and the Media Access Control (MAC) layer. This paper presents a cross-layer mapping algorithm to improve the quality of transmission of H.264 (a recently-developed video coding standard of the ITU-T Video Coding Experts Group) video stream over IEEE 802.11e-based wireless networks. The major goals of H.264 standard were on improving the rate distortion and the enhanced compression performance. Our proposed cross-layer design involves the mapping of H.264 video slices (packets) to appropriate access categories of IEEE 802.11e according to their information significance. We evaluate the performance of our proposed cross-layer design and the results obtained demonstrate its effectiveness in exploiting characteristics of the MAC and application layers to improve the video transmission quality.     

IPv6 based relay gateway protocol in hybrid networks

During the past few years, WMAN (Wireless Metropolitan Area Network) and WLAN (Wireless Local Area Network) provided fast, convenient and reliable solution for all kinds of users. With the development of the communication technology, the future wireless networks will allow mobile users access Internet anywhere, anytime. Therefore, it is a natural trend to combine different wireless networks to provide a seamless roaming for users. It is feasible for mobile terminal which have multiple modes to handoff between different networks. A relay gateway protocol for the WLAN-mode nodes (mono-mode WLAN nodes and dual-mode nodes) in the hybrid network of IEEE 802.16 WMANs and IEEE 802.11 WLANs is proposed to provide wider mobility. Dual-mode mobile nodes with good service can provide WLAN links to nearby WLAN-mode nodes with bad service or out of the WLAN coverage. Simulation results show that the relay gateway scheme can improve network performance of the hybrid networks. Moreover, the relay gateway method can bring additional service coverage and increase the capacity of WMANs and WLANs.