无线局域网中的接入点AP负载分配研究

基于IEEE802．11的无线局域网结点工作在基础设施模式时,通常选择收到的信号强度（RSSI）最大的AP建立连接,但这种方法会造成系统资源的浪费和性能的下降,而目前的解决方法要么没有考虑结点的真实负载大小,要么增大结点的切换次数,对系统性能的改善并不是很理想。对此,文章提出了一种基于RSSI、AP当前负载和带宽大小等因素的负载分配方法,仿真试验结果表明,该方法在整体性能上要优于其它方法。     

基于AP预先转发的802.11无线局域网切换机制研究

IEEE802．11无线局域网(WLAN)规范包括MAC子层和物理层(PHY)两个协议层，支持结点的移动通信，但它并没有提供移动主机(MH)的无缝切换算法．当MH从当前的接入点(AP)覆盖小区移动到另一个AP小区时，由于通信的暂时中断会对系统的性能造成一定的影响．目前的切换算法包括硬切换(hard-handoff)和平滑切换(smooth-handoff)等，这些算法要么丢包率大，要么切换延迟长，对切换的性能改善程度并不是很高．从切换延迟和丢包率两方面人手，提出了一种AP预先转发的WLAN切换算法，并对其进行了仿真实验研究，实验结果表明，该切换算法的整体性能优于其他算法．     

共享WiFi系统中MAC层快速切换方案的研究与实现

基于IEEE 802.11协议的无线局域网(WLAN)能够为无线多媒体应用提供足够的带宽,由于无线接入点AP(Access Point)发射功率受限,其服务覆盖范围也因此受限,这使得终端在移动过程中需要在不同AP之间频繁地切换,由此带来的时延严重影响了无线多媒体应用的服务质量。因此,提出一种基于信道掩码和优先级的选择扫描策略和选择性邻居图缓存算法,增大了命中缓存的概率,减少了扫描的信道数量和系统信令开销。仿真结果表明,该切换方案可以有效降低移动节点MN(Mobile Node)在共享WiFi(Wireless Fidelity)系统中不同AP间的切换时延,满足了无线多媒体业务(如VoIP)的传输要求。     

IEEE 802.11e无线局域网中VoIP容量分析及改进

IEEE 802.11e提供了对不同的业务流进行优先级区分的机制,提高了无线局域网（WLAN）内VoIP等实时业务相对于其他业务流的优先性,从而在一定程度上保障其QoS要求,但并没有对WLAN中移动节点（MH）和接入节点（AP）进行优先级的区分,导致AP处上下行业务流量的不平衡,并造成瓶颈效应,限制了VoIP容量。针对这一问题,在对WLAN中VoIP进行容量分析的基础上,提出一种改进AP信道接入能力的策略。通过对AP中队列长度的检测来判断网络拥塞程度,从而动态调整AP的仲裁帧间间隔（AIFS）,以达到根据网络拥塞状况来动态调整AP的优先级,弱化AP的瓶颈效应对VoIP容量制约的目的。仿真结果表明,所提出的改进方案能够有效地提高WLAN中的VoIP容量。     

基于分离式MAC架构的WLAN漫游认证机制研究

根据分离式MAC架构WLAN的结构特点,提出了一种新的漫游切换认证方案,减少了切换时无线用户的重新认证时间,满足了VOIP等实时业务的需要。     

面向机器人场景的带宽保证快速切换技术

802.11规定的切换过程十分耗时,同时移动终端根据接收到的信号强度来选择AP往往不能满足通信带宽的需求,这两点对于需要依靠不间断通信来进行作业的工业机器人十分不利.本文面向工业机器人场景提出一种基于动态无线地图的带宽保证快速切换技术.利用预先建立和下载的无线地图,机器人不需扫描信道即可获得附近可用的AP基站,同时利用...     

提高WLAN语音传输容量的MAC协议研究

在传统的无线局域网(Wireless Local Area Network,WLAN)环境下,VoIP(Voice over Internet Protocol)业务的传输容量严重受限,大大限制了无线VoIP的发展.通过分析与仿真,发现接入点(Access Point,AP)为限制无线局域网中语音传输容量的瓶颈,从而有针对性地提出了动态调整AP竞争信道优先级的方法.该方法在AP队列过长时提高AP发送数据的优先级,简单易行,并可有效缓解AP的瓶颈效应.多场景下的仿真结果表明,无线局域网中语音传输容量可提高20%,而信道接入延时可降低60%左右.     

RSVP协议在移动IPv6网络中的扩展性研究

针对无线移动通信的特点,提出了一种在移动IPv6网络中保障用户通信服务质量的资源预留新方案Fast RSVP.该方案采用跨层设计的思想,将两个不同层次的模块:移动IPv6模块和RSVP模块结合起来,通过在两个模块之间增加一些原语使得二者配合工作以保证移动用户的通信业务质量.Fast RSVP方案引入了邻居隧道提前资源预留、优化路径资源预留、切换预留、路径融合等一系列新机制.仿真实验结果表明,与其他移动环境中的RSVP扩展方案相比,该Fast RSVP方案在支持无线移动通信方面具有如下优势:(1)能够实现移动节点带有服务质量保证的快速切换;(2)能够避免移动IP切换过程中三角路由和重复预留造成的资源浪费;(3)能够区分不同类型的切换预留请求,在保证网络整体性能的前提下显著降低因为切换而导致的服务中断率.     

无线多媒体网络中一种基于测量网络状态的动态呼叫接纳控制算法

提出了一种无线多媒体网络中基于测量网络状态的动态呼叫接纳控制算法.它区分了实时和非实时业务,在网络带宽资源不足时可通过降低非实时业务带宽确保实时业务呼叫连接的可靠性;还可根据当前网络状况调整预留带宽大小,使小区实时业务切换呼叫掉线率低于设定的门限值.大量仿真结果显示该算法具有低实时业务切换呼叫掉线率和与固定预留方案相当的带宽利用率,而只以略高的新呼叫阻塞率为代价,适合各种不同概率发生时实际应用的情况.     

WLAN上跑语音——谈VoIP基于无线局域网的应用

虽然 WLAN(无线局域网)由于安全标准的不确定性,严重阻碍了其发展的步伐,但利用基于 WLAN 的 VoIP 来代替或补充目前现有的电话系统,如通过笔记本电脑、PDA 或者新型的 Wi-Fi 电话与 WLAN 语音服务器连接,将成为了促使 WLAN 广泛应用的亮点之一, 换占之,无线网络上跑语音即将成为现实。作为一种成熟的网络通信技术,VoIP 在有线网络已逐渐开始广泛应用,而基于 WLAN 的 VoIP 技术以与之相补充显示了其市场潜力。虽然目前在无线网络上实现 VoIP 还面临着无线吞吐量的波动、热点间漫游、Wi- Fi 电话终端缺乏等问题的困扰,但基于 WLAN 的 VoIP 应用代表了市场发展的趋势。可软硬件实现基于 WLAN 的 VoIP 实现可通过软硬来实现,其工作方式分为两个部分。一是基于 SpectraLink、Symbol Technologies Inc．Cisco 等公司的     

全IP移动网络中基于信令预测优先级排队的动态资源预留排序

无线移动网络中的QoS是最近的研究热点之一,其中资源预留协议(ResourceReserVationProtocol,RSVP)是一个关键技术.为了提供更多的服务,蜂窝在变小,这样切换更频繁了.因此为了保证切换时的QoS,需要进行资源预留.当有多个移动主机需要进行预留时,就需要对它们进行优先级排序.本文提出一种在全IP移动网络中基于信令预测优先级排队的动态排序算法,以有效地对多个移动主机进行资源预留.仿真结果表明本文提出的算法既保证了快速切换,又实现了资源的有效预留,有效地分配了有限的资源.     

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。     

Predictive channel reservation for handoff prioritization in wireless cellular networks

In wireless cellular networks, a roaming mobile station (MS) is expected to move from one cell to another. In order to ensure that ongoing calls are not dropped while the owner mobile stations roam among cells, handoff calls may be admitted with a higher priority than newly generated calls. Predictive channel reservation (or pre-reservation) schemes allow the reservation of a channel for an ongoing call in an adjacent cell before its owner MS moves into that cell, so that the call is sustained when the MS moves into the adjacent cell. Pre-reservations are made when the MS is within some distance of the new cell boundary. This distance determines the area in which the MS can make channel reservations. In this paper, we study the effect of the pre-reservation area size on handoff performance in wireless cellular networks. Our studies show that if the reserved channels are strictly mapped to the MSs that made the corresponding reservations, as we increase the pre-reservation area size, the system performance (in terms of the probability that the handoff calls are dropped) improves initially. However, beyond a certain point, the performance begins to degrade due to a large number of false reservations. The optimal pre-reservation area size is closely related to the traffic load of the network and the MSs’ mobility pattern (moving speed). We provide an analytical formulation that furthers understanding with regard to the perceived behavior in one-dimensional networks (in which all cells are along a line).With the objective of improving handoff performance and alleviating this sensitivity to the area size, we propose an adaptive channel pre-reservation scheme. Unlike prior pre-reservation methods, the key idea in our scheme is to send a channel pre-reservation request for a possible handoff call to a neighboring cell not only based on the position and orientation of the owner MS, but also as per its speed towards the target cell. The newly proposed scheme uses GPS measurements to determine when channel pre-reservation requests are to be made. Simulation results show that the adaptive channel pre-reservation scheme performs better in all typical scenarios than a previously proposed popular pre-reservation method that does not take mobility into account.     

Mobility-based predictive call admission control and bandwidth reservation in wireless cellular networks

This paper presents call admission control and bandwidth reservation schemes in wireless cellular networks that have been developed based on assumptions more realistic than existing proposals. In order to guarantee the handoff dropping probability, we propose to statistically predict user mobility based on the mobility history of users. Our mobility prediction scheme is motivated by computational learning theory, which has shown that prediction is synonymous with data compression. We derive our mobility prediction scheme from data compression techniques that are both theoretically optimal and good in practice. In order to utilize resource more efficiently, we predict not only the cell to which the mobile will handoff but also when the handoff will occur. Based on the mobility prediction, bandwidth is reserved to guarantee some target handoff dropping probability. We also adaptively control the admission threshold to achieve a better balance between guaranteeing handoff dropping probability and maximizing resource utilization. Simulation results show that the proposed schemes meet our design goals and outperform the static-reservation and cell-reservation schemes.     

Dynamically adaptive channel reservation scheme for cellular networks

In personal communications networks (PCN) supporting network-wide handoffs, new and handoff requests compete for connection resources in both mobile and backbone networks. Forced call terminations due to handoff call blocking are generally more objectionable than new call blocking. In general, most of the previously proposed schemes for radio channel allocation in cellular networks reduce handoff call blocking probability substantially at the expense of increasing the new call blocking probability by giving higher priority to handoff calls over new calls in admission control. This reduces the total admitted traffic and results in inefficient utilization of wireless channels. The tradeoff between the new and handoff calls blocking probabilities should be defined on importance basis. In this paper, we propose a performance metric equation that makes a trade off between the two probabilities depending on the network preferences. Using this equation, we study the performance of various proposed channel reservation schemes. Also in this paper, a new dynamically adaptive channel reservation scheme (DACRS) is developed and compared with other schemes proposed in the literature. The DACRS assigns handoff-reserved channels to new calls depending on the locality principle in which the base station with the help of location estimation algorithms in the mobile location center predicts the position of the mobile terminal. Eventually, the DACRS is designed to improve channel utilization while satisfying the QoS of the calls. As will be shown analytically and through simulation, the DACRS outperforms current reservation schemes and results in more statistical gain, and powerful channel utilization.     

一种基于无线局域网的移动IP快速切换方案

为了使移动节点在无线局域网环境中进行快速有效的切换，我们提出了一种基于管理域的快速切换方案。该方案将多个相邻的无线局域网组成一个单独的域，采用层次管理方式，使域内各个子网通过专用的Mac桥连接，减少了移动节点域内切换过程中产生的注册时延和数据包的丢失。     

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.