Modeling finite buffer effects on TCP traffic over an IEEE 802.11 infrastructure WLAN

The network scenario is that of an infrastructure IEEE 802.11 WLAN with a single AP with which several stations (STAs) are associated. The AP has a finite size buffer for storing packets. In this scenario, we consider TCP-controlled upload and download file transfers between the STAs and a server on the wireline LAN (e.g., 100 Mbps Ethernet) to which the AP is connected. In such a situation, it is well known that because of packet losses due to finite buffers at the AP, upload file transfers obtain larger throughputs than download transfers. We provide an analytical model for estimating the upload and download throughputs as a function of the buffer size at the AP. We provide models for the undelayed and delayed ACK cases for a TCP that performs loss recovery only by timeout, and also for TCP Reno. The models are validated in comparison with NS2 simulations.     

太赫兹技术在室内无线通信中的应用

该文通过对太赫兹通信相关器件的总结和相关信道特性的分析,调研了近年来国内外相关实验的结果,对重点实验结果做了着重描述,总结出了未来室内无线通信可能的发展方向和亟待解决的问题。太赫兹波段是介于毫米波与远红外的电磁辐射频段。室内无线局域网的高数据率传输要求通信系统运行于更高的频率以获得更大的带宽。太赫兹频率范围未受军事管制,可实现Gbit/s数据率,并且太赫兹频域不像光频受环境噪声干扰那么严重。全固态室温太赫兹器件的研究为太赫兹无线通信系统奠定了基础。已探明适于未来太赫兹室内通信系统运行的大气窗口。基于飞秒激光门控光导天线的太赫兹通信链路实现了太赫兹波载波音频信号的调制和解调。     

3G用户在WLANs间的快速认证方法

EAP-AKA协议可以实现用户和3G网络的双向认证,但缺乏对无线局域网(WLAN)接入网络的认证和用户主密钥的更新机制,并且3G用户在WLAN间漫游时代价较高。针对上述问题,利用身份加密技术和门票技术改进EAP-AKA协议,设计无需3G网络参与的WLAN间快速漫游协议,实现对WLAN接入网络的认证和主密钥更新。     

Exploiting the channel using uninterrupted collision‐free MAC adaptation over IEEE 802.11 WLANs

IEEE 802.11 wireless local area networks (WLANs) have reached an important stage and become a common technology for wireless access due to its low cost, ease of deployment, and mobility support. In parallel with the extensive growth of WLANs, the development of an efficient medium access control protocol that provides both high throughput performance for data traffic and quality of service support for real‐time applications has become a major focus in WLAN research. The IEEE 802.11 Distributed Coordination Functions (DCF/EDCA) provide contention‐based distributed channel access mechanisms for stations to share the wireless medium. However, performance of these mechanisms may drop dramatically because of high collision probabilities as the number of active stations increases. In this paper, we propose an adaptive collision‐free MAC adaptation. The proposed scheme prevents collisions and allows stations to enter the collision‐free state regardless of the traffic load (saturated or unsaturated) and the number of stations on the medium. Simulation results show that the proposed scheme dramatically enhances the overall throughput and supports quality of service for real‐time services over 802.11‐based WLANs. Copyright © 2013 John Wiley & Sons, Ltd.     

Performance analysis of differentiated QoS in IEEE 802.11e WLANs

This paper proposes a multi‐dimensional Markov model to analyse the performance of the IEEE 802.11e EDCF MAC protocol. Based on this model, we present extensive performance evaluation in terms of throughput, throughput ratios, and access delay of flows of distinct priorities under RTS/CTS mode. We also provide quantitative analysis of the impact of prioritized parameters, i.e. Arbitration InterFrame Space (AIFS), Contention Window (CW) on Quality of Service (QoS) differentiation. The accuracy of the proposed model is verified by means of comparing the numerical results obtained from both analytical model and simulations. Our research can be used as a guideline for the prediction of how flows belonging to a certain Traffic Category (TC) perform with their TC‐specific parameters, as well as designing EDCF‐based WLANs and tuning the parameters to achieve the desirable differentiated QoS objectives. Copyright © 2005 John Wiley & Sons, Ltd.     

Cross‐Layer Resource Allocation Scheme for WLANs with Multipacket Reception

Tailored for wireless local area networks, the present paper proposes a cross‐layer resource allocation scheme for multiple‐input multiple‐output orthogonal frequency‐division multiplexing systems. Our cross‐layer resource allocation scheme consists of three stages. Firstly, the condition of sharing the subchannel by more than one user is studied. Secondly, the subchannel allocation policy which depends on the data packets’ lengths and the admissible combination of users per subchannel is proposed. Finally, the bits and corresponding power are allocated to users based on a greedy algorithm and the data packets’ lengths. The analysis and simulation results demonstrate that our proposed scheme not only achieves significant improvement in system throughput and average packet delay compared with conventional schemes but also has low computational complexity.     

无线局域网的速率自适应算法设计与仿真分析

目前的IEEE802.11在物理层提供了不同的传输速率,因此根据不同的信道条件可以选择一个合适的传输速率,以使系统吞吐量达到最大。在WLANs中,当有多个用户向同一个接入点传输数据时,碰撞就会产生。Collision-Aware Rate Adaptation（CARA）算法能够有选择地启用Request-To-Send/Clear-To-Send（RTS/CTS）识别出这种碰撞,很大程度上避免了由于碰撞引起的吞吐量下降。但随着碰撞的增加,CARA吞吐量还是有明显下降。为了提高吞吐量,提出了一种新的速率自适应算法Adaptive Avoid Collision Rate Adaptation（Adaptive-ACRA）,该算法根据信道条件不同改变门限值,有效地提高了大量碰撞存在情况下的吞吐量。最后,通过大量NS2仿真证明了新算法提高系统吞吐量的有效性。     

Resource allocation for cognitive WLAN over fiber

In this paper,a novel WLAN system,Cognitive WLAN over Fiber (CWLANoF),is introduced in the first place.Moreover,when CWLANoF has more channels than STAs,a new channel allocation scheme is proposed using the Hungarian algorithm,which is demonstrated to be the optimal one.Furthermore,when CWLANoF has fewer channels than STAs,it is possible for more than one STA to share the same channel simultaneously based on the new features of CWLANoF.And the power control scheme is proposed for this kind of sharing,considering efficiency and fairness.Finally,extensive simulation results illustrate the significant performance improvement of the proposed channel allocation scheme and power control scheme.     

TCP performance optimization in multi-cell WLANs

Though significant attention has been given to understanding the performance of a single-cell WLAN, performance evaluation of a group of interfering basic service sets (BSSs) within an extended service set (ESS) is still an open area. In this paper, we first demonstrate that a severe throughput imbalance occurs between downlink TCP flows even in the simplest of multi-cell WLANs via simulation and real world experiments; then, to solve this unfairness problem, we derive an analytical model that describes the interaction between TCP flows at the MAC layer, and formulate a throughput allocation problem as a nonlinear optimization problem subject to certain fairness requirements. Our formulation considers real world complexity such as hidden terminals, packet transmission retry limit, and the unique characteristics of TCP traffic. Solving our optimization problem yields the optimal MAC layer contention window settings that can lead each TCP flow to its target end-to-end throughput without the need for any per-flow queuing nor modification of the TCP sender. Simulation results show that our approach can achieve a fair allocation on the end-to-end throughput and attest to the accuracy of our proposed method.