摘 要

近年来，随着无线网络的飞速发展，无线局域网应用遍及各个领域，在人们的生活中处处都有。现在有很多地方，比如咖啡馆，电影院，酒店，商场都能够为人们提供免费使用的无线上网服务，人们可以随时随地访问无线局域网进行数据通信和网络通信，给人们的生活带来极大的方便。

作为有线局域网，WLAN中的站点是通过共享信道资源来实现通信的，存在争用信道的情况。在IEEE 802.11局域网中，信道如何共享资源，站点发送和接收数据的时间由MAC层协议协调功能决定。MAC层协议是802.11系列标准中非常重要的协议之一，分布式协调功能是MAC层协议的主要功能。信道资源分配的分布式协调功能不提供集中控制，而是在每个站点采用分布式接入的载波侦听多址接入算法，即CSMA/CA协议退避算法。这样，每个站点只通过执行退避算法获得信道资源和通信权。

本文重点介绍CSMA/CA协议，并改进CSMA/CA退避算法的不足之处。为了便于理解CSMA/CA协议与网络性能之间的关系，本文引用了目前广泛使用的CSMA/CA协议理论分析方法：Bianchi提出的二维马尔可夫模型分析方法。在该模型中，针对每个网络饱和吞吐量和网络推导出站点状态和转移关系表达式，然后对该分析的结果进行深入分析。

关键词：MAC协议；CSMA/CA协议；退避算法

Abstract

In recent years, with the rapid development of wireless networks, wireless LAN applications throughout various fields, in people's lives everywhere. Now there are lots of places, such as cafes, cinemas, hotels, shopping malls are able to provide people with free use of wireless Internet service, people can access the wireless local area network for data communication and network communication anytime, anywhere, as brought great convenience to people's life.

As wired LAN, the site in WLAN is by sharing channel resources to achieve the communication, there are cases where contention channel. In the IEEE 802.11 LAN, the channel how to share resources, the time of site sends and receives data by the MAC layer protocol coordination function determined. The MAC layer protocol is one of very important protocol in the 802.11 series standard, and the distributed coordination function is the main function of the MAC layer protocol. Distributed Coordination Function for channel resources allocation does not provide a centralized control, but in each site, using the algorithm of carrier sense multiple access of distributed access, namely CSMA/CA protocol backoff algorithm. In this way, each site only to obtain channel resources and the right to communicate by performing backoff algorithm.

This paper focuses on CSMA/CA protocol and to improve the CSMA/CA backoff algorithm deficiencies. In order to facilitate understanding of the links between the CSMA / CA protocol and network performance from theory, the article cites a widely used CSMNCA protocols theory analytical method at present: a two-dimensional Markov model analysis method put forward by Bianchi. In this model, the site status and metastasis relational expression is derived for each network saturation throughput and network nd then to the results of this analysis do in-depth analysis.

KeyWords: MAC Protocol; CSM/CA Protocol; Backoff Algorith

目 录

摘 要 I

Abstract II

Chapter 1 Introduction 1

1.1 Topic Selection Background and Research Significance 1

1.2 Wireless LAN Development and Research Status 3

1.3 Dissertation research content and chapter arrangement 4

Chapter 2 Protocol of The IEEE802.11 5

Chapter 3 Two Communication Methods in the CSMA/CA Protocol 10

3.1 Basic Communication Method 10

3.2 RTS/CTS Communication Mode 12

Chapter 4 CSMA/CA Protocol Performance Analysis 18

4.1 CSMA/CA Protocol Performance Specifications 18

4.1.1 Throughput 18

4.1.2 age delay 18

4.2 Performance Analysis of CSMA/CA Protocol 19

4.2.1 Model Establishment 19

4.2.2 Performance Analysis of the CSMA/CA Protocol 20

In conclusion 22

References 23

Acknowledgements 24

Chapter 1 Introduction

1.1 Topic Selection Background and Research Significance

In recent years, with the advancement of science and technology and the rapid development of computer communication networks, network users have been

There is a higher demand for convenience and mobility, so people have a very urgent need for mobile networks.It promotes the rapid development of wireless networks.

Wireless Local Area Network (WLAN) has many advantages: First, the use of the low cost, simple and easy maintenance, high maintenance costs, and freedom from cable restrictions relative to wired networks Reduces installation complexity and has a good economic advantage. Secondly, the wireless LAN has a low requirement for the environment of the installation networking. It is not limited by space and time. The networking and access methods are flexible. Users can install the WLAN in any environment and can be within a certain area. The use of the network resources anytime, anywhere, which to a large extent make up for the lack of wired networks, making wireless LAN has a very strong application areas and scope. Second, because the configuration of the wireless LAN is simple and flexible, the wireless LAN can be easily extended and roaming can also be achieved^[1].

The wireless access function of the wireless LAN facilitates the connection of the portable terminal to the network, making the mobile device The scope of application is also constantly expanding and has become an important tool for people's daily lives. For example In addition to basic communications, computers, tablets, etc. can also be used for online shopping, watching videos, Handling emails, etc. Now people’s lives have become a “wireless life” throughout the wireless network.

There are many places, such as cinemas, stations, restaurants, hotels, shopping malls, etc.Fees use Wi-Fi (Wireless-Fidelity) service, people can connect anytime anywhere Into the wireless LAN for data communication and network communication. Wireless LAN Development and Popularization for People's Lives

Bring great convenience, wireless LAN WLAN plays an indispensable corner in modern communication networks color.

Although wireless LANs solve the difficulties that wired networks cannot overcome and have many advantages, they still have one.Some deficiencies. For example: Wireless LANs rely on radio waves for transmission. When they encounter obstacles in the propagation process, they will affect network performance. In addition, wireless local area networks (WLANs) currently exhibit the disadvantages of insufficient adaptability to heavy load networks and low site access fairness. Therefore, as people's demands for networks become higher and higher, solving some of the existing problems in wireless local area networks has great significance for the future development of wireless local area networks. How to improve the WLAN's lack of adaptability to heavy-load networks and low site access fairness has become an important research direction^[2].

Wireless LAN, like wired LAN, is a kind of network that realizes communication through shared channels.There is also a case of contention for channels. In the IEEE802.11 LAN, the media access control layer MAC (Medium Access Control) layer protocol (Xie Xiren, 2008) is an important part of the 802.1 1 series of standards

One of the protocols, and the most important and indispensable function in the MAC protocol is the distribution coordination function. DCF (Distributed Coordination Function) 0 The DCF protocol establishes a network of sites through contention letters.The mechanism for obtaining transmission rights, the performance of the mechanism plays a decisive role in the level of network throughput. effect. Therefore, understanding the working principle of the MAC layer protocol does not improve the adaptability of the wireless LAN to the heavy load network.Enough, the disadvantage of low site access fairness is of great significance.

This article discusses wireless LANs based on the 802.11 series of standards and focuses on such networks.The CSMA/CA (Carrier Sense Multiple) used in the DCF sublayer of the MAC layer Access/Col1ision Avoidance) protocol. The purpose of this study is to address the CSMA/CA protocol backoff The method of improving the CSMA/CA protocol backoff algorithm with the disadvantages of insufficient adaptability and low fairness when the network is heavily loaded has the following implications: Overall,

(1) Improve the problem of unfair competition of channel resources at sites in heavily loaded networks.

(2) Improve the competition window oscillation problem.

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