电信基础论文 - 关于WLAN的简介中英文互译版 - Talking - About

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浅谈无线局域网技术： 802.11 x

Xu Shumin

国际学校，类 2009215102 09212759

北京邮电大学

Email: jp092759@bupt.edu.cn

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摘要：

在刚刚过去的几年，无线局域网来占据重要生态位在局域网络市场。越来越多，各组织都发现无线局域网不可或缺的兼职到传统的有线局域网，以满足移动、搬迁，ad hoc 网络和覆盖难线的位置的要求。

关键词：无线局域网；IEEE 802.11 ；电信

1、引言

顾名思义，无线局域网（W局域网）是一个让使用的一种无线传输媒介。它是非常方便的数据传输系统，以取代传统的同轴电缆，形成本地局域网 (局域网) 一直采用射频（RF），使得无线局域网使用简单访问体系结构允许用户通过它。

2.无线局域网技术

根据所使用的传输技术无线局域网是一般分类。所有当前的无线局域网产品分为以下几类（表 1）。

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2.1 红外线 (IR) 局域网：

因为红外灯没有穿透不透明的墙，限于一间单人房，红外局域网的单个单元格。

2.1.1 长处和弱点光谱红外的是无限的其中介绍了实现极高的数据速率的可能性。此外，红外共享的可见光一些吸引力的局域网配置某些类型的属性。该设备是比较便宜、简单的红外的另一种力量。不过，红外媒体还展示了一些缺陷。许多室内环境经验相当激烈的红外辐射，从阳光和室内照明。 2.1.2 传输技术有是通常用于红外数据传输的三个备选传输技术：传输的信号可以集中和目的（例如在电视遥控）；它可以被辐射舒缓；或者，它可以反映在光色的天花板。红外定向梁可用于创建点对点链接。在此模式中，范围取决于发射功率和聚焦的程度。全方位的配置涉及单一的基站内的局域网上的所有其他地点的视线。在弥漫的配置中，所有的红外发射器是集中，并旨在漫反射的天花板上的一个点。 2.2 扩频局域网：

这种类型的局域网的扩频传输技术的利用。在大多数情况下，这些局域网运行 ISM （工业、科学和医疗）频带以便不需要联邦通信委员会 (FCC) 许可是他们在美国的使用需要。

2.2.1 配置除了很小的办事处，使无线局域网的扩频使用的多个单元格的安排（表 2）。相邻的单元格进行同一组别内不同的中心频率的使用，以避免干扰。

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在给定的单元格中，拓扑结构可以是集线器或点对点。在集线器的拓扑中，集线器是通常安装在天花板上，连接到有线局域网提供连接到连接有线局域网和无线局域网中的其他单元格的一部分的站站的骨干。端到端拓扑是没有枢纽之一。如 CSMA MAC 算法用来控制访问。此拓扑是适当的特设的局域网。 2.2.2 传输问题的理想，虽然不必要、无线局域网的特色是它而不必透过发牌程序将可用。

2.3 窄带微波：这些局域网微波频率下运行，但不是使用扩频。这些产品的一些工作需要催化裂化许可，而其他人使用一个无牌的 ISM 频段的频率。 2.3.1 持牌可用的语音、数据和视频传输的窄带射频微波无线电频率颁发执照及在特定的地理区域，以避免潜在的干扰系统之间的协调。好处是它保证无干扰的通信。

2.3.2 无牌窄带第一供应商介绍窄带射频无线局域网中使用的无牌的 ISM 谱是无线局域网。它利用的端对端的配置和不稳定。 3.IEEE 802.11 的体系结构和服务

3.1 IEEE 802.11 建筑最小的构建基块的 W局域网是基本服务集 (BSS)，其中包括一定数量的站执行相同的 MAC 协议和访问同一争夺共享无线媒体。BSS 可能孤立，或者它可能连接到骨干分布系统 (DS) 通过接入点（AP）。MAC 协议可能完全分发，或由设在非典型肺炎的中央协调功能控制。BSS 通常对应于什么被称为文学中的单元格。DS 可以开关、有线的网络或无线网络。最简单的配置如图 3 所示。

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3.2 IEEE 802.11 服务 IEEE 802.11 定义了九个服务，需要提供无线局域网功能提供相同的固有的有线局域网（表 4）。

服务提供商可以站或 DS。三服务用来控制 IEEE 802.11 局域网访问和保密。六服务用于支持的 MAC 服务数据单元 (MSDUs)，车站之间传递。 4.IEEE 802.11 介质访问控制

4.1 可靠数据传递 IEEE 802.11 包括框架交换协议解决数据传输的可靠性。当一个工作站接收从另一站点的数据帧时，它会返回到源站的 ACK 框架。如果

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源短的时间内没有收到确认，源重新传输的帧。进一步提高可靠性，源首先发出请求发送 (RTS) 框架上的目标，然后清除发送 (CTS) 以响应的目标。CTS 后，源传输的数据帧，然后确认

4.2 MAC 帧 MAC 帧类型目标响应：控制框架协助提供可靠的数据帧。数据帧：有八个的数据帧子类型，分成两组。前四个定义进行高级别的数据源站到目的地车站的帧。其余四个实际上不执行任何用户数据。管理框架用来管理站和接入点之间的通信。覆盖的职能包括管理协会（请求、响应，再结合、分离和身份验证）。

4.3 访问控制 802.11 工作组审议了两种类型的 MAC 算法的建议：分布式访问协议，其中，以太网，如分发传输载波侦听机制；使用的所有节点上的决定集中式访问协议，涉及传输集中的决策者的规管。802.11 的最终结果是一种称为 DFWMAC 的 MAC 算法（分布式的基础无线 MAC），提供可选的集中控制，最重要的是建立了一种分布式的访问控制机制。图 5 说明了体系结构。

5.IEEE 802.11 物理层

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5.1 原始 IEEE 802.11 物理层三物理介质中原始的 802.11 标准定义：直接序列扩频通信在 2.4 GHz ISM 频段，1 Mbps 的数据传输速率在经营 2 Mbps。跳频扩频在 2.4 GHz ISM 波段操作在 1 Mbps 和 2 Mbps。红 1 Mbps，操作之间 850 和 950 nm 波长的 2 Mbps 的数据传输速率。

5.2 IEEE 802.11a IEEE 802.11a 规格，5 GHz 频段的利用。

5.3 IEEE 802.11b IEEE 802.11b 是 IEEE 802.11 扩计划，扩展提供数据速率的 5.5 和 11 Mbps。

5.4 IEEE 802.11g IEEE 802.11g 是 IEEE 802.11b、相结合的各种编码技术的物理层的高速度扩展。

? 参考文献

1. http://baike.http://www.wodefanwen.com//view/32856.htm 2. http://ossie.wireless.vt.edu/

3. Data and Computer Communications, Higher Education Press, Seventh Edition, P543 - P568 Chapter 17.

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Talking About WLAN technology：802.11x

Xu Shumin

International School, Class 2009215102, 09212759 Beijing University of Post and Telecommunication

Email: jp092759@bupt.edu.cn

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? Abstract

In just the past few years, wireless LANs have come to occupy a significant niche in the local area network market. Increasingly, organizations are finding that wireless LANs are an indispensable adjunct to traditional wired LANs, to satisfy requirements for mobility, relocation, ad hoc networking, and coverage of locations difficult to wire.

? Key Words: WLAN; IEEE 802.11; Telecommunication ? 1. Introduction

As the name suggests, a Wireless Local Area Network (WLAN) is one that makes use of a wireless transmission medium. It is quite convenient data transmission system, which has been using Radio Frequency (RF) to replace the traditional Coaxial that form the Local Area Network (LAN), makes the WLAN use simple access architecture allows users to pass through it.

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? 2. Wireless LAN Technology

Wireless LANs are generally categorized according to the transmission technique that is used. All current wireless LAN products fall into one of the following categories (Table 1).

? 2.1 Infrared (IR) LANs: An individual cell of an IR LAN is limited to a single

room, because infrared light does not penetrate opaque walls. ? 2.1.1 Strengths and Weakness

The spectrum for infrared is virtually unlimited, which presents the possibility of achieving extremely high data rates. In addition, infrared shares some properties of visible light that make it attractive for certain types of LAN configurations. Another strength of infrared is that the equipment is relatively inexpensive and simple.

However, the infrared medium also exhibits some drawbacks. Many indoor environments experience rather intense infrared background radiation, from sunlight and indoor lighting. ? 2.1.2 Transmission Techniques

There are three alternative transmission techniques commonly used for IR data transmission: the transmitted signal can be focused and aimed (as in a remote TV control); it can be radiated omnidirectionally; or it can be reflected from a light colored ceiling.

Directed-beam IR can be used to create point-to-point links. In this mode, the range depends on the emitted power and on the degree of focusing.

An omnidirectional configuration involves a single base station that is within line of sight of all other stations on the LAN.

In a diffused configuration, all of the IR transmitters are focused and aimed at a point on a diffusely reflecting ceiling.

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? 2.2 Spread spectrum LANs: This type of LAN makes use of spread spectrum

transmission technology. In most cases, these LANs operate in the ISM (industrial, scientific, and medical) bands so that no Federal Communications Commission (FCC) licensing is required for their use in the United States.

? 2.2.1 Configuration

Except for quite small offices, a spread spectrum wireless LAN makes use of a multiple-cell arrangement (Table 2). Adjacent cells make use of different center frequencies within the same band to avoid interference.

Within a given cell, the topology can be either hub or peer to peer. In a hub topology, the hub is typically mounted on the ceiling and connected to a backbone wired LAN to provide connectivity to stations attached to the wired LAN and to stations that are part of wireless LANs in other cells. A peer-to-peer topology is one in which there is no hub. A MAC algorithm such as CSMA is used to control access. This topology is appropriate for ad hoc LANs. ? 2.2.2 Transmission Issues

A desirable, though not necessary, characteristic of a wireless LAN is that it be usable without having to go through a licensing procedure.

? 2.3 Narrowband microwave: These LANs operate at microwave frequencies but

do not use spread spectrum. Some of these products operate at frequencies that require FCC licensing, while others use one of the unlicensed ISM bands. ? 2.3.1 Licensed Narrowband RF

Microwave radio frequencies usable for voice, data, and video transmission are licensed and coordinated within specific geographic areas to avoid potential interference between systems. The advantage is that it guarantees interference-free communication.

? 2.3.2 Unlicensed Narrowband RF

The first vendor to introduce a narrowband wireless LAN using the unlicensed ISM spectrum is the Radio LAN. It makes use of a peer-to-peer configuration and it is unstable.

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? 3. IEEE 802.11 Architecture and Service

? 3.1 IEEE 802.11 Architecture

The smallest building block of a WLAN is a basic service set (BSS), which consist of some number of stations executing the same MAC protocol and competing for access to the same shared wireless medium. A BSS may be isolated or it may connect to a backbone distribution system (DS) through an access point (AP). The MAC protocol may be fully distributed or controlled by a central coordination function housed in the AP. The BSS generally corresponds to what is referred to as a cell in the literature. The DS can be a switch, a wired network, or a wireless network. The simplest configuration is shown in Figure 3.

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? 3.2 IEEE 802.11 Services

IEEE 802.11 defines nine services that need to be provided by the wireless LAN to provide functionally equivalent to that which is inherent to wired LANs (Table 4).

? The service provider can be either the station or the DS.

? Three of the services are used to control IEEE 802.11 LAN access and

confidentiality. Six of the services are used to support delivery of MAC service data units (MSDUs) between stations.

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? 4. IEEE 802.11 Medium Access Control

? 4.1 Reliable Data Delivery

IEEE 802.11 includes a frame exchange protocol to solve the reliability of data delivery. When a station receives a data frame from another station, it returns an ACK frame to the source station. If the source does not receive an ACK within a short period of time, the source retransmits the frame. To further enhance reliability, a source first issues a Request to Send (RTS) frame to the destination, then the destination responds with a Clear to Send (CTS). After receiving the CTS, the source transmits the data frame, and the destination responds with an ACK.

? 4.2 MAC Frame

MAC Frame Types:

? Control Frames assist in the reliable delivery of data frames.

? Data Frames: There are eight data frame subtypes, organized into two groups. The

first four define frames that carry upper-level data from the source station to the destination station. The remaining four do not in fact carry any user data.

? Management Frames are used to manage communications between stations and

APs. Functions covered include management of associations (request, response, reassociation, dissociation, and authentication).

? 4.3 Access Control

The 802.11 working group considered two types of proposals for a MAC algorithm: ? Distributed Access Protocols, which, like Ethernet, distribute the decision to

transmit over all the nodes using a carrier-sense mechanism;

? Centralized Access Protocols, which involve regulation of transmission by a

centralized decision maker.

? The end result for 802.11 is a MAC algorithm called DFWMAC (distributed

foundation wireless MAC) that provides a distributed access control mechanism with an optional centralized control built on top of that. Figure 5 illustrates the architecture.

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? 5. IEEE 802.11 Physical Layer

? 5.1 Original IEEE 802.11 Physical Layer

Three physical media are defined in the original 802.11 standard:

? Direct-sequence spread spectrum operating in the 2.4 GHz ISM band, at data rates

of 1 Mbps and 2 Mbps.

? Frequency-hopping spread spectrum operating in the 2.4 GHz ISM band, at data

rates of 1 Mbps and 2 Mbps.

? Infrared at 1 Mbps and 2 Mbps operating at a wavelength between 850 and 950 nm. ? 5.2 IEEE 802.11a

The IEEE 802.11a specification makes use of the 5-GHz band.

? 5.3 IEEE 802.11b