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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