9无线网络(08版).ppt

9 Wireless 无线网络（802.11） The possibilities building wireless networks are almost endless ranging, from using infrared signals with in a single building , to constructing a global network from a grid of low-orbit satellites. 建立无线网络的可能性几乎是无止境的，从在一个建筑内使用红外线信号，到使用低轨卫星栅格构造全球网络。 For example: frequency hopping is one kind of spread spectrum, it is transmission signal in a random frequency sequence: first send over one frequency then next and then third and go on by order. 例如，跃频是一项扩展频谱技术，它在一个随机的频率序列上传输信号：即，首先以一个频率传输，然后是第二个，然后是第三个，以此类推。 2. Medium access adopted in wireless LAN 在无线局域网上采用的媒体接入技术 802.11 addresses these two problems with an algorithm called Multiple Access with Collision Avoidance (MACA). The idea is for the sender and receiver to exchange control frames with each other before the sender actually transmits any data. This exchange informs all nearby nodes that a transmission is about to begin. 802.11用一个称为避免冲突的多路访问的算法来解决这两个问题。其思想是，在发送方真正传输数据之前，发送方和接收方彼此交换控制帧。该交换通知所有邻近结点传输就要开始了。 Any node that sees the RTS frame but not the CTS frame is not close enough to the receiver to interfere with it, and so is free to send a frame . 看到RTS帧的任意结点都知道它离发送方很近，因此在发送指定长度帧的这段时间内可以发送数据。 Each of AP services the nodes in some region. Each of these regions is analogous to a cell in a cellular phone system, with the APs playing the same role as a base station. 每个访问点在某个区域内为各个结点服务。这些区域的每一个区域类似蜂窝电话系统中的一个单元，而AP扮演着与结点相同的角色。 The idea behind this configuration is that each node associates itself with one access point. 分布式系统的思想是：每个节点与一访问点相联系。 Figure Access points connected to a distribution network 连接到一分布式网络的访问点 * * 9 Wireless 9.1 Physical Properties 9.2 Collision Avoidance 9.3 Distribution System 9.4 Frame Format 9.1 Physical Properties 物理特征 802.11 was designed to run over three different physical media-two based on spread spectrum radio and one