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1.CSMA协议思想
<1>CSMA是指载波监听多路访问
<2>CSMA协议的思想:发送帧之前,监听信道
<3>监听结果:
(1)信道空闲:发送完整帧
1^ 1-坚持CSMA :空闲则直接传输;忙则一直监听,知道空闲马上传输
2^ 非坚持CSMA:空闲直接传输,忙则等待一个随机的时间之后再监听
3^ P-坚持CSMA:空闲则以P概率直接传输,概率1-P等待到下一个时间槽再传输
(2)信道忙:推迟发送
2.CSMA/CD协议 (带冲突检测的载波监听多路访问)
<1>CSMA/CD通常用于总线型拓扑结构和星型拓扑结构的局域网中。它的每个站点都能独立决定发送帧,若两个或多个站同时发送,即产生冲突。每个站都能判断是否有冲突产生,如果有冲突发生,则等待随机事件间隔后重发,来避免再次发生冲突
<2>CSMA/CD工作原理的概括:先听后发,边发边听,冲突停止,随机延迟后重发
当一个站点想要发送数据的时候,它检测网络查看是否有其他站点正在传输,即监听信道是否空闲。
如果信道忙就等待,直到信道空闲。
如果信道闲,站点就传输数据。在发送数据的同时,站点继续监听网络确保没有其他站点发送数据;如果有两个或多个站点同时发送数据,就会产生冲突。当一个传输节点识别出一个冲突,它就会发送一个拥塞信号,其他节点收到拥塞信号后都停止传输,等待一个随机产生的时间间隙后重发。
3.CSMA/CA协议(对碰撞的避免的载波监听多路访问)
工作原理:
使用无线局域网先检测信道是否空闲,空闲则发出RTS(RTS包括发生端/接收端/的地址、下一份数据将持续发送的时间等信息)
接收端收到RTS后,将相应CTS。发送端收到CTS后,开始发送数据帧(同时预约信道),接收端收到数据帧后将用CRC来检验数据是否正确,正确则相应ACK帧