以下关于CSMA/CD协议的叙述中，正确的是（ ）。
- A.每个结点按照逻辑顺序占用一个时间片轮流发送
- B.每个结点检查介质是否空闲，如果空闲则立即发送
- C.每个结点想发就发，如果没有冲突则继续发送
- D.得到令牌的结点发送，没有得到令牌的结点等待
正确答案及解析
正确答案
解析
每个以太网节点利用总线发送数据时，首先需要侦听总线是否空闲。以太网的物理层规定发送的数据采用曼彻斯特编码方式。如果总线上已经没有数据在传输，总线的电平将不会发生跳变，可以判断此时为“总线空闲”。如果一个节点已准备好发送的数据帧，并且总线此时处于空闲状态，则这个节点就可以“启动发送”。
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