HDLC协议通信过程如图2所示，其中属于U帧的是（）。
- A.仅SABME
- B.SABME和UA
- C.SABME、UA和REJ,1
- D.SABM
- E.UA和I,0,0
正确答案及解析
正确答案
解析
无编号帧（U帧）。该帧可以实现链路控制、连接管理。该帧可以分为传输信息的命令和响应帧；用于链路恢复的命令和响应帧；设置数据传输方式的命令和响应帧；其它命令和响应帧。包含建立连接的SABME帧、次站对主站命令的确认的UA帧。
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