单选

●计算机通过电话网拨号方式上网时，异步传输的字符同步，下列选项 (18) 的说法是正确的；采用数据位为8位的异步起止方式传输数据时，其效率最高为 (19) ，高级数据链路控制规程(HDLC)是 (20) 提出的标准；HDLC帧同步标志是 (21) ；HDLC协议为保证帧同步标志在线路上传输时的惟一性，对数据进行的处理是 (22) 字节。
(18)

A．异步传输起始位一般有2位，而停止位有1位或2位 &n
bsp; B．异步传输都采用单一单位编码，且起始位、停止位常为1位
C．异步传输常采用单一单位编码，但可用5、7、8位制中的任一种
D．异步传输起始位一般有1位、2位，而停止位只有1位 (19) A．73％
B．60％
C．70％
D．80％ (20) A．IEEE
B．ISO
C．ANSI
D．CCITT (21) A．10101011
B．01111111
C．01111110
D．10101010 (22) A．在连续5个1之后自动插入一个0
B．在连续6个1之后自动插入一个0
C．将数据用补码表示
D．将数据进行异或处理

试题出自试卷《2011年网络管理员考试考前密卷（三）-上午试题》

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