1、搜索出数据流中的帧同步字信号，并给出帧同步标志。

2、系统工作开始后，要连续3次确认帧同步字进入锁定状态后才输出帧同步标志。

3、在锁定状态时，如连续出现3次错误的帧同步字，则帧同步标志输出无效，系统重新进入搜索状态；否则继续输出有效的帧同步标志。

-1, the search for the data stream signal in the frame synchronization word and frame synchronization flag is given. 2, the system works after the start of 3 consecutive times to confirm the frame synchronization word into the locked state after the output frame sync flag. 3, in the locked state, such as 3 consecutive incorrect frame synchronization word, the output frame sync flag is invalid, the system re-entering the search state　otherwise continue to output a valid frame sync flag.