数字通信中的消息字流总是用若干码元组成一个“字”，又用若干“字”组成一“句”。因此，在接收这些数字流时，同样也必须知道这些“字”、“句”的起始时刻。在接收端产生与“字”、“句”起始时刻相一致的定时脉冲序列，称为“字”同步和“句”同步，统称为群同步或帧同步。同步是保证信息可靠传输的前提，它是通信系统的一个重要问题。-In digital communication, message streams are composed with sentences. These sentences are composed with words which are composed with codes. So, when receiving these digital streams, we have to identify the beginning of these words and sentences. The method that to produce time impulse which has the same initial time with these message streams is called word synchronization and sentence synchronization, and go by the general name of group synchronization or frame synchronization. Synchronization is the assurance of communication’s reliability, so it is very important in communication system.