Each class is defined with an average data rate. 每一类都定义有一平均数据速率。
For synchronization the receiver has to know the data rate, which has to be set in advance, so that when it detects the start bit it is able to receive the few fol lowing bits. 对于同步传输时，接收者必须要提前知道数据速率，然后当它发现开始位时，它才能接收接着的二进制位。
