阅读理解。

     My name's Charlie Street. I'm not twelve. I'm eleven. I'm not in Grade Eight. I'm in Grade Seven. I'm in

Class Two. My teacher isn't Miss Black. My teacher is Mr Grey. My phone number is 5642389.

阅读以上句子，然后根据内容填表。

解释占用带宽的定义，一般β／2应取多少？