(zz)比特位，帧，包，段(bit, frame, packet, segment)

（这些通信中的术语，区别是什么？我发现Wikipedia的是比较正确的，网络其他上地方的则良莠不齐，所以转载于此。）

OSI Model
	Data unit	Layer	Function
Host layers	Data	7. Application	Network process to application
		6. Presentation	Data representation and encryption
		5. Session	Interhost communication
	Segment	4. Transport	End-to-end connections and reliability
Media layers	Packet	3. Network	Path determination and logical addressing
	Frame	2. Data Link	Physical addressing
	Bit	1. Physical	Media, signal and binary transmission

来源：http://en.wikipedia.org/wiki/OSI_model
(我发现中文版WIKI上有个对理解七层模型很有帮助的图，所以推荐一下：http://zh.wikipedia.org/wiki/OSI%E6%A8%A1%E5%9E%8B)