Hierarchy

Using Example 2.2, there are three different activities at the sender site and another three activities at the receive site.

使用2.2示例，在发送端网址有三次不同的活动，同样，在接收端也有三次活动。

The task of transporting the letter between the sender and the receiver is done by the carrier. 

从发传者到接收者之间的信件传输任务，被载体完成。

carrier /k'/ (carriers) 1. [N-count] carrier is a vehicle  that is used for carrying people, especially soldiers, or things.

carrier是一辆被用来装人的车辆，特别是，士兵，或者货物。

2.【n-count】 A carrier is a passenger airline . Carrier是一种乘客空机，