网络不习惯于用较大的数据块或较小的字节来表示数据，而习惯于以包为单位。
Networks tend to consider data not in large chunks, nor in tiny bytes, but in packets.
在不同的章节中，我们将会看到数据单位如何通过协议在给定的网络堆栈层来接收、传输。
In different chapters, we will see how data units are received and transmitted by the protocols that sit at a given layer in the network stack.
“帧”是一种数据单位，它在特定的网络中，在各主机之间传递。
A frame is the unit of data that travels between hosts on a specific network.
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