数据通信入门必读。涵盖数据通信的基本概念：数据、信号、信道、通信方式等，传输介质、编解码、调制和解调、多路复用、数据交换、差错控制等(The required reading of data communication. It covers the basic concepts of data communication: data, signal, channel, communication mode, transmission medium, codec, modulation and d