【期末考试复习】计算机网络英文版

第一章计算机网络概述

第二章数据和信号

第三章传输介质

第四章

●What is Encoding? Why must encoding be used for data transmission?

(1)【什么是编码？】Convert internal data of a sender into line signals suitable for transmission is called encoding.

(2)【数据发送时为什么要编码？】

1) Electromagnetic signals in information process devices are not allowed transmitting directly over communication lines.

2) The signal type of a device differs from the signal type of a communication line.

3) The parameter requirements of device signals differs from the one of line signals.

4) Specific requirements in synchronization, efficiency , error control, etc..

★Because electromagnetic signals in information process devices are not allowed transmitting directly over communication lines, so encoding must be used for data transmission to converts internal data of a sender into line signals suitable for transmission.

2.3.4.机内信号与线路类型不一样；机内信号参数与线路传输的要求不一样；同步、效率、纠错等方面的特殊要求

●Which two techniques are used when analog data transmitted by digital signals?

Which of them is the most important? Why?

用数字信号传输模拟数据常用技术有哪两种？最常用的是？为什么？

(1) Pulse Code Modulation(PCM) and Delta Modulation(DM).

(2) Pulse Code Modulation(PCM) is the most important.

(3) The principal advantage of DM over PCM is the simplicity of its implementation. In general, PCM exhibits better SNR characteristics at the same data rate.

●What processes will be involved when analog data are digitized by PCM

technique?

用PCM技术将模拟数据数字化时，要经过哪些处理步骤？

Sampling (PAM)---->quantization---->binary encoding---->digital to digital coding

●Q4-1 List three techniques of digital-to-digital conversion.

●Q4-4 Define baseline wandering and its effect on digital transmission.

(1)In decoding a digital signal, the receiver calculates a running average of the received signal power. This average is called the baseline.(2)The incoming signal power is evaluated against this baseline to determine the value of the data element.

(3)A long string of 0s or 1s can cause a drift in the baseline (baseline wandering) and make it difficult for the receiver to decode correctly.

●Q4-5 Define a DC component and its effect on digital transmission.

(Define) When the voltage level in a digital signal is constant for a while, the specturm creates very low frequencies(results of Fourier analysis). These frequences around zero,called DC(derect-current) components, that present problems for a system that can not pass low frequencies or a system that uses electrical coupling.

(Effect) DC component means 0/1 parity that can cause base-line wondering. 直流分量的影响不仅仅是基线偏移，还有更重要的原因。看看教材和课件，或看看原书作业答案。

第五章

●Which modulation techniques are used in analog transmission? Which of them

is the most susceptible to noise? Which of them is the mechanism commonly used in all modern modems.

1.Amplitude shift keying ( ASK)

Frequency shift keying ( FSK )

Phase shift keying ( PSK)

Quadrature Amplitude Modulation(QAM)

2.The most susceptible to noise of the four digital-to-analog conversion techniques is ASK.

3.The mechanism commonly used in all modern modems of them is QAM.

●Q5-1 Define analog transmission.