期末考试复习计算机网络英文版

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1、-第一章 计算机网络概述l Identify the five ponents of a data munications system. (数据通信系统5个组成局部：报文 发送方 承受方 传输介质 协议)message,sender,receiver,transmissionmediumandprotocol.l What are the three criteria necessary for an effective and efficient networkperformance, reliability,and security.l What is an internet(互联网)

2、What is the Internet因特网*An internetis an interconnection of networks which is a collection of networks connected by internetworking devices such as routers. *The Internetis the name of a specific worldwide network that uses the TCP/IP protocol suite and is the largest internet in the world.第二章 数据和信号

3、l Which practical methods are used for analysis of a signal And which is suitable for network techniques 研究电磁信号的方法是哪两种？适用于网络技术的？ Time domain and frequency domain frequency domainl Why cant any digital signal be transferred with no distortion in munication transmission数字信号为什么不能无失真传输？1.Because to rece

4、ive an e*act replica of the digital signal, all of the frequency ponents must be faithfully through the transmission medium, if certain harmonic cannot transfer successful, the signal we receive will be distorted. However, any medium transfers signals only within certain frequency ranges, that is, h

5、as a limited bandwidth.2.Transmission impairments of signals (attenuation, distortion and noise) always e*ist, so signal errors unavoidable.l Whats the bandwidth(带宽) E*plain briefly what correlation do bandwidth, data rate and cost have in munication process带宽、数据率、本钱之间的关系？The range of frequencies co

6、ntained in a posite signal is itsbandwidth.In networking, we use the term bandwidth in two conte*ts with two different measuring values: Bandwidth in Hertz is the range of frequencies in a posite signal or the range of frequencies that a channel can pass, usually used to present the performance of a

7、nalog channels and transmission media. (Another is) Bandwidth in bits per second is the speed of bit transmission in a channel,a link,or even a network, usually used to present the performance of links and munication devices of a network.(课本P84)The greater the transmission bandwidth,the greater the

8、data rate, and the higher the cost.l Q3-3: How can a posite signal be deposed into its individual frequencies 复位信号如何分解成单独的频率成分？If the posite signal is periodic, we can ues Fourier Series depose it into a series of signals with discrete frequencies;if the posite signal is nonperiodic, we can ues Four

9、ier Transform depose it into a bination of sine waves with continuous frequencies.l Q3-4: Name three types of transmission impairment. 三种传输减损？Attenuation, distortion,and noise.衰减、失真和噪声l Q3-7: What does the Nyquist theorem have to do with munications奈奎斯特公式在通信中的作用？Estimate the channel capacity of nois

10、e-free digital channels, , when the channel is greater than the data rate transmitted on channel capacity, it will be failure due to severe distortion. The Nyquist theorem defines the theoretical ma*imum bit rate of a noiseless channel.l Q3-8: What does the Shannon capacityhave to do with munication

11、s香农容量原理在通信中的作用？Estimate the thermal noise interference channel capacity (theoretical upper limit), the data rate is higher, damage caused by noise will be more serious. The Shannon capacity defines the theoretical highest data rate for noisy channel.第三章 传输介质l Briefly describe the main applications o

12、f three types of guided transmission media.三种重要的有向传输介质的应用总的说：Twisted-pair cable is used for voice and data munications. Coa*ial cable is used in cable TV networks and traditional Ethernet LANs. Fiber-optic cable is used in backbone networks, cable TV networks, and Fast Ethernet networks.分别说：twisted-

13、pair cable: 双绞线 广泛用于楼布线1.Used as telephone lines to provide voice and data channels connecting subscribers to the central telephone office.2.Used as DSL (digital subscriber line) loop to provide high-data-rate Internet connection the so called Family Broad Band.3.Used in LANs to provide high-data-ra

14、te baseband transmission (10 and 100 Mbps).coa*ial cable: 同轴电缆 不用了，目前主要用于有线电视1. Was widely used in analog telephone networks (10000 voice signals) and in digital telephone networks (data rate up to 600 Mbps). However, it has largely been replaced with fiber-optic cable.2.Was widely used in cable TV

15、networksHowever, cable TV providers replaced most of coa* with fiber-optic cable.3.Was widely used in early traditional Ethernet LANs. However, it has been replaced with UTP which has data rate up to 100 Mbps. Now, mainly used for CATVfiber-optic cable: 光纤广泛用于楼间布线和长途传输widely used for wirings between

16、 buildings or long-distance transmission.such as：1.High-speed backbone networks because its wide bandwidth is cost-effective.2.Cable TV networks use usually a bination of optical fiber and coa*ial cable.3.Backbone channels in LANs, CANs and WANs4.Lightning-proof networks5.Secrecy networks against wi

17、retappingl What types of optical fiber transmission modes are there Which of them is suitable for long distance transmissionTypes:sigal mode and multimode, and the multimode include two types:Step-inde* and Graded-inde*.The sigal mode is suitable for long distance transmission.第四章l What is Encoding

18、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 munication lines.2) The sig

19、nal type of a device differs from the signal type of a munication 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 ar

20、e not allowed transmitting directly over munication 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.机信号与线路类型不一样；机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l Which two techniques are used when analog data transmitted

21、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 e*h

22、ibits better SNR characteristics at the same data rate.l 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 codingl Q4-1 List three techniques of digital-to-digital conversion.They ar

23、eline coding,block coding and scrambling coding.l 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 ining signal power is evaluated

24、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.l Q4-5 Define a DC ponent and its effect on digital transmission.(Define) When the voltage

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

26、fect) DC ponent 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。l Q4-9 Define scrambling and give its purpose.(Define) Scrambling is a way to avoid a long sequence of 0s in the original stream,Scrambling is a technique that does not increase the number

27、 of bits and does provide synchronization,Scrambling is a solution that substitutes long zero-lever pulse with a bination of other lever to provide synchronization.(Purpose) substitutes long zero-lever pulse with a bination of other lever to provide synchronization. Its purpose is to provide synchro

28、nization without increasing the number of bits.l Q4-10 pare and contrast PCM and DM. PCM is the most mon technique to change an analog signal to digital data edit is very ple*.Although DM is not perfect,the quantization error of DM is much less than that for PCM. In general, PCM e*hibits better SNR

29、characteristics at the same data rate. Both of them are use sampling to convert the analog signal to digital signal.PCM finds the value of the signal amplitude for each sample; DM finds the change from the previous sample(between two consecutive samples).第五章l Which modulation techniques are used in

30、analog transmission Which of them is the most susceptible to noise Which of them is the mechanism monly 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 digi

31、tal-to-analog conversion techniques is ASK.3.The mechanism monly used in all modern modems of them is QAM.l Q5-1 Define analog transmission.(1) Analog transmission refers to the transmission of analog signals using a band-pass channel. Analog transmission is the only choice if we have a bandpass cha

32、nnel.(2) While digital transmission is very desirable, a low-pass channel is needed.Baseband digital or analog signals are converted to a ple* analog signal with a range of frequencies suitable for the channel.l Q5-2 Define carrier signal and e*plain its role in analog transmission.(Define) A sender

33、 produces an analog signal as fundamental wave to carry digital signals. The fundamental wave is called carrier signal.(e*plain) Digital data are converted into modulated signals through changing one or more of three characteristics of a carrier signal: amplitude, frequency and phase.Most familiar a

34、pplication is for transmitting digital data through the public telephone network via a modem.(A carrier is a single-frequency signal that has one if its characteristic (amplitude, frequency, or phase) changed to represent the baseband signal.【In analog transmission, the sending device produces a hig

35、h frequency analog signal that acts as a base for the information signals (data). The base signal is called the carrier signal. It is to represent the baseband signal.】)l Q5-3 Definne digital-to-analog conversion.Digital-to-analog conversion is the process of changing one of the characteristics of a

36、n analog signal based on the information in digital data. It is also called modulation of a digital signal. The baseband digital signal representing the digital data modulations the carrier to create a broadband analog signal.l Q5-4 Which of characteristics of an analog signal are changed to represe

37、nt the digital signal in each of the following digital-analog conversionsa.ASK changes the amplitude of the carrier.b.FSK changes the frequency of the carrier.c.PSK changes the phase of the carrier.d.QAM changes both the amplitude and phase of the carrier.QAM is the most efficient of these options a

38、nd is the mechanism monly used in all modern modems.l Q5-5 Which of the four digital-to-analog conversion techniques(ASK,FSK,PSK or QAM) is the most susceptible to noiseDefend your answer.(Answer) The most susceptible to noise of the four digital-to-analog conversion techniques is ASK.(Define) Noise

39、 usually affects the amplitude; therefore, ASK is the modulation technique most affected by noise than phase or frequency.第六章l Which of the four multiple*ing techniques is (are) suitable for analog channels Which of them is (are) suitable for digital channels See P181 Questions Q6-3)Frequency-divisi

40、on and wavelength-division multiple*ing are suitable for analog channels Synchronous time-division multiple*ing and asynchronous time-division multiple*ing are suitable for digital channels.l Which of the four multiple*ing techniques is (are) suitable for electrical links Which of them is (are) suit

41、able for optical linksFrequency-division , Synchronous time-division and asynchronous time-division multiple*ingare suitable forelectrical links .Wavelength-division multiple*ing is suitable for optical links.光传输信道都属于模拟信道，而时分复用只适用于数字信道。l Describe the main differences between FDM and TDM.In FDM signa

42、ls generate by each sending device modulate different carrier frequencies.In TDM instead of sharing a portion of the bandwidth as in FDM,time is share.Each connections occupies a portion of time in the link.一个是通过frequency (carrier modulation)实现复用，另一个是通过 time (slot allocation) 实现复用。4. P181-182 6.4.2

43、Questions：Q6-3.Distinguish between a link and a channel in multiple*ing.In multiple*ing,The word link refers to the physical path.The word channel refers to the portion of a link that carriers a transmission between a give pair of lines .One link can have many (n)channels.l Q6-7.Which of the three m

44、ultiple*ing techniques is mon for fiber-optic linksIt is the Wavelength -division multiple*ing.(WDM)because it allows the multiple*ing of signals with a very high frequencyl Q6-9.Distinguish between synchronous and statistical TDM. In synchronous TDM each input connection has an allotment in the out

45、put even if it is not sending data.Time slots preassigned to sources and fi*ed.Time slots allocated even if no data. In synchronous TDM, each input has a reserved slot in the output frame. This can be inefficient if some input lines have no data to send. In statistical TDM slots are dynamically allo

46、cated to improve bandwidth efficient.can improve the efficient by remove the empty slots from the frame.Only when an input lines has a slots worth of data to send is it given a slot in the output frame and the number of slots in each frame is less than the number of input lines.第七章l Whats bit-level

47、errors Describe the basic principle and the requirement for detection of bit-level errors.* The bit-level errors means one bit or more bits of a given data unit (such as a byte, character or packet) isare changed from 1 to 0 or from 0 to 1. The central concept in detecting or correcting errors is re

48、dundancy, i.e. send some e*tra bits with our data. These redundant bits, based on specific algorithm, are added by the sender and removed by the receiver. Their presence allows the receiver to detect or correct corrupted bits.l Which techniques are monly used in bit-level error detection Which of th

49、em is(are) implemented by hardware such as gate circuits Which of them is(are) implemented by software Give their main applications.(1) Parity Check, CRC and Checksum are monly used in bit-level error detection.(2) Parity Check and CRC are implemented by hardware such as gate circuits. Parity Check

50、is used in peripherals and CRC is used in networks such as LANs and WANs.(3) Checksum is implenmented by software. Mainly used for the Internet.l Which of FEC and ARQ is monly used in puter networksARQ is monly used in puter networks.l Define the Hamming distance and the minimum Hamming distance.(1)

51、 The Hamming distance between two words(of the same size) is the number of differences between corresponding bits.(2) The minimum Hamming distance is the smallest Hamming distance between all possible pairs in a set of words.最小海明距的计算见P286l Briefly describe the principle of CRC.CRC的工作原理CRC is a most

52、powerful of the redundancy checking techniques based on binary division. At the encoder, the redundant bits are derived by dividing the augmented data unit with the predetermined divisor, and the remainder is the CRC. Then the CRC is appended to the end of a datawordand create the transmitted codewo

53、rd.Also,to be valid, a CRC must have e*actly one less bit than the divisor, and the resulting codeword bees e*actly divisible by the same divisor. At the decoder, the received codeword is divided by the same divisor. And the remainder is called syndrome. If there is no remainder, the data unit is as

54、sumed to be intact and is therefore accepted. Otherwise, it means that the data unit has been corrupted in transmission and therefore the remainder must be rejected.The divisors used in CRC are normally called the generator polynomial(生成多项式) or simply the generator, which are given by some internati

55、onal organizations.l Q10-4 In a codeword, we add two redundant bits to each 8-bits data word. Find the number ofa.valid codewords. b.invalid codewordsWe have k=8, r=2, n=8+2=10.a.The number of valid codewords is 2k=28=256;b.The number of invalid codewords is 2n-2k=210-28=768;Q10-6 If we want to be a

56、ble to detect two-bit errors, what should be the minimum Hamming distanceThe minimum Hamming distance d=s+1,since s=2,d=3.l Q10-8 In CRC, if the dataword is 5 bits and the codeword is 8 bits, how many 0s need to be added to the dataword to make the dividend What is the size of the remainder What is

57、the size of the divisor(1) We need argment the datawords with three 0s.(2) The size of the remainder is 3.(3) The size of the divisor is 4.(We have k=5 and n=8. The size of the dividend is the same as the size of the codeword (8 bits). We need to augment the dataword with three 0s. The size of the r

58、emainder is r=n-k=8-5=3bits.The size of the divisor is: r+1=4 bits)第八章l Why do we need the data link layer And what is a data linkThe data link layer is needed because it is necessary to impose a layer of control in each municating device that provides functions such as flow control, error control,

59、addressing and link management. When a data link control protocol is used, the transmission medium between systems is referred to as a data link. l Briefly describe the two main functions of data link layer.The two main functions:(1) Data link control (node-to-node munication) Provide smooth and rel

60、iable transmission of frames between nodes. - Framing (organize the transmitted bits) - Flow control ( automatically regulate transfer rate) - Error control ( handle frames lost or damaged)(2) Media access control (not used if point-to-point) How to share the link - Addressing control - Sending righ

61、t control Only used in broadcast networks, e.g. LANs l Briefly describe three version of ARQ.(1)Stop-and-Wait ARQ,Send one frame at a time. (2)Design of Go-Back-N ARQ,Send ma* W frames at a time,retranmit the error frames and all succeeding ones. 3)Selective Repeat ARQ ,Send ma* W frames at a time,r

62、etranmit rejected frames only.l Briefly describe the main difference of three HDLC frames (I-,S- and U-frames).I-frame used to transport user data and control information relation to user data (piggybacking) S-frame used only to transport control informationU-frame used to transport link management

63、information (Information carried by U-frames is intended for managing link itself.)l Briefly describe the main functions of four S-frames of HDLC (RR, RNR,REJ,SREJ).RR stands for receiving ready. REJ stands for rejecting. REJ stands for receiving not ready. SREJ stands for selective-reject.【RR: This

64、 kind of frame acknowledges the receipt of a safe and sound frame or group of frames.RNR: It acknowledges the receipt of a frame or group of frames, and it announces that the receiver is busy and cannot receive more frames. It acts as a kind of congestion-control mechanism by asking the sender to slow down.REJ: It is a NAK that can be used in Go-Back-N ARQ to improve the efficiency of the process by informing the sender, before the sender timer e*pires, that the last frame is lost or damaged.SREJ: This is a NAK frame used in S