【期末考試復習】計算機網絡英文版

1、第一章 計算機網絡概述l Identify the five components of a data communications system. (數據通信系統(tǒng)5個組成部分：報文 發(fā)送方 接受方 傳輸介質 協(xié)議)message,sender,receiver,transmission medium and protocol.l What are the three criteria necessary for an effective and efficient network?performance, reliability,and security.l What is an inter

2、net(互聯(lián)網)? What is the Internet（因特網）? An internet is an interconnection of networks which is a collection of networks connected by internetworking devices such as routers. The Internet is the name of a specific worldwide network that uses the TCP/IP protocol suite and is the largest internet in the w

3、orld.第二章 數據和信號l Which practical methods are used for analysis of a signal? And which is suitable for network techniques? 研究電磁信號的方法是哪兩種？適用于網絡技術的？ Time domain and frequency domain frequency domainl Why can't any digital signal be transferred with no distortion in communication transmission?（數字信號為什

4、么不能無失真?zhèn)鬏敚浚?.Because to receive an exact replica of the digital signal, all of the frequency components 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 certai

5、n frequency ranges, that is, has a limited bandwidth.2.Transmission impairments of signals (attenuation, distortion and noise) always exist, so signal errors unavoidable.l What's the bandwidth(帶寬)? Explain briefly what correlation do bandwidth, data rate and cost have in communication process?帶寬

6、、數據率、成本之間的關系？The range of frequencies contained in a composite signal is its bandwidth. In networking, we use the term bandwidth in two contexts with two different measuring values: Bandwidth in Hertz is the range of frequencies in a composite signal or the range of frequencies that a channel can pa

7、ss, usually used to present the performance of analog 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 communication devices of a network.(課本P84)The

8、greater the transmission bandwidth,the greater the data rate, and the higher the cost.l Q3-3: How can a composite signal be decomposed into its individual frequencies? 復位信號如何分解成單獨的頻率成分？If the composite signal is periodic, we can ues Fourier Series decompose it into a series of signals with discrete

9、frequencies;if the composite signal is nonperiodic, we can ues Fourier Transform decompose it into a combination of sine waves with continuous frequencies.l Q3-4: Name three types of transmission impairment. 三種傳輸減損？Attenuation, distortion,and noise. 衰減、失真和噪聲l Q3-7: What does the Nyquist theorem have

10、 to do with communications? 奈奎斯特公式在通信中的作用？ Estimate the channel capacity of noise-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 maximum bit rate of a no

11、iseless channel.l Q3-8: What does the Shannon capacityhave to do with communications? 香農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 h

12、ighest data rate for noisy channel.第三章 傳輸介質l Briefly describe the main applications of three types of guided transmission media. 三種重要的有向傳輸介質的應用總的說：Twisted-pair cable is used for voice and data communications. Coaxial cable is used in cable TV networks and traditional Ethernet LANs. Fiber-optic cable

13、 is used in backbone networks, cable TV networks, and Fast Ethernet networks.分別說：twisted-pair cable: 雙絞線 （廣泛用于樓內布線）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-rat

14、e Internet connection the so called Family Broad Band.3.Used in LANs to provide high-data-rate baseband transmission (10 and 100 Mbps).coaxial cable: 同軸電纜 （不用了，目前主要用于有線電視）1. Was widely used in analog telephone networks (10000 voice signals) and in digital telephone networks (data rate up to 600 Mbps

15、). However, it has largely been replaced with fiber-optic cable.2.Was widely used in cable TV networksHowever, cable TV providers replaced most of coax 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 Mb

16、ps. Now, mainly used for CATVfiber-optic cable: 光纖（廣泛用于樓間布線和長途傳輸）widely used for wirings between 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 combination of optical fiber and coaxial cabl

17、e.3.Backbone channels in LANs, CANs and WANs4.Lightning-proof networks5.Secrecy networks against wiretappingl What types of optical fiber transmission modes are there? Which of them is suitable for long distance transmission?Types:sigal mode and multimode, and the multimode include two types:Step-in

18、dex and Graded-index.The sigal mode is suitable for long distance transmission.第四章l What is Encoding? Why must encoding be used for data transmission?(1)【什么是編碼？】Convert internal data of a sender into line signals suitable for transmission is called encoding.(2)【數據發(fā)送時為什么要編碼？】1) Electromagnetic signal

19、s 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 synchro

20、nization, 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

21、.3.4.機內信號與線路類型不一樣；機內信號參數與線路傳輸的要求不一樣；同步、效率、糾錯等方面的特殊要求l Which two techniques are used when analog data transmitted by digital signals? Which of them is the most important? Why? 用數字信號傳輸模擬數據常用技術有哪兩種？最常用的是？為什么？(1) Pulse Code Modulation(PCM) and Delta Modulation(DM).(2) Pulse Code Modulation(PCM) is the m

22、ost 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.l What processes will be involved when analog data are digitized by PCM technique? 用PCM技術將模擬數據數字化時，要經過哪些處理步驟？Sampling (PAM)->

23、quantization->binary encoding->digital to digital codingl Q4-1 List three techniques of digital-to-digital conversion.They are line 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 ca

24、lculates 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 dif

25、ficult for the receiver to decode correctly.l 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(

26、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. 直流分量的影響不僅僅是基線偏移，還有更重要的原因。看看教材和課件，或看看原書作業(yè)答案。l Q4-9 Define scrambling and give its purpo

27、se.(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 of bits and does provide synchronization,Scrambling is a solution that substitutes long zero-lever pulse with a combination of other lever to provide sync

28、hronization.(Purpose) substitutes long zero-lever pulse with a combination of other lever to provide synchronization. Its purpose is to provide synchronization without increasing the number of bits.l Q4-10 Compare and contrast PCM and DM. PCM is the most common technique to change an analog signal t

29、o digital data edit is very complex.Although DM is not perfect,the quantization error of DM is much less than that for PCM. In general, PCM exhibits better SNR 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 t

30、he 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 analog transmission? Which of them is the most susceptible to noise? Which of them is the mechanism commonly used in all modern modems.1.Amp

31、litude 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.l Q5-1 Define analog trans

32、mission.(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 channel.(2) While digital transmission is very desirable, a low-pass channel is needed.Baseband digital or analog signals are converted

33、 to a complex analog signal with a range of frequencies suitable for the channel.l Q5-2 Define carrier signal and explain its role in analog transmission.(Define) A sender produces an analog signal as fundamental wave to carry digital signals. The fundamental wave is called carrier signal.(explain)

34、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 application is for transmitting digital data through the public telephone network via a modem.(A carrier is a single-frequency sig

35、nal that has one if its characteristic (amplitude, frequency, or phase) changed to represent the baseband signal.【In analog transmission, the sending device produces a high frequency analog signal that acts as a base for the information signals (data). The base signal is called the carrier signal. I

36、t 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 an analog signal based on the information in digital data. It is also called modulation of a digital signal. The baseband digital

37、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 represent the digital signal in each of the following digital-analog conversions?a.ASK changes the amplitude of the carrier.b.FSK change

38、s 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 and is the mechanism commonly used in all modern modems.l Q5-5 Which of the four digital-to-analog conversion techniques(ASK,FSK,

39、PSK or QAM) is the most susceptible to noise?Defend your answer.(Answer) The most susceptible to noise of the four digital-to-analog conversion techniques is ASK.(Define) Noise usually affects the amplitude; therefore, ASK is the modulation technique most affected by noise than phase or frequency.第六

40、章l Which of the four multiplexing techniques is (are) suitable for analog channels? Which of them is (are) suitable for digital channels? （See P181 Questions Q6-3)Frequency-division and wavelength-division multiplexing are suitable for analog channels Synchronous time-division multiplexing and async

41、hronous time-division multiplexing are suitable for digital channels.l Which of the four multiplexing techniques is (are) suitable for electrical links? Which of them is (are) suitable for optical links?Frequency-division , Synchronous time-division and asynchronous time-division multiplexingare sui

42、table forelectrical links .Wavelength-division multiplexing is suitable for optical links.光傳輸信道都屬于模擬信道，而時分復用只適用于數字信道。l Describe the main differences between FDM and TDM.In FDM signals generate by each sending device modulate different carrier frequencies.In TDM instead of sharing a portion of the ba

43、ndwidth as in FDM,time is share.Each connections occupies a portion of time in the link.一個是通過frequency (carrier modulation)實現復用，另一個是通過 time (slot allocation) 實現復用。4. P181-182 6.4.2 Questions：Q6-3.Distinguish between a link and a channel in multiplexing.In multiplexing,The word link refers to the phy

44、sical 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 multiplexing techniques is common for fiber-optic links?It is the Wavelength -division multiplexing.(WDM)because it allo

45、ws the multiplexing 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 output even if it is not sending data.Time slots preassigned to sources and fixed.Time slots allocated even if no data

46、. 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 allocated to improve bandwidth efficient.can improve the efficient by remove the empty slots from the frame.Only when a

47、n input lines has a slot's 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 What's bit-level errors? Describe the basic principle and the requirement for detection of bit-level errors. The bit-level

48、 errors means one bit or more bits of a given data unit (such as a byte, character or packet) is（are） changed from 1 to 0 or from 0 to 1. The central concept in detecting or correcting errors is redundancy, i.e. send some extra bits with our data. These redundant bits, based on specific algorithm, a

49、re added by the sender and removed by the receiver. Their presence allows the receiver to detect or correct corrupted bits.l Which techniques are commonly used in bit-level error detection? Which of them is(are) implemented by hardware such as gate circuits ? Which of them is(are) implemented by sof

50、tware? Give their main applications.(1) Parity Check, CRC and Checksum are commonly used in bit-level error detection.(2) Parity Check and CRC are implemented by hardware such as gate circuits. Parity Check is used in peripherals and CRC is used in networks such as LANs and WANs.(3) Checksum is impl

51、enmented by software. Mainly used for the Internet.l Which of FEC and ARQ is commonly used in computer networks?ARQ is commonly used in computer networks.l Define the Hamming distance and the minimum Hamming distance.(1) The Hamming distance between two words(of the same size) is the number of diffe

52、rences between corresponding bits.(2) The minimum Hamming distance is the smallest Hamming distance between all possible pairs in a set of words.最小海明距的計算見P286l Briefly describe the principle of CRC.CRC的工作原理CRC is a most powerful of the redundancy checking techniques based on binary division. At the

53、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 codeword.Also,to be valid, a CRC must have exactly one less bit than the divisor, and

54、the resulting codeword becomes exactly 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 assumed to be intact and is therefore accepted. Otherwise, it means that the da

55、ta unit has been corrupted in transmission and therefore the remainder must be rejected.The divisors used in CRC are normally called the generator polynomial(生成多項式) or simply the generator, which are given by some international organizations.l Q10-4 In a codeword, we add two redundant bits to each 8

56、-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 able to detect two-bit errors, what should be the minimum Hamming distance?The

57、 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 the size of the divisor?(1) We need argment the datawords with three 0s.(2

58、) 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 remainder 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 link"?The data link layer is needed because it is necessary to imp