计算机网络习题答案(二)英文.doc

Chapter 5: Data CodingFill In the Blank5-1.It is much easier for a machine to process a code if all the characters in the code have _.5-2.The number of possible combinations or characters in a coding system are called _.5-3.The minimum number of bits that would be required to encode only the 26 characters of the alphabet is _.5-4.The three different groups of character assignments that are normally made in a coding system are _, _, and _.5-5.The coding system that has enough bit combinations to encode all of the characters in all of the worlds languages is called _.5-6.Three major types of data compression are called _, _, and _.5-7.Arguably, the most important data coding scheme because it is the mostly widely used is _.5-8.In an even parity system, a 1 bit is added to a characters code when necessary to make the total number of 1 bits representing the character an _ number.5-9.Character compression is also known as _.5-10.Character stripping removes the leading and trailing _ characters from a message and adds them back at the receiving end.5-11.Run length encoding replaces _ characters or repetitive groups of characters with a different, much shorter group of characters.5-12.A measure of how few bits are required to accurately convey the meaning of a character is called _ _.5-13.Bits that are used to determine the code points in a code are called _ _.5-14.The _ code is an 8-bit code that has 256 code points.True or False5-1.The term binary digit is abbreviated bit.5-2.Machines cannot send Morse code because the coded characters have different numbers of code elements.5-3.It is more difficult for a machine to process a code if all of the bits are of the same duration.5-4.When even parity is used, a 0 bit is added, if necessary, to make the total number of 0 bits in the character an even number.5-5.The escape character in a coding system is a different character than the one that is generated when you press the ESC key on the keyboard of a PC.5-6.ASCII is a 7-bit code.5-7.Unicode provides enough code points so that all characters in all languages of the world have a unique 16-bit code point.5-8.Code efficiency is a measure of how many extra bits are used beyond those required to convey the meaning of a character.5-9.Data compression is the process of reducing the number of bits used to represent a character, or reducing the number of characters before they are transmitted.5-10.Character stripping is the process of removing control characters from a message before it is transmitted.5-11.The binary digits are 1 and 2.5-12.A code is a predetermined set of symbols that have variable meanings.5-13.Morse code is not considered a binary code, because its two elements are of different length.5-14.A binary code works well for machines communicating by electrical means because the 1 bits and 2 bits can be represented by a current flow that is either on or off.5-15.For transmission efficiency, it is ideal to have a coding system that uses a minimum number of bits to represent each character.5-16.Most coding systems include a technique called an escape character.5-17.The necessity to support escape characters complicates the design of equipment that is to code and decode the data.5-18.The ASCII code is the most widely used code in computers and communications networks today.5-19.Unicode is a 16-bit character code.Multiple Choice5-1.Characters that control the positioning of information on a workstation screen or paper are called _.a.page control charactersb.format effector charactersc.device control charactersd.screen control characterse.a and b5-2.It is much easier for a machine to process a code if the code has the following attributes: _.a.it is a true binary codeb.all of the characters have the same number of bitsc.all of the bits are perfectly formedd.all of the bits are the same duratione.a and cf.All of the above.5-3.The unique sequence of bits assigned to represent the various characters of a code are called _.a.code pointsb.character assignmentsc.control charactersd.character algorithmse.None of the above.5-4.The purpose of escape characters is to _.a.allow a program to abort a message prematurelyb.implement a priority system by allowing one message to be prematurely ended so another can be sent on the circuitc.act as an alternate escape control character when more than 8 bits are neededd.indicate that the characters that follow are to be interpreted as having an alternate meaninge.indicate that a PDU has been received correctly5-5.The ASCII code is also known as the _.a.International Telegraph Alphabet 5b.extended ASCII codec.8-bit coded.PC codee.EBCDIC code5-6.The EBCDIC code _.a.was developed by IBMb.is an 8-bit codec.has 256 code pointsd.is used on older IBM mainframe computerse.All of the above.f.None of the above.5-7.Unicode _.a.is a 16-bit codeb.was developed by a group of companiesc.supports the characters of all languagesd.All of the above.e.None of the above.5-8.Code efficiency is calculated _.a.by minimizing the number of bits transmitted for a given messageb.by dividing the information bits by the total number of bits in a characterc.by dividing the total number of bits by the information bits in a characterd.by measuring the error rate at the receiving end of a transmission5-9.Three major types of data compression are _.a.character removal, run length encoding, and bit compressionb.bit stripping, double clocking, and character strippingc.character compression, Huffman coding, and character stripping d.character stripping, run length encoding, and character compression e.over clocking, transmission digitizing, and double ACKing5-10.A 10-bit code could represent how many characters?a.100b.210 -1c.29d.512e.2105-11.The number of possible combinations or characters in a coding system is called the _.a.character assignmentb.code pointsc.control characterd.code limite.binary code5-12.Many coding systems include an extra bit, called a _ , for checking purposes.a.parity bitb.padding bitc.control bitd.binary digite.checking bit5-13.When an ESC character is sent as a part of the data, it means that _.a.the transmission is being abortedb.this is the end of the transmissionc.the characters that follow are to be interpreted as having an alternate meaningd.the characters that follow are encryptede.the transmission should be ignored5-14.Graphic characters _.a. can be displayed on a workstation screenb. are special characters only used by graphic artistsc. are only present in specialized code systemsd. can also be used as format effector characters5-15Code conversion _.a. changes one character to another for security purposesb. changes the characters in one coding system to those of another coding systemc. can be readily performed by a computerd. All of the above.e. b and cChapter 6: Data Communication FundamentalsFILL IN THE BLANK6-1. The three attributes of a sine wave are its _ , _ , and _.6-2. The FM radio band in the United States extends from 88 MHz to 108 MHz. Its bandwidth is _.6-3. A signal drop of 10 dB represents a drop in power of _ times.6-4. A radio signal with a frequency of 101 MHz falls within the _ radio band.6-5. Speech with frequencies above _ is attenuated and not transmitted through the public telephone network.6-6. Changing a communication signal by altering its amplitude, frequency, or phase is called _.6-7. The unit of signaling rate on a communication circuit is called _.6-8. The unit of measure of a circuits data carrying rate is called _6-9. Sending a tribit on a circuit requires _ (how many) unique signal changes to be transmitted?6-10. A circuit that allows transmission in one direction only is called a _ circuit.6-11. A circuit that allows transmission in either direction but not at the same time is called a _ circuit.6-12. A circuit that allows transmission in both directions simultaneously is called a _ circuit.6-13. A circuit that allows all of the bits in a character to be transmitted simultaneously is called a _ circuit.6-14. A circuit in which the bits of a character are transmitted one after the other on a single communication path is called a _ circuit.6-15. The mechanism by which character synchronization occurs in asynchronous transmission is called _ and _ bits.6-16. The means by which the receiver knows which bits to group together to form a character is called _.6-17. When each block of data to be transmitted is surrounded with synchronizing and other control characters the transmission is called _.6-18. When each character to be transmitted is surrounded with start and stop bits, the transmission is called _.6-19. The _ condition is the idle condition on a line. Conventionally it is the same as the signal for a 1 bit.6-20. The purpose of the _ in synchronous transmission is to maintain synchronization between the transmitting node and the receiving node.6-21. _ modulation is better suited for high-speed analog transmission because the receiver more easily detects changes in _ than changes in amplitude or frequency.6-22. _ equalizers assume that the circuit meets an average set of loss, noise, and distortion parameters, and shape the transmitted signal accordingly.6-23. _ equalizers examine the actual parameters of the circuit being used, and shape the signal to match its specific characteristics.6-24. Modems that use the V.34 standard use a sophisticated modulation technique called _, and they assume that most of the actual transmission will occur on _ circuits.6-25. High-speed modems achieve a high throughput rate by using sophisticated modulation and _ _ techniques.6-26. A _ ensures that the signal entering a digital circuit has properly shaped, square pulses, and is precisely timed.6-27. The time it takes the modems on an HDX circuit to reverse their function the time for the transmitting modem to switch to being the receiver, and the receiving modem to switch to being the transmitteris called _.6-28. The advantages of _ transmission over _ transmission include better data integrity; higher capacity circuits; easier integration of voice, data and other signals; better security and privacy; and lower cost.6-29. The type of digital modulation that takes 8,000 samples per second but uses only 4 bits per sample to code the difference between the values of two samples is called _.6-30. An _ converts analog signals to a digital format.6-31. The difference between the actual value (height) of an analog signal and the nearest integer value, which is assigned when the signal is digitized, is called _.6-32. When the number of data bits in a character is divided by the total number of bits required to transmit the character, the _ _ is being calculated.TRUE OR FALSE6-1. Signaling is the propagation of a signal on a medium.6-2. Playing the A key above middle C on a piano results in a digital signal that has 880 pulses per second.6-3. Sound waves, electrical waves, and electromagnetic waves have essentially different characteristics.6-4. A digital signal level is measured in decibels.6-5. The loss of signal strength between two points on a communication circuit is called attenuation.6-6. The amplitude and phase of a voice signal remain constant while its frequency changes.6-7. ADPCM is accomplished by shifting each channel to a different part of the frequency spectrum.6-8. The speed of a circuit is measured in baud.6-9. A 2,400 baud signal can carry data at a maximum rate of 7,200 bps.6-10. When 2 bits of information are coded into one signal change, they are called bi-bits.6-11. The most common digital signals are unipolar.6-12. When transmitting digital signals on an analog circuit, a modem is used.6-13. An analog signal that has been digitized can be closely restored to its original analog shape by a D/A converter.6-14. PCM is the ITU-Ts recommended method for digitizing voice at 32 Kbps.6-15. In a modem, equalizer circuitry compensates for the variability of the actual transmission line used.6-16. Line turnaround time in a modem can be a significant part of the total transmission time.6-17. Modems can never handle full-duplex transmission.6-18. Phase modulation is the technique of changing a digital signals phase in order to modulate it.6-19. V.34-bis modems assume that most data transmission occurs on digital lines.6-20. The transmission speed between a pair of V.90 modems is symmetric.6-21. Cable modems for each CATV system are unique.6-22. Modem eliminators are a low-cost alternative to modems on LAN circuits.6-23. There are very few practical uses for simplex transmission.6-24. Parallel mode transmission is widely used on WANs to achieve high throughput.6-25. Asynchronous transmission is widely used today.6-26. Start bits precede every data block in synchronous transmission.6-27. To jam a spread spectrum signal, one simply broadcasts a jamming signal on the spread spectrum signals carrier frequency.MULTIPLE CHOICE6-1. The reasons that digital transmission is superior to analog transmission are _.a. better data integrityb. higher capacityc. easier integrationd. better security and privacye. lower costf. All of the above.g. only a, b, c, and d6-2. A modem is a form of a _.a. A/D converterb. D/A converterc. codecd. DTEe. None of the above.f. a and b6-3. The three attributes of an analog signal discussed in the chapter are _.a. frequency, decibels, and modulationb. phase, frequency, and amplitudec. amplitude, carrier, and sined. bit rate, speed, and error ratee. unipolarity, quantization, and equilibrium6-4. Parallel transmission occurs _.a. during spread spectrum transmissionb. when a PC sends data to a printer using a serial cablec. on most WAN circuitsd. All of the above.e. None of the above.6-5. Differential Manchester coding _.a. is often used on WANsb. is almost never usedc. is subservient to Manchester codingd. requires an analog circuit to work properlye. None of the above.6-6. The most widely used interface standard between PCs and modems is _.a. point-to-pointb. USBc. RS-232-Dd. RS-232-Ce. X.256-7. The X.21 interface standard _.a. is a replacement for RS-232-Cb. defines the interface to a digital circuitc. uses a PL-259 connectord. All of the above.e. None of the above.6-8. A network interface card _.a. provides the interface from a PC to a networkb. provides the interface from one network to anotherc. provides the interface from a modem to a networkd. converts signals from analog to digitale. None of the above.6-9. When two modems send signals to each other for the process of establishing transmission parameters, it is called _.a. synchronizingb. call setupc. call establishmentd. MOST (acronym for modem startup)e. handshaking6-10. Hertz is the unit of measure for _.a. baudb. bit ratec. trellis code modulationd. cycles per seconde. data ratef. None of the above.6-11. The difference between the exact height of an analog signal and the nearest integer value when a digitizing sample is taken is called _.a. digitizing discrepancyb. digitizing errorc. quantizing noised. digitizing discernmente. a and cf. b and c6-12. QAM uses a combination of _ and _ to generate quadbits.a. phase changes and relative amplitudesb. phase changes and dibitsc. phase changes and frequency changesd. amplitude changes and frequency changese. amplitude changes and dibitsChapter 7: Data Link Control ProtocolsFILL IN THE BLANK7-1. A set of rules that define the exact format of messages exchanged between computers or between computers and people is called a _.7-2. Data link protocols are necessary so that all of the devices on the network are operating by the same set of rules and are able to _ with each other.7-3. The part of a protocol consisting of the rules that specify the way the receiving terminal signals the sending terminal if it has received data correctly, how and under what circumstances the line will be turned around, and whether the receiving terminal can accept more data is called _.7-4. The part of the protocol consisting of rules specifying what happens when an error is detected, what to do if communications suddenly and unexplainably cease, and the way communications are reestablished after they are broken is called _.7-5. The part of a protocol consisting of rules for ending the communications under normal and abnormal circumstances is called _.7-6. The ability of data terminal equipment (DTE) to be able to transmit and receive any bit pattern as data is called _.7-7. Predetermined communicati