计算机网络英文版——提供给学生部分习题答案.doc

Solution of Selected Exercises from the End of Chapter Exercises Chapter 1 - Introduction And Overview 1.4 To what aspects of networking does data communications refer? Answer: Data communications refers to the study of low-level mechanisms and technologies used to send information across a physical communication medium, such as a wire, radio wave, or light beam. 1.5 What is packet-switching, and why is packet switching relevant to the Internet? Answer: Packet switching divides data into small blocks, called packets, and includes an identification of the intended recipient in each packet. Packet switching changed networking in a fundamental way, and provided the basis for the modern Internet. Packet switching allows multiple senders to transmit data over a shared network. 1.8 What is a communication protocol? Conceptually, what two aspects of communication does a protocol specify? Answer: A communication protocol refer to a specification for network communication.Major aspects of a protocol are syntax (format) and semantics (meaning) of the protocol. 1.9 What is a protocol suite, and what is the advantage of a suite? Answer: protocols are designed in complete, cooperative sets called suites or families, instead of creating each protocol in isolation. Each protocol in a suite handles one aspect of communication; together, the protocols in a suite cover all aspects of communication. The entire suite is designed to allow the protocols to work together efficiently. 1.11 List the layers in the TCP/IP model, and give a brief explanation of each. (See Textbook)1.14 Give a brief explain of the layers in the ISO Open System Interconnection model. (See Textbook) Chapter 3 - Internet Applications And Network Programming3.1 What are the two basic communication paradigms used in the Internet? Answer: There are various approaches, but according to textbook, we can specify them as Stream Paradigm and Message Paradigm. 3.2 Give six characteristics of Internet stream communication. (See Textbook)3.3 Give six characteristics of Internet message communication. (See Textbook)3.4 If a sender uses the stream paradigm and always sends 1024 bytes at a time, what size blocks can the Internet deliver to a receiver? Answer: stream paradigm does not provide any guarantees for block sizes, so all depends on individual transfer. 3.6 What are the three surprising aspects of the Internets message delivery semantics? Answer: The Internets message delivery has the following undesirable characteristics: * Messages can be lost * Messages can be duplicated * Messages can be delivered out-of-order 3.8 When two applications communicate over the Internet, which one is the server? Answer: The application that waits for some other applications to contact is called server, and the application that contact other one is called client. 3.14 What two identifiers are used to specify a particular server? Answer: A particular server is identified by the following identifiers: * An identifier for the computer on which a server runs (IP Address) * An identifier for a particular service on the computer (Port Number) Chapter 4 - Traditional Internet Applications4.1 What details does an application protocol specify? (See Textbook)4.3 What are the two key aspects of application protocols, and what does each include? (See Textbook)4.6 What are the four parts of a URL, and what punctuation is used to separate the parts? Answer: The URL into four components: a protocol, a computer name, a document name, and parameters. The computer name and protocol port are used to form a connection to the server on which the page resides. And the document name and parameters are used to request a specific page. 4.7 What are the four HTTP request types, and when is each used? (See Textbook)4.12 When a user requests an FTP directory listing, how many TCP connections are formed? Explain. Answer: FTP uses two types of connections to perform its functionality, namely * A control connection is reserved for commands. Each time the server needs to download or upload a file, the server opens a new connection. * A data connection is used to transfer files. 4.16 List the three types of protocols used with email, and describe each. (See Textbook)4.17 What are the characteristics of SMTP? (See Textbook)4.20 What are the two main email access protocols? Answer: Two major email access protocols are: * Post Office Protocol (POP) * Internet Mail Access Protocol (IMAP) Chapter 6- Information Sources and Signals 6.4 State and describe the four fundamental characteristics of a sine wave. (See Textbook)6.9 What is the analog bandwidth of a signal? Answer: Analog bandwidth of signal can be defined as to be the difference between the highest and lowest frequencies of the constituent parts (i.e., the highest and lowest frequencies obtained by Fourier analysis) 6.11 Suppose an engineer increases the number of possible signal levels from two to four. How many more bits can be sent in the same amount of time? Explain. Answer: The number of levels that can be represented by n bits is given by 2n . So if number of levels changes from 24, it means number of bits goes from 12 612. What is the definition of baud? Answer: Baud is defined as the number of times that a signal can change per second.6.14 What is the bandwidth of a digital signal? Explain. Answer: According to the definition of analog bandwidth, a digital signal has infinite bandwidth because Fourier analysis of a digital signal produces an infinite set of sine waves with frequencies that grow to infinity. 6.18 What is the chief advantage of a Differential Manchester Encoding? Answer: The most important property of differential encoding is that the encoding works correctly even if the two wires carrying the signal are accidentally reversed. 6.20 If the maximum frequency audible to a human ear is 20,000 Hz, at what rate must the analog signal from a microphone be sampled when converting it to digital? Answer: The sampling rate = 2 fmax, so the signal should be sampled at 2x20,000 = 40,000 Hz Chapter 7 - Transmission Media7.2 What are the three energy types used when classifying physical media according to energy used? Answer: Three types of energy used when classifying physical media are electrical, electromechanical (radio), and light 7.4 What three types of wiring are used to reduce interference form noise? (See Textbook) 7.10 List the three forms of optical fiber, and give the general properties of each. (See Textbook)7.21 What is the relationship between bandwidth, signal levels, and data rate? Answer: If a transmission system uses K possible signal levels and has an analog bandwidth B, Nyquists Theorem states that the maximum data rate in bits per second, D, is: D = 2 B log2K 7.22 If two signal levels are used, what is the data rate that can be sent over a coaxial cable that has an analog bandwidth of 6.2 MHz? Answer: Using the D= 2 B log2 K relationship, D = 2*6.2*log22 = 2*6.2*1 = 12.4 Mbps 7.24 If a system has an input power level of 9000, and an output power level of 3000, what is the difference when expressed in dB? Answer: Decibel is expressed as 10log10(Pout/Pin) 10log10(3,000/9,000) = to be determined by reader 7.23 If a system has an average power level of 100, an average noise level of 33.33, and a bandwidth of 100 MHz, what is the effective limit on channel capacity? Answer: Shannon theorem specify the maximum data rate that could be achieved over a transmission system that experiences noise: C = Blog2 (1 + S/N) = 100,000,000 * log2 (1 + 100/33.33) = 100,000,000 * log24 = 200,000,000 = 200 Mbps 7.25 If a telephone system can be created with a signal-to-noise ratio of 40 dB and an analog bandwidth of 3000 Hz, how many bits per second could be transmitted? Answer: First we should convert 40 dB to a real number, namely if 40 = 10 log10S/NS/N = 10,000 , Using the Shannons capacity expression C = B log2 (1 + S/N) C = 3,000 log2 (1+ 10,000) = to be determined by reader Ch 8 - Reliability And Channel Coding8.1 List and explain the three main sources of transmission errors. (See Textbook)8.3 In a burst error, how is burst length measured? Answer: For a burst error, the burst size, or length, is defined as the number of bits from the start of the corruption to the end of the corruption. 8.4 What is a codeword? Answer: We can define the set of all possible messages to be a set of datawords, and define the set of all possible encoded versions to be a set of codewords. So each possible code sequence is considered to be a codeword. 8.8 Compute the Hamming distance for the following pairs: (0000, 0001), (0101, 0001), (1111, 1001), and ( 0001, 1110). (See Textbook)8.11 Generate a RAC parity matrix for a (20, 12) coding of the dataword 100011011111. (See Textbook)8.15 Express the two values in the previous exercise as polynomials. Answer: X10 + X7 + X5 + X3 + X X4 + X2 + 1 Ch 9 - Transmission Modes 9.1 Describe the difference between serial and parallel transmission. Answer: Transmission modes can be divided into two fundamental categories: * Serial: one bit is sent at a time * Parallel: multiple bits are sent at the same time9.2 What are the advantages of parallel transmission? What is the chief disadvantage? Answer: A parallel mode of transmission has two chief advantages: * High speed: Because it can send N bits at the same time, a parallel interface can operate N times faster than an equivalent serial interface. * Match to underlying hardware: Internally, computer and communication hardware uses parallel circuitry. Thus, a parallel interface matches the internal hardware well. The main disadvantage of parallel transmission is number of cables required, for long distance communication, this is an important consideration. 9.4 What is the chief characteristic of asynchronous transmission? Answer:Asynchronous transmission can occur at any time, with an arbitrary delay between the transmission of two data items, it allows the physical medium to be idle for an arbitrary time between two transmissions. Chapter 11 - Multiplexing And Demultiplexing 11.2 What are the four basic types of multiplexing? (See Textbook) 11.4 What is a guard band? Answer: For proper communication without interference, we should choose a set of carrier frequencies with a gap between them known as a guard band. The guard band reduces or eliminates the possible interference between neighboring carrier signals.11.8 Explain how a range of frequencies can be used to increase data rate. Answer:To increase the overall data rate, a sender divides the frequency range of the channel into K carriers, and sends 1 /K of the data over each carrier.11.12 Suppose N users compete using a statistical TDM system, and suppose the underlying physical transport can send K bits per second. What is the minimum and maximum data rate that an individual user can experience? Answer: If we neglect the overhead generated by statistical TDM, a system will have two possibilities: * Minimum: If all channels have equal data then the rate will be K/N bps * Maximum: If only one channel active and the others are passive, then rate will be K bps Chapter 13 - Local Area Networks: Packets, Frames, And Topologies 13.1 What is circuit switching, and what are its chief characteristics? Answer: The term circuit switching refers to a communication mechanism that establishes a path between a sender and receiver with guaranteed isolation from paths used by other pairs of senders and receivers. The circuit switching has the following main characteristics: * Point-to-point communication * Separate steps for circuit creation, use, and termination * Performance equivalent to an isolated physical path 13.3 In a packet switching system, how does a sender transfer a large file? Answer: The packet switching system requires a sender to divide each message into blocks of data that are known as packets . The size of a packet varies; each packet switching technology defines a maximum packet size. So, a large file will be divided into smaller pieces and sent. 13.5 What are the characteristics of LANs, MANs, and WANs? Answer: There are lots of details that can be said and discussed for categorization of network types based on geography, few points are highlighted below: * Local Area Network (LAN): Least expensive; spans a single room or a single building * Metropolitan Area Network (MAN) Medium expense; spans a major city or a metroplex * Wide Area Network (WAN) Most expensive; spans sites in multiple cities 13.6 Name the two sublayers of Layer 2 protocols defined by IEEE, and give the purpose of each. Answer: The Layer 2 protocols defined by IEEE defines two sub-layers as mentioned below: * Logical Link Control (LLC) Addressing and demultiplexing * Media Access Control (MAC) Access to shared media 13.8 What are the four basic LAN topologies? Answer: The four basic LAN topologies are star, ring, mesh and bus. 13.10 In a mesh network, how many connections are required among 20 computers? Answer: The expression to calculate number of connections in a mesh network is given by n (n-1)/2. So for 20 computers then number of connections required will be = 20 (20 1)/2 =19013.15 Give a definition of the term frame . Answer: In a packet-switched network, each frame corresponds to a packet processed at data link layer. Chapter 14 - The IEEE MAC Sub-Layer 14.1 Explain the three basic approaches used to arbitrate access to a shared medium. (See Textbook)14.3 List the three main types of channelization and the characteristics of each. (See Textbook)14.6 What is a token, and how are tokens used to control network access? Answer: A special control message is called a token. In a token passing system, when no station has any packets to send, the token circulates among all stations continuously. When a station captures the token, it sends its data, and when transmission completed, it releases the token. 14.8 Expand the acronym CSMA/CD, and explain each part. Answer: The acronym CSMA/CD stands for Carrier Sense Multi-Access with Collision Detection, which means the following: * Carrier Sense: Instead of allowing a station to transmit whenever a packet becomes ready, Ethernet requires each station to monitor the cable to detect whether another transmission is already in progress. * Multiple Access: The system allows multiple users/hosts to make use of a common/shared media * Collision Detection. A collision can occur if two stations wait for a transmission to stop, find the cable idle, and both start transmitting. 14.10 Why does CSMA/CD use a random delay? (Hint: think of many identical computers on a network.) Answer: Randomization is used to avoid having multiple stations transmit simultaneously as soon as the cable is idle. That is, the standard specifies a maximum delay, d, and requires each station to choose a random delay less than d after a collision occurs. In most cases, when two stations each choose a random value, the station that chooses the smallest delay willChapter 15 - Wired LAN Technology (Ethernet And 802.3)15.1 How large is the maximum Ethernet frame, including the CRC? Answer: According to Fig. 15.1 a conventional Ethernet frame has the following fields: * Header: 14 bytes (fixed) * Payload: 46-1500 bytes (there is a minimum frame size because of collision detection) * CRC: 4 bytes (fixed) Accordingly an Ethernet frame will be maximum 1518 bytes and minimum 64 bytes 15.3 In an 802.3 Ethernet frame, what is the maximum payload size? Answer: The 802.3 Ethernet makes use of 8-bytes of the original/conventional Ethernet for Logical Link Control / Sub- Network Attachment Point (LLC / SNAP) header instead of extending/increasing the header. This is for sake of backward compatibility. So the maximum pay load is reduced from 1500 bytes to 1492 bytes. 15.6 How did a computer attach to a Thicknet Ethernet? Answer: Hardware used with Thicknet was divided into two major parts: * Transceiver: A network interface card (NIC) handled the digital aspects of communication, and a separate electronic device called a transceiver connected to the Ethernet cable and handled carrier detection, conversion of bits into appropriate voltages for transmission, and conversion of incoming signals to bits. * AUI: A physical cable known as an Attachment Unit Interface (AUI) connected a transceiver to a NIC in a computer. A transceiver was usually remote from a computer. 15.7 How were computers attached to a Thinnet Ethernet? Answer: Thinnet Ethernet (formally named 10Base2) uses a thinner coaxial cable that was more flexible than Thicknet. The wiring scheme differed dramatically from Thicknet. Instead of using AUI connections between a computer and a transceiver, Thinnet integrates a transceiver directly on the NIC, and runs a coaxial cable from one computer to another. 15.8 What is an Ethernet hub, and what wiring is used with a hub? Answer: An electronic device that serves as the central interconnection is known as a hub. Hubs were available in a variety of sizes, with the cost proportional to size. The hubs are becoming old-fashioned, and being replaced with switches. 15.3 What category of twisted pair wiring is needed for a 10 Mbps network? 100 Mbps?