计算机网络 习题 英文版(4版).pdf

1 Chap 1 1 Imagine that you have trained your St Bernard Bernie to carry a box of three 8mm tapes instead of a flask of brandy When your disk fills up you consider that an emergency These tapes each contain 7 gigabytes The dog can travel to your side wherever you may be at 18 km hour For what range of distances does Bernie have a higher data rate than a transmission line whose data rate excluding overhead is 150 Mbps 2 An alternative to a LAN is simply a big timesharing system with terminals for all users Give two advantages of a client server system using a LAN 3 The performance of a client server system is influenced by two network factors the bandwidth of the network how many bits sec it can transport and the latency how many seconds it takes for the first bit to get from the client to the server Give an example of a network that exhibits high bandwidth and high latency Then give an example of one with low bandwidth and low latency 4 Besides bandwidth and latency what other parameter is needed to give a good characterization of the quality of service offered by a network used for digitized voice traffic 5 A factor in the delay of a store and forward packet switching system is how long it takes to store and forward a packet through a switch If switching time is 10 sec is this likely to be a major factor in the response of a client server system where the client is in New York and the server is in California Assume the propagation speed in copper and fiber to be 2 3 the speed of light in vacuum 6 A client server system uses a satellite network with the satellite at a height of 40 000 km What is the best case delay in response to a request 7 In the future when everyone has a home terminal connected to a computer network instant public referendums on important pending legislation will become possible Ultimately existing legislatures could be eliminated to let the will of the people be expressed directly The positive aspects of such a direct democracy are fairly obvious discuss some of the negative aspects 8 A collection of five routers is to be connected in a point to point subnet Between each pair of routers the designers may put a high speed line a medium speed line a low speed line or no line If it takes 100 ms of computer time to generate and inspect each topology how long will it take to inspect all of them 9 A group of 2 n 1 routers are interconnected in a centralized binary tree with a router at each tree node Router i communicates with router j by sending a message to the root of the tree The root then sends the message back down to j Derive an approximate expression for the mean number of hops per message for large n assuming that all router pairs are equally likely 2 10 A disadvantage of a broadcast subnet is the capacity wasted when multiple hosts attempt to access the channel at the same time As a simplistic example suppose that time is divided into discrete slots with each of the n hosts attempting to use the channel with probability p during each slot What fraction of the slots are wasted due to collisions 11 What are two reasons for using layered protocols 12 The president of the Specialty Paint Corp gets the idea to work with a local beer brewer to produce an invisible beer can as an anti litter measure The president tells her legal department to look into it and they in turn ask engineering for help As a result the chief engineer calls his counterpart at the other company to discuss the technical aspects of the project The engineers then report back to their respective legal departments which then confer by telephone to arrange the legal aspects Finally the two corporate presidents discuss the financial side of the deal Is this an example of a multilayer protocol in the sense of the OSI model 13 What is the principal difference between connectionless communication and connection oriented communication 14 Two networks each provide reliable connection oriented service One of them offers a reliable byte stream and the other offers a reliable message stream Are these identical If so why is the distinction made If not give an example of how they differ 15 What does negotiation mean when discussing network protocols Give an example 16 In Fig 1 19 a service is shown Are any other services implicit in this figure If so where If not why not 17 In some networks the data link layer handles transmission errors by requesting damaged frames to be retransmitted If the probability of a frame s being damaged is p what is the mean number of transmissions required to send a frame Assume that acknowledgements are never lost 18 Which of the OSI layers handles each of the following a a Dividing the transmitted bit stream into frames b b Determining which route through the subnet to use 19 If the unit exchanged at the data link level is called a frame and the unit exchanged at the network level is called a packet do frames encapsulate packets or do packets encapsulate frames Explain your answer 20 A system has an n layer protocol hierarchy Applications generate messages of length M bytes At each of the layers an h byte header is added What fraction of the network bandwidth is filled with headers 21 List two ways in which the OSI reference model and the TCP IP reference model are the same Now list two ways in which they differ 22 What is the main difference between TCP and UDP 23 The subnet of Fig 1 25 b was designed to withstand a nuclear war How many bombs would it take to partition the nodes into two disconnected sets Assume that any bomb wipes out a node and all of the links connected to it 3 24 The Internet is roughly doubling in size every 18 months Although no one really knows for sure one estimate put the number of hosts on it at 100 million in 2001 Use these data to compute the expected number of Internet hosts in the year 2010 Do you believe this Explain why or why not 25 When a file is transferred between two computers two acknowledgement strategies are possible In the first one the file is chopped up into packets which are individually acknowledged by the receiver but the file transfer as a whole is not acknowledged In the second one the packets are not acknowledged individually but the entire file is acknowledged when it arrives Discuss these two approaches 26 Why does ATM use small fixed length cells 27 How long was a bit on the original 802 3 standard in meters Use a transmission speed of 10 Mbps and assume the propagation speed in coax is 2 3 the speed of light in vacuum 28 An image is 1024 x 768 pixels with 3 bytes pixel Assume the image is uncompressed How long does it take to transmit it over a 56 kbps modem channel Over a 1 Mbps cable modem Over a 10 Mbps Ethernet Over 100 Mbps Ethernet 29 Ethernet and wireless networks have some similarities and some differences One property of Ethernet is that only one frame at a time can be transmitted on an Ethernet Does 802 11 share this property with Ethernet Discuss your answer 30 Wireless networks are easy to install which makes them inexpensive since installation costs usually far overshadow equipment costs Nevertheless they also have some disadvantages Name two of them 31 List two advantages and two disadvantages of having international standards for network protocols 32 When a system has a permanent part and a removable part such as a CD ROM drive and the CD ROM it is important that the system be standardized so that different companies can make both the permanent and removable parts and everything still works together Give three examples outside the computer industry where such international standards exist Now give three areas outside the computer industry where they do not exist 33 Make a list of activities that you do every day in which computer networks are used How would your life be altered if these networks were suddenly switched off 34 Find out what networks are used at your school or place of work Describe the network types topologies and switching methods used there 35 The ping program allows you to send a test packet to a given location and see how long it takes to get there and back Try using ping to see how long it takes to get from your location to several known locations From thes data plot the one way transit time over the Internet as a function of distance It is best to use universities since the location of their servers is known very accurately For example berkeley edu is in Berkeley California 4 mit edu is in Cambridge Massachusetts vu nl is in Amsterdam The Netherlands www usyd edu au is in Sydney Australia and www uct ac za is in Cape Town South Africa 36 Go to IETF s Web site www ietf org to see what they are doing Pick a project you like and write a half page report on the problem and the proposed solution 37 Standardization is very important in the network world ITU and ISO are the main official standardization organizations Go to their Web sites www itu org and www iso org respectively and learn about their standardization work Write a short report about the kinds of things they have standardized 38 The Internet is made up of a large number of networks Their arrangement determines the topology of the Internet A considerable amount of information about the Internet topology is available on line Use a search engine to find out more about the Internet topology and write a short report summarizing your findings 5 Chap 2 1 Compute the Fourier coefficients for the function f t t 0 t 1 2 A noiseless 4 kHz channel is sampled every 1 msec What is the maximum data rate 3 Television channels are 6 MHz wide How many bits sec can be sent if four level digital signals are used Assume a noiseless channel 4 If a binary signal is sent over a 3 kHz channel whose signal to noise ratio is 20 dB what is the maximum achievable data rate 5 What signal to noise ratio is needed to put a T1 carrier on a 50 kHz line 6 What is the difference between a passive star and an active repeater in a fiber network 7 How much bandwidth is there in 0 1 micron of spectrum at a wavelength of 1 micron 8 It is desired to send a sequence of computer screen images over an optical fiber The screen is 480 x 640 pixels each pixel being 24 bits There are 60 screen images per second How much bandwidth is needed and how many microns of wavelength are needed for this band at 1 30 microns 9 Is the Nyquist theorem true for optical fiber or only for copper wire 10 In Fig 2 6 the lefthand band is narrower than the others Why 11 Radio antennas often work best when the diameter of the antenna is equal to the wavelength of the radio wave Reasonable antennas range from 1 cm to 5 meters in diameter What frequency range does this cover 12 Multipath fading is maximized when the two beams arrive 180 degrees out of phase How much of a path difference is required to maximize the fading for a 50 km long 1 GHz microwave link 13 A laser beam 1 mm wide is aimed at a detector 1 mm wide 100 m away on the roof of a building How much of an angular diversion in degrees does the laser have to have before it misses the detector 14 The 66 low orbit satellites in the Iridium project are divided into six necklaces around the earth At the altitude they are using the period is 90 minutes What is the average interval for handoffs for a stationary transmitter 15 Consider a satellite at the altitude of geostationary satellites but whose orbital plane is inclined to the equatorial plane by an angle To a stationary user on the earth s surface at north latitude does this satellite appear motionless in the sky If not describe its motion 16 How many end office codes were there pre 1984 when each end office was named by its three digit area code and the first three digits of the local number Area codes started with a digit in the range 2 9 had a 0 or 1 as the second digit and ended with any digit The first two digits of a local number were always in the range 2 9 The third digit could be any digit 17 Using only the data given in the text what is the maximum number of telephones that the existing U S system can support without changing the numbering plan 6 or adding additional equipment Could this number of telephones actually be achieved For purposes of this problem a computer or fax machine counts as a telephone Assume there is only one device per subscriber line 18 A simple telephone system consists of two end offices and a single toll office to which each end office is connected by a 1 MHz full duplex trunk The average telephone is used to make four calls per 8 hour workday The mean call duration is 6 min Ten percent of the calls are long distance i e pass through the toll office What is the maximum number of telephones an end office can support Assume 4 kHz per circuit 19 A regional telephone company has 10 million subscribers Each of their telephones is connected to a central office by a copper twisted pair The average length of these twisted pairs is 10 km How much is the copper in the local loops worth Assume that the cross section of each strand is a circle 1 mm in diameter the density of copper is 9 0 grams cm 3 and that copper sells for 3 dollars per kilogram 20 Is an oil pipeline a simplex system a half duplex system a full duplex system or none of the above 21 The cost of a fast microprocessor has dropped to the point where it is now possible to put one in each modem How does that affect the handling of telephone line errors 22 A modem constellation diagram similar to Fig 2 25 has data points at the following coordinates 1 1 1 1 1 1 and 1 1 How many bps can a modem with these parameters achieve at 1200 baud 23 A modem constellation diagram similar to Fig 2 25 has data points at 0 1 and 0 2 Does the modem use phase modulation or amplitude modulation 24 In a constellation diagram all the points lie on a circle centered on the origin What kind of modulation is being used 25 How many frequencies does a full duplex QAM 64 modem use 26 An ADSL system using DMT allocates 3 4 of the available data channels to the downstream link It uses QAM 64 modulation on each channel What is the capacity of the downstream link 27 In the four sector LMDS example of Fig 2 30 each sector has its own 36 Mbps channel According to queueing theory if the channel is 50 loaded the queueing time will be equal to the download time Under these conditions how long does it take to download a 5 KB Web page How long does it take to download the page over a 1 Mbps ADSL line Over a 56 kbps modem 28 Ten signals each requiring 4000 Hz are multiplexed on to a single channel using FDM How much minimum bandwidth is required for the multiplexed channel Assume that the guard bands are 400 Hz wide 29 Why has the PCM sampling time been set at 125 sec 30 What is the percent overhead on a T1 carrier that is what percent of the 1 544 Mbps are not delivered to the end user 31 Compare the maximum data rate of a noiseless 4 kHz channel using a a Analog encoding e g QPSK with 2 bits per sample 7 b b The T1 PCM system 32 If a T1 carrier system slips and loses track of where it is it tries to resynchronize using the 1st bit in each frame How many frames will have to be inspected on average to resynchronize with a probability of 0 001 of being wrong 33 What is the difference if any between the demodulator part of a modem and the coder part of a codec After all both convert analog signals to digital ones 34 A signal is transmitted digitally over a 4 kHz noiseless channel with one sample every 125 sec How many bits per second are actually sent for each of these encoding methods a a CCITT 2 048 Mbps standard b b DPCM with a 4 bit relative signal value c c Delta modulation 35 A pure sine wave of amplitude A is encoded using delta modulation with x samples sec An output of 1 corresponds to a signal change of A 8 and an output signal of 1 corresponds to a signal change of A 8 What is the highest frequency that can be tracked without cumulative error 36 SONET clocks have a drift rate of about 1 part in 10 9 How long does it take for the drift to equal the width of 1 bit What are the implications of this calculation 37 In Fig 2 37 the user data rate for OC 3 is stated to be 148 608 Mbps Show how this number can be derived from the SONET OC 3 parameters 38 To accommodate lower data rates than STS 1 SONET has a system of virtual tributaries VT A VT is a partial payload that can be inserted into an STS 1 frame and combined with other partial payloads to fill the data frame VT1 5 uses 3 columns VT2 uses 4 columns VT3 uses 6 columns and VT6 uses 12 columns of an STS 1 frame Which VT can accommodate a a A DS 1 service 1 544 Mbps b b European CEPT 1 service 2 048 Mbps c c A DS 2 service 6 312 Mbps 39 What is the essential difference between message switching and packet switching 40 What is the available user bandwidth in an OC 12c connection 41 Three packet switching networks each contain n nodes The first network has a star topology with a central switch the second is a bidirectional ring and the third is fully interconnected with a wire from every node to every other node What are the best average and worst case transmission paths in hops 42 Compare the delay in sending an x bit message over a k hop path in a circuit switched network and in a lightly loaded packet switched network The circuit setup time is s sec the propagation delay is d sec per hop the packet size is p bits and the data rate is b bps Under what conditions does the packet network have a lower delay 8 43 Suppose that x bits of user data are to be transmitted over a k hop path in a packet switched network as a series of packets each containing p data bits and h header bits with x p h The bit rate of the lines is b bps and the propagation delay is negligible What value of p minimizes the total delay 44 In a typical mobile phone system with hexagonal cells it is forbidden to reuse a frequency band in an adjacent cell If 840 frequencies are available how many can be used in a given cell 45 The actual layout of cells is seldom as regular that as shown in