通信网基本概念与主体结构(第二版)答案Ch1

Solutions to Chapter 11a. Describe the step-by-step procedure that is involved from the time you deposit a letter in a mailbox to the time the letter is delivered to its destination. What role do names, addresses and mail codes (such as ZIP codes or postal codes) play? How might the letter be routed to its destination? To what extent can the process be automated?Solution:The steps involved in mailing a letter are:1. The letter is deposited in mailbox.2. The letter is picked up by postal employee and placed in sack.3. The letter is taken to a sorting station, where it is sorted according to destination, asdetermined by the mail code and grouped with other letters with the same destination mailcode. (If there is no mail code, then it is determined by the largest geographical unit, forexample, country (if specified), otherwise state (if specified), otherwise city (if specified).)4. The letter is shipped to the post office that handles the mail for the specific mail code (orcountry or city).5. The letter is then sorted by street address.6. The letter is picked up at the post office by the postal worker responsible for delivering to thespecified address.7. The letter is delivered according to the number and street.The name is not really used, unless the street address is missing or incorrect. The name is at the destination to determine who the letter belongs to. (Unless of course the letter is being sent to a small town, where most inhabitants are known to the postal worker.)The mail delivery process can be automated by using optical recognition on the mail code. The letter can then be sorted and routed to the destination postal station, and even to the destination neighborhood, depending on the amount of geographical detail built into the mail code.1b. Repeat part (a) for an e-mail message. At this point, you may have to conjecture different approaches about what goes on inside the computer network.Solution:The steps involved in e-mailing a message are:1. The message is sent electronically by clicking 'Send'. (In Chapter 2 we see that the SimpleMail Transfer Protocol (SMTP) is used to do this.)2. The mail provider of the sender sends a request to a name server for the network address ofthe mail provider of the recipient. The mail provider is determined by the information followingthe @ symbol.3. If the mail provider finds the network address of the recipient's mail provider, then it sends themessage to that address.4. Upon receipt of the message the mail provider either informs the recipient automatically thatthere is mail or waits until mail is requested by the recipient, depending on how the mailservice is set up. The recipient is determined by the information before the @ symbol.1c. Are the procedures in parts (a) and (b) connection-oriented or connectionless?Solution:The transfer service of letters in a postal system is connectionless. Users do not set upconnections prior to the transfer of each letter. (Note however that the postal system may have regularly scheduled shipments of bundles of mail from city to city, because the volume of mailbetween cities is predictable. These regular shipments can be viewed as pre-arranged"connections.")E-mail transfer is also a connectionless service in that the user does not set up end-to-endconnections to send a message. We will see in Chapter 2, however, that e-mail protocols useconnection-oriented procedures to exchange messages.NOTE: For parts (a) and (b) other step-by-step procedures are possible.2a. Describe what step-by-step procedure might be involved inside the network in making a telephone connection.Solution:1. The telephone number specifies an "address" where the receiver is located. In the NorthAmerican telephone numbering system the first three digits are the area code which specifiesthe main geographical region of the receiver; the next three digits specify a particulartelephone office in the North American network. The final four digits identify the particularlocation of the receiver.2. When a telephone number is dialed equipment at the other end of the telephone line uses thesequence of dialed numbers to determine a route across the telephone network from the call-originating phone to the destination phone. A circuit is established between the originatingand destination phone along the identified route.3. A ringing tone is then applied at the destination to indicate that there is an incoming call. If thedestination party (a person, an answering machine, or some other device) is ready to answer,the call is completed through the lifting of the phone set or some equivalent action. Thesetting up of telephone calls is discussed in Chapter 4.2b. Now consider a personal communication service that provides a user with a personal telephone number. When the number is dialed, the network establishes a connection to wherever the user is located at the given time. What functions must the network now perform in order to implement this service?Solution:1. The key difference here is that the personal telephone number is not tied to a specificlocation. Instead the personal number is associated with one or more pieces of equipment,for example, a cell phone, that can request service from various points in the network. Thisnecessitates the translation of the personal telephone number to a number that correspondsto a specific location in the network at a given time.2. When the telephone number is dialed, a message requesting a connection setup is sent tothe "home" location of the personal number.3. The home location must somehow be able to redirect the connection setup process to thecurrent location of the user. For example, the user may register one or more forwarding"addresses" to which calls are to be redirected. The signaling system is used to locate theuser. In cellular telephony, for example, requests for connections to a given mobiletelephone are broadcast over specific signaling channels.4. Once the location of the destination is identified a connection is established.3. Explain how the telephone network might modify the way calls are handled to provide the following services:Solutions follow questions:a.Call Display: the number and/or name of the calling party is listed on a screen before the call is answered.Along with the request for connection setup, the system sends identifying information regarding the originator's phone.b.Call Waiting: a special sound is heard when the called party is on the line and another user is trying toreach the called party.When a second caller tries to access a busy line, the system applies an audible sound, for example, a "beep" or a tone, to the voice signal that is being sent to the called party. The tone alerts the called party that there is an incoming call and provides the choice of answering the second call without disconnecting the first call.c.Call Answer: if the called party is busy or after the phone rings a prescribed number of times, the networkgives the caller the option of leaving a voice message.If the call is not answered after so many rings, the system establishes a connection to anothernumber (that is, forwards the call) that is associated with the answering system. The call is then automatically answered by an answering system.d.Three-Way-Calling: allows a user to talk with two other people at the same time.After the establishment of a call between two parties, the caller can then indicate to the telephone system that he or she wants to call a third party. Without disconnecting the first pair, the system sets up a connection to the second called party, and once established, transmits both conversations to all parties simultaneously. Special equipment is required to combine and distribute the voice signals of the three parties.4a. Suppose that the letter in problem 1 is sent by fax. Is this mode of communications connectionless or connection-oriented? Real-time or non-real time?Solution:In order to send the letter by fax, a telephone connection must first be established. Therefore the mode of communications is connection-oriented. The transfer of information across the network occurs in real-time.4b. Repeat part (a) if a voice mail message is left at a given telephone.Solution:Again, the process of leaving a voice mail message is connection-oriented, since there must be anend-to-end connection between the caller and the receiver with the answering service. The leaving of voice mail is also done in real-time. However, the mode of communication between the person that leaves the voice mail and the person that listens to the voice mail is connectionless and non-real-time. The people involved do not need to establish a simultaneous connection with each other in order to communicate; also, they record and listen to the messages at different times.5. Suppose that network addresses are scarce, so addresses are assigned so that they are not globally unique; in particular suppose that the same block of addresses may be assigned to different organizations. How can the organizations make use of these addresses? Can users from two such organizations communicate with each other?Solution:To make the example concrete suppose that two organizations are assigned the same set oftelephone numbers. Clearly, users within each organization can communicate with each other as long as they have a unique address within the organization. However, communications outside anorganization poses a problem since any given address is no longer unique across multipleorganizations.A possible approach to enabling communications between users in different organizations is to use atwo-step procedure as follows. Each organization has a special gateway to communicate outside the organization. Internal users contact the gateway to establish calls to other organizations. Gateways have procedures to establish connections with each other. This enables gateways to establishconnections between their internal users and users in other networks.6a. Describe the similarities and differences in the services provided by (1) a music program delivered over broadcast radio and (2) music delivered by a dedicated CD player.Solution:Both broadcast radio and a dedicated CD player provide users with similar types of information and with almost the same performance. However the broadcast radio offers its service in a real-time fashion with no interaction with users. A CD player, on the other hand, stores its information on a CD and delivers it on demand.6b. Describe how these services might be provided and enhanced by providing them through a communications network.Solution:Broadcast radio is typically transmitted in real time using radio waves "over the air." Broadcast audio programs can also be transmitted over any communication network. Furthermore, when stored in servers that are attached to a network, "broadcast" audio programs can be retrieved for listening at a later time. Indeed a server can also store CD audio material and retrieve it on demand. If the network and server are sufficiently responsive, it may also be possible to provide the interactivity of a personal CD player through a network-based service.7a. Use the World Wide Web to visit the sites of several major newspapers. How are these newspapers changing the manner in which they deliver news services?Solution:Traditionally, newspapers deliver news in printed "hard-copy" form in various editions during the day.The information is primarily in text form, supplemented by still pictures and graphics. The information in a newspaper is organized in several sections: main page, business, local, sports, classifieds, etc.Some of the revenue of newspapers is from subscriptions, but most of the newspaper revenue is usually from advertising.In the traditional model, getting the news from a particular newspaper required getting the physical printed form. The WWW and the Internet allow newspapers to deliver information in electronic form and at any time that a user requests it. Newspaper information on the Web still consists mostly of text, images, and graphics that can be retrieved and displayed readily by a Web browser. In most cases, subscription is not required and advertising is displayed along with news information. In order to be competitive, newspapers can no longer work in terms of editions, especially for late-breaking news. Instead, news items are updated continuously day and night. The use of a Web browser allowsa user to quickly find the news items of interest. In addition, newspapers can provide more in-depthcoverage in their web page on topics of special interest than is possible in print medium.7b. Now visit the websites of several major television networks. How are they changing the manner in which they deliver news over the Internet? What differences, if any, are there to the approach taken by television networks and newspapers?Solution:Traditionally, television networks deliver news using audio, video, and graphics, with very little text.News is traditionally delivered in programs that are broadcast at various times of the day. Again, the information is organized in several sections: international, business, sports, entertainment, etc. The revenue in television news is almost entirely from advertising. The WWW and the Internet allow users to retrieve news information in electronic form and at any time. However, the transmission of audio and video over the Internet poses a challenge, so this information must be transmitted in compressed form and in relatively low quality. Consequently, more of the information is provided in text form. The dependence on audio and video also required the development of applications for the decompression and display of such information. Again, in order to be competitive, news networks update their Web pages continuously day and night as news develops. The higher cost involved in providing video service has led to the introduction of subscription fees.8. Discuss the advantages and disadvantages of transmitting fax messages over the Internet instead of the telephone network.Solution:The comparison of fax transmission over the Internet and over the telephone network reflect the differences between e-mail and telephone calls. The transmission of a fax message as an attachment to e-mail means that the transfer of the message is not in real time and that delivery is not necessarily confirmed. The transmission of a fax message over the telephone network, on the other hand, is real time and is confirmed with fairly high certainty. On the other hand, the cost of transmission of a fax message over the Internet is very inexpensive and is not distance-sensitive. The transmission of fax messages over telephone networks may involve long distance charges.9a. Suppose that an interactive video game is accessed over a communication network. What requirements are imposed on the network if the network is connection-oriented? connectionless?Solution:We suppose that the game involves the interaction between a player and a server across a network.To support an interactive video game over a communications network, the network, whetherconnection-oriented or connectionless, must provide real-time delivery of the player's commands to the server, and of the server's responses to the player. With a connection-oriented network,connections between the player and the servers transfer the sequence of commands and responsesthroughout the game with very little delay. In a connectionless network, user commands may be delivered to the other end with variable delay, out-of-sequence, or not at all. The user’s network software is responsible for ensuring the ordered and correct delivery of game commands. In-time delivery of commands cannot be assured.9b. Repeat part (a) if the game involves several players located at different sites.Solution:The requirements on the network depend on how the game is implemented. In the centralizedapproach the players interact through a central server that processes the commands from all of the players, maintains a view of the state of the overall system, and issues appropriate responses to all the players. Alternatively, the game could be implemented in a decentralized fashion, where each player receives commands from some or all of the players, maintains a local view of the system state, and transmits responses to some or all of the players.In the centralized approach, the network requirements are essentially the same as those in part (a). In the decentralized approach, the real-time response requirement may apply only to a subset of players when they happen to be interacting with each other. If players are located at different sites, thenetwork could provide multicast capability so messages can be exchanged among the server and all the players.9c. Repeat part (b) if one or more of the players is in motion, for example, kids in the back of the van during a summer trip.Solution:If one or more players is in motion, then the network must also be able to locate each mobile player and then deliver and receive information to and from such player. Additional delay and loss issues come into play when a mobile user is “handed-off” from one radio coverage area to another.10. Discuss the similarities between the following national transportation networks and a communications network. Is the transportation system more similar to a telephone network or to a packet network?Solution:Transportation networks are designed to transfer people and goods; communications networks are designed to transfer information. Like communication networks, all transportation systems involve links, in the form of roads, rails, or air corridors, and switching points, in the form of stations, airports, and highway interchanges. Addressing and geographical names are used in transportation networks to identify destinations, and routing of various forms is required to direct goods to their destinations.Transportation resources, in the forms of cars or airplanes, are shared or "multiplexed" amongvarious goods that traverse common parts of the network.a.Railroad network.Railroad network: Goods or people arrive at train stations and are loaded for transport to specific places. Tickets are purchased for a given destination, usually guaranteeing that there will be a place in a car from the starting station all the way to the destination station. This is similar to establishing a connection across a telephone network. Alternatively, a passenger may choose to purchase a ticket at each station along the way. This corresponds more closely to a connectionless packet network model.b.Airline network.Airline network: In this case passengers purchase tickets that guarantee a seat all the way to the destination, even if transfers are made at intermediate airports. This is similar to the establishment of connections across a telephone network.c.Highway system.Highway systems: Trucks or cars enter the highway without making reservations ahead of time and without informing any central authority of their destination or route. This mode of operationcorresponds closely to the operation of a connectionless packet network.bination of (a), (b), and (c).Combination of transportation systems: The combination of air, rail, and highway transportationsystems can and are used jointly for the transfer of people and goods. Each transportation system is different in how it is organized and how transfers are accomplished. Nevertheless, it is possible to use these systems in combination to provide a higher degree of connectivity between sources anddestinations of people and goods. We will see that the Internet plays a role similar to the combined transportation systems in that the Internet enables the transfer of information across multipledissimilar networks that may differ in how they are organized and how they operate.11. In the 1950s, standard containers were developed for the transportation of goods. These standard containers could fit on a train car, on a truck, or in specially designed container ships. The standard size of the containers makes it possible to load and unload them much more quickly than using non-standard containers of different sizes. Draw an analogy to packet switching communications networks. In your answer identify what might constitute a container and speculate on the advantages that may come from standard-size information containers.Solution:In a packet-switching communications network, the component that is analogous to the container is a constant-size packet (preferably of short length) that can be used for the transport of information.Transmission systems and networks of various types can be designed to transfer information of the given standardized size, much like trucks, trains, and ships can be designed to carry standardcontainers. Packing and unpacking of fixed-size units is simpler than packing and unpacking ofvariable-length units. Consequently, it is simpler to schedule the transfer of packets across switches that use constant-size packets than across switches that make use of variable-length packets.12. The requirements of world commerce led to the building of the Suez and Panama canals. What analogous situations might arise in communication networks?Solution:The Suez and Panama canals were built to provide a shorter path between geographical areas with major trade relationships. The analogous situation arises in communication networks when the path between two areas in the network are too "long" either in terms of excessive delay or inadequate capacity to transfer the required volumes of traffic. The addition of network resources to provide shorter paths for lower delay or wider paths for higher capacity corresponds to the building of a canal.13. Two musicians located in different cities wish to have a jam session over a communications network. Find the maximum possible distance between the musicians if they are to interact in real-time, in the sense of experiencing the same delay in hearing each other as if they were 10 meters apart. The speed of sound is approximately 330 meters/second. Assume that the network transmits the sound at the speed of light in cable, 2.3 x 108 meters/second.Solution:The first step is to find the delay for the sound when the musicians are 10 meters apart: t10 = 10/330 = 30.30 millisecondsThe maximum distance is the time required for a real-time 'experience' times the cable speed:d = (2.3 x 108) x (30.30 x 10-3) = 6,969,000 meters = 6969 kilometers14. The propagation delay is the time that is required for the energy of a signal to propagate from one point to another.a. Find the propagation delay for a signal traversing the following networks at the speed of light in cable (2.3 x 108 meters/second):• a circuit board 10 cm• a room 10 m• a building 100 m• a metropolitan area 100 km• a continent 5000 km•up and down to a geostationary satellite 2 x 36000 kmSolution:To find the propagation delay, divide distance by the speed of light in cable. Thus we have:• a circuit board t prop = 4.347 x 10-10 seconds• a room t prop = 4.3478 x 10-8 seconds• a building t prop = 4.3478 x 10-7 seconds• a metropolitan area t prop = 4.3478 x 10-4 seconds• a continent t prop = 0.02174 seconds•up and down to a geostationary satellite t prop = 0.31304 seconds14b. How many bits are in transit during the propagation delay in the above cases, if bits are entering the above networks at the following transmission speeds: 10,000 bits/second; 1 megabit/second; 100 megabits/second; 10 gigabits/second.Solution:The number of bits in transit is obtained by multiplying the transmission rate R by the propagation delay:Distance (m)10 Kbps 1 Mbps100 Mbps10 Gbps0.1 4.347 x 10−6 4.347 x 10−40.04347 4.347810 4.3478 x 10−40.043478 4.3478434.780100 4.3478 x 10−30.4347843.4784347.800100 000 4.3478434.7843478 4.3478x1065 000 000217.4217402174000 2.174x10872 000 0003130.431304031304000 3.1304x10915. In problem 14, how long does it take to send an L-byte file and to receive a 1-byte acknowledgment back? Let L=1, 103, 106, and 109 bytes.Solution:The total time required to send a file and receive an acknowledgment of its receipt is given by:t total = L message/R + L ack/R +2* t prop = L message/R + L ack/R + 2*d/cwhere L message is the message length in bits, L ack is the acknowledgment length in bits, R is thetransmission bit rate, d is the distance traversed, and c is the speed of light.The above equation shows that there are two main factors that determine total delay:1. Message and ACK transmission time, which depends on the message length and thetransmission bit rate;2. Propagation delay, which depends solely on distance.When the propagation delay is small, message and ACK transmission times determine the total delay. On the other hand, when the bit rate becomes very large, the propagation delay provides a delay component that cannot be reduced no matter how fast the transmission rate becomes.The tables below show the two main components of the total delay in microseconds. The message transmission time is shown in red and the propagation delay is shown in blue. The entries in the total delay tables are colored according to which delay component is dominant.Message + ACK delay @10 kbps Message +ACK delay@1 Mbpsmessage+ACK delay@100 Mbpsmessage+ACK delay@10 Gbps1600160.160.0016Distance (meters) 2 * prop. delay(microseconds)Total delay@10 kbpstotal delay@1 Mbpstotal delay@100 Mbpstotal delay@10 Gbps0.1 0.00087 1600.0008716.00086960.160870.0024710 0.086957 1600.08695716.08695650.2469570.088557100 0.869565 1600.86956516.8695652 1.0295650.871165 100000 869.5652 2469.565217885.565217869.7252869.5668 5000000 43478.26 45078.2608743494.260943478.4243478.26 72000000 626087 627686.9565626102.957626087.1626087 Table 1: Message length = 1 byteMessage + ACK delay @10 kbps message +ACK delay@1 MbpsMessage+ACK delay@100 Mbpsmessage+ACK delay@10 Gbps800800800880.080.8008Distance (meters) 2 * prop. delay(microseconds)Total delay@10 kbpstotal delay@1 MbpsTotal delay@100 Mbpstotal delay@10 Gbps0.1 0.00087 800800.00098008.0008780.080870.8016710 0.086957 800800.0878008.0869680.166960.887757100 0.869565 800800.86968008.8695780.94957 1.670365 100000 869.5652 801669.56528877.56522949.6452870.366 5000000 43478.26 844278.260951486.260943558.3443479.06 72000000 626087 1426886.957634094.957626167626087.8 Table 2: Message length = 1000 bytesMessage + ACK delay @10 kbps message +ACK delay@1 Mbpsmessage+ACK delay@100 Mbpsmessage+ACK delay@10 Gbps8.00E+088.00E+0680000.08800.0008distance (meters) 2 * prop. delay(microseconds)Total delay@10 kbpstotal delay@1 Mbpstotal delay@100 Mbpstotal delay@10 Gbps0.1 0.00087 8.00E+088.00E+0680000.08800.001710 0.086957 8.00E+088.00E+0680000.17800.0878100 0.869565 8.00E+088.00E+0680000.95800.8704 100000 869.5652 8.00E+088.00E+0680869.651669.566 5000000 43478.26 8.00E+088.04E+06123478.344278.26 72000000 626087 8.01E+088.63E+06706087626887 Table 3: Message length = 106 bytes。