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Communication Networks 2E Sol Manual
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2011-12-18
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转头厚的通信网英文版Communication Networks全习题解答。
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Communication Networks (2
nd
Edition) Chapter 1 Solutions
Solutions to Chapter 1
1a. 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, as
determined by the mail code and grouped with other letters with the same destination mail
code. (If there is no mail code, then it is determined by the largest geographical unit, for
example, 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 (or
country 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 the
specified 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 Simple
Mail 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 of
the mail provider of the recipient. The mail provider is determined by the information following
the @ symbol.
3. If the mail provider finds the network address of the recipient's mail provider, then it sends the
message to that address.
Leon-Garcia/Widjaja 1
Communication Networks (2
nd
Edition) Chapter 1 Solutions
4. Upon receipt of the message the mail provider either informs the recipient automatically that
there is mail or waits until mail is requested by the recipient, depending on how the mail
service 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 up
connections 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 mail
between 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-end
connections to send a message. We will see in Chapter 2, however, that e-mail protocols use
connection-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 North
American telephone numbering system the first three digits are the area code which specifies
the main geographical region of the receiver; the next three digits specify a particular
telephone office in the North American network. The final four digits identify the particular
location of the receiver.
2. When a telephone number is dialed equipment at the other end of the telephone line uses the
sequence 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 originating
and 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 the
destination 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. The
setting 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 specific
location. 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. This
necessitates the translation of the personal telephone number to a number that corresponds
to 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 to
the "home" location of the personal number.
Leon-Garcia/Widjaja 2
Communication Networks (2
nd
Edition) Chapter 1 Solutions
3. The home location must somehow be able to redirect the connection setup process to the
current 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 the
user. In cellular telephony, for example, requests for connections to a given mobile
telephone 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 to
reach 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 network
gives 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 another
number (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 an
Leon-Garcia/Widjaja 3
Communication Networks (2
nd
Edition) Chapter 1 Solutions
end-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 of
telephone 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 an
organization poses a problem since any given address is no longer unique across multiple
organizations.
A possible approach to enabling communications between users in different organizations is to use a
two-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 establish
connections 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.
Leon-Garcia/Widjaja 4
Communication Networks (2
nd
Edition) Chapter 1 Solutions
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 allows
a user to quickly find the news items of interest. In addition, newspapers can provide more in-depth
coverage 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, whether
connection-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 responses
Leon-Garcia/Widjaja 5
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