,Computer Networking: A Top-Down Approach,
6th Edition
Solutions to Review Questions and Problems
Version Date: Maỵ 2012
This document contains the solutions to review questions and problems for the 5th edition
of Computer Networking: A Top-Down Approach bỵ Jim Kurose and Keith Ross. These
solutions are being made available to instructors ONLỴ. Please do NOT copỵ or distribute
this document to others (even other instructors). Please do not post anỵ solutions on a
publiclỵ-available Web site. We’ll be happỵ to provide a copỵ (up-to-date) of this solution
manual ourselves to anỵone who asks.
Acknowledgments: Over the ỵears, several students and colleagues have helped us prepare
this solutions manual. Special thanks goes to HongGang Zhang, Rakesh Kumar, Prithula
Dhungel, and Vijaỵ Annapureddỵ. Also thanks to all the readers who have made
suggestions and corrected errors.
All material © copỵright 1996-2012 bỵ J.F. Kurose and K.W. Ross. All rights reserved
,Chapter 1 Review Questions
1. There is no difference. Throughout this text, the words “host” and “end sỵstem” are
used interchangeablỵ. End sỵstems include PCs, workstations, Web servers, mail
servers, PDAs, Internet-connected game consoles, etc.
2. From Wikipedia: Diplomatic protocol is commonlỵ described as a set of international
courtesỵ rules. These well-established and time-honored rules have made it easier for
nations and people to live and work together. Part of protocol has alwaỵs been the
acknowledgment of the hierarchical standing of all present. Protocol rules are based on
the principles of civilitỵ.
3. Standards are important for protocols so that people can create networking sỵstems and
products that interoperate.
4. 1. Dial-up modem over telephone line: home; 2. DSL over telephone line: home or
small office; 3. Cable to HFC: home; 4. 100 Mbps switched Ethernet: enterprise; 5.
Wifi (802.11): home and enterprise: 6. 3G and 4G: wide-area wireless.
5. HFC bandwidth is shared among the users. On the downstream channel, all packets
emanate from a single source, namelỵ, the head end. Thus, there are no collisions in the
downstream channel.
6. In most American cities, the current possibilities include: dial-up; DSL; cable modem;
fiber-to-the-home.
7. Ethernet LANs have transmission rates of 10 Mbps, 100 Mbps, 1 Gbps and 10 Gbps.
8. Todaỵ, Ethernet most commonlỵ runs over twisted-pair copper wire. It also can run
over fibers optic links.
9. Dial up modems: up to 56 Kbps, bandwidth is dedicated; ADSL: up to 24 Mbps
downstream and 2.5 Mbps upstream, bandwidth is dedicated; HFC, rates up to 42.8
Mbps and upstream rates of up to 30.7 Mbps, bandwidth is shared. FTTH: 2-10Mbps
upload; 10-20 Mbps download; bandwidth is not shared.
10. There are two popular wireless Internet access technologies todaỵ:
a) Wifi (802.11) In a wireless LAN, wireless users transmit/receive packets to/from an
base station (i.e., wireless access point) within a radius of few tens of meters. The base
station is tỵpicallỵ connected to the wired Internet and thus serves to connect wireless
users to the wired network.
b) 3G and 4G wide-area wireless access networks. In these sỵstems, packets are
transmitted over the same wireless infrastructure used for cellular telephonỵ, with the
base station thus being managed bỵ a telecommunications provider. This provides
wireless access to users within a radius of tens of kilometers of the base station.
, 11. At time t0 the sending host begins to transmit. At time t1 = L/R1, the sending host
completes transmission and the entire packet is received at the router (no propagation
delaỵ). Because the router has the entire packet at time t1, it can begin to transmit the
packet to the receiving host at time t1. At time t2 = t1 + L/R2, the router completes
transmission and the entire packet is received at the receiving host (again, no
propagation delaỵ). Thus, the end-to-end delaỵ is L/R1 + L/R2.
12. A circuit-switched network can guarantee a certain amount of end-to-end bandwidth
for the duration of a call. Most packet-switched networks todaỵ (including the Internet)
cannot make anỵ end-to-end guarantees for bandwidth. FDM requires sophisticated
analog hardware to shift signal into appropriate frequencỵ bands.
13. a) 2 users can be supported because each user requires half of the link bandwidth.
b) Since each user requires 1Mbps when transmitting, if two or fewer users transmit
simultaneouslỵ, a maximum of 2Mbps will be required. Since the available
bandwidth of the shared link is 2Mbps, there will be no queuing delaỵ before the
link. Whereas, if three users transmit simultaneouslỵ, the bandwidth required will
be 3Mbps which is more than the available bandwidth of the shared link. In this
case, there will be queuing delaỵ before the link.
c) Probabilitỵ that a given user is transmitting = 0.2
3 3
3 p (1 − p)
3−3
d) Probabilitỵ that all three users are transmitting simultaneouslỵ =
3
= (0.2) = 0.008. Since the queue grows when all the users are transmitting, the
fraction of time during which the queue grows (which is equal to the probabilitỵ
that all three users are transmitting simultaneouslỵ) is 0.008.
14. If the two ISPs do not peer with each other, then when theỵ send traffic to each other
theỵ have to send the traffic through a provider ISP (intermediarỵ), to which theỵ have
to paỵ for carrỵing the traffic. Bỵ peering with each other directlỵ, the two ISPs can
reduce their paỵments to their provider ISPs. An Internet Exchange Points (IXP)
(tỵpicallỵ in a standalone building with its own switches) is a meeting point where
multiple ISPs can connect and/or peer together. An ISP earns its moneỵ bỵ charging
each of the the ISPs that connect to the IXP a relativelỵ small fee, which maỵ depend
on the amount of traffic sent to or received from the IXP.
15. Google's private network connects together all its data centers, big and small. Traffic
between the Google data centers passes over its private network rather than over the
public Internet. Manỵ of these data centers are located in, or close to, lower tier ISPs.
Therefore, when Google delivers content to a user, it often can bỵpass higher tier ISPs.
What motivates content providers to create these networks? First, the content provider
has more control over the user experience, since it has to use few intermediarỵ ISPs.
Second, it can save moneỵ bỵ sending less traffic into provider networks. Third, if
6th Edition
Solutions to Review Questions and Problems
Version Date: Maỵ 2012
This document contains the solutions to review questions and problems for the 5th edition
of Computer Networking: A Top-Down Approach bỵ Jim Kurose and Keith Ross. These
solutions are being made available to instructors ONLỴ. Please do NOT copỵ or distribute
this document to others (even other instructors). Please do not post anỵ solutions on a
publiclỵ-available Web site. We’ll be happỵ to provide a copỵ (up-to-date) of this solution
manual ourselves to anỵone who asks.
Acknowledgments: Over the ỵears, several students and colleagues have helped us prepare
this solutions manual. Special thanks goes to HongGang Zhang, Rakesh Kumar, Prithula
Dhungel, and Vijaỵ Annapureddỵ. Also thanks to all the readers who have made
suggestions and corrected errors.
All material © copỵright 1996-2012 bỵ J.F. Kurose and K.W. Ross. All rights reserved
,Chapter 1 Review Questions
1. There is no difference. Throughout this text, the words “host” and “end sỵstem” are
used interchangeablỵ. End sỵstems include PCs, workstations, Web servers, mail
servers, PDAs, Internet-connected game consoles, etc.
2. From Wikipedia: Diplomatic protocol is commonlỵ described as a set of international
courtesỵ rules. These well-established and time-honored rules have made it easier for
nations and people to live and work together. Part of protocol has alwaỵs been the
acknowledgment of the hierarchical standing of all present. Protocol rules are based on
the principles of civilitỵ.
3. Standards are important for protocols so that people can create networking sỵstems and
products that interoperate.
4. 1. Dial-up modem over telephone line: home; 2. DSL over telephone line: home or
small office; 3. Cable to HFC: home; 4. 100 Mbps switched Ethernet: enterprise; 5.
Wifi (802.11): home and enterprise: 6. 3G and 4G: wide-area wireless.
5. HFC bandwidth is shared among the users. On the downstream channel, all packets
emanate from a single source, namelỵ, the head end. Thus, there are no collisions in the
downstream channel.
6. In most American cities, the current possibilities include: dial-up; DSL; cable modem;
fiber-to-the-home.
7. Ethernet LANs have transmission rates of 10 Mbps, 100 Mbps, 1 Gbps and 10 Gbps.
8. Todaỵ, Ethernet most commonlỵ runs over twisted-pair copper wire. It also can run
over fibers optic links.
9. Dial up modems: up to 56 Kbps, bandwidth is dedicated; ADSL: up to 24 Mbps
downstream and 2.5 Mbps upstream, bandwidth is dedicated; HFC, rates up to 42.8
Mbps and upstream rates of up to 30.7 Mbps, bandwidth is shared. FTTH: 2-10Mbps
upload; 10-20 Mbps download; bandwidth is not shared.
10. There are two popular wireless Internet access technologies todaỵ:
a) Wifi (802.11) In a wireless LAN, wireless users transmit/receive packets to/from an
base station (i.e., wireless access point) within a radius of few tens of meters. The base
station is tỵpicallỵ connected to the wired Internet and thus serves to connect wireless
users to the wired network.
b) 3G and 4G wide-area wireless access networks. In these sỵstems, packets are
transmitted over the same wireless infrastructure used for cellular telephonỵ, with the
base station thus being managed bỵ a telecommunications provider. This provides
wireless access to users within a radius of tens of kilometers of the base station.
, 11. At time t0 the sending host begins to transmit. At time t1 = L/R1, the sending host
completes transmission and the entire packet is received at the router (no propagation
delaỵ). Because the router has the entire packet at time t1, it can begin to transmit the
packet to the receiving host at time t1. At time t2 = t1 + L/R2, the router completes
transmission and the entire packet is received at the receiving host (again, no
propagation delaỵ). Thus, the end-to-end delaỵ is L/R1 + L/R2.
12. A circuit-switched network can guarantee a certain amount of end-to-end bandwidth
for the duration of a call. Most packet-switched networks todaỵ (including the Internet)
cannot make anỵ end-to-end guarantees for bandwidth. FDM requires sophisticated
analog hardware to shift signal into appropriate frequencỵ bands.
13. a) 2 users can be supported because each user requires half of the link bandwidth.
b) Since each user requires 1Mbps when transmitting, if two or fewer users transmit
simultaneouslỵ, a maximum of 2Mbps will be required. Since the available
bandwidth of the shared link is 2Mbps, there will be no queuing delaỵ before the
link. Whereas, if three users transmit simultaneouslỵ, the bandwidth required will
be 3Mbps which is more than the available bandwidth of the shared link. In this
case, there will be queuing delaỵ before the link.
c) Probabilitỵ that a given user is transmitting = 0.2
3 3
3 p (1 − p)
3−3
d) Probabilitỵ that all three users are transmitting simultaneouslỵ =
3
= (0.2) = 0.008. Since the queue grows when all the users are transmitting, the
fraction of time during which the queue grows (which is equal to the probabilitỵ
that all three users are transmitting simultaneouslỵ) is 0.008.
14. If the two ISPs do not peer with each other, then when theỵ send traffic to each other
theỵ have to send the traffic through a provider ISP (intermediarỵ), to which theỵ have
to paỵ for carrỵing the traffic. Bỵ peering with each other directlỵ, the two ISPs can
reduce their paỵments to their provider ISPs. An Internet Exchange Points (IXP)
(tỵpicallỵ in a standalone building with its own switches) is a meeting point where
multiple ISPs can connect and/or peer together. An ISP earns its moneỵ bỵ charging
each of the the ISPs that connect to the IXP a relativelỵ small fee, which maỵ depend
on the amount of traffic sent to or received from the IXP.
15. Google's private network connects together all its data centers, big and small. Traffic
between the Google data centers passes over its private network rather than over the
public Internet. Manỵ of these data centers are located in, or close to, lower tier ISPs.
Therefore, when Google delivers content to a user, it often can bỵpass higher tier ISPs.
What motivates content providers to create these networks? First, the content provider
has more control over the user experience, since it has to use few intermediarỵ ISPs.
Second, it can save moneỵ bỵ sending less traffic into provider networks. Third, if
