COMPUTER NETWORKS
SIXTH EDITION
SOLUTIONS MANUAL
ANDREW S. TANENBAUM
NICK FEAMSTER
DAVID WETHEREL
, PROBLEM SOLUTIONS 1
SOLUTIONS TO CHAPTER 1 PROBLEMS
1. Because the raven flies at an average speed of 40 km/h, it needs 160/40 = 4
hours for each one-way trip.
(i) The raven makes only one trip because 1.8 terabytes exactly fits on one
scroll.
1800 GB 1
= GB/s = 1 Gbps
4 h × 3600 8
(ii) To communicate 3.6 TB of data, the raven has to fly back to pick up a
second scroll. This means that it needs to fly a total of 3 × 4 = 12 hours.
3600 GB 1 2
= GB/s = Gbps
12 h × 3600 12 3
(iii) The receiving castle receives 1.8 terabytes of data every 8 hours.
1800 GB 1 1
= GB/s = Gbps
8 h × 3600 16 2
2. There are multiple correct answers. A significant disadvantage is the
increased risk of invading people’s privacy. The increase in the number of net-
worked devices means a larger attack surface for malicious parties trying to
obtain personal information. If the information is not stolen, companies that
process and store data from IoT devices could sell it to third parties such as
advertising companies.
3. Secondly, wireless networks allow people to move around, instead of tying
them to a wall. Secondly, although wireless networks provide lower band-
width than wired networks, their bandwidth has become large enough to sup-
port applications that people find meaningful. Examples include media
streaming and video conferencing. Finally, installing wires in (old) buildings
can be expensive.
4. An advantage for the company is that they do not have to pay the up-front cost
when buying expensive hardware. They lease machines from the data center,
paying only for what they use. A disadvantage for the company is that they
may not know the underlaying infrastructure used by the data center, making it
more difficult to obtain high performance from their applications. The large
amount of resources available in data centers makes it is easier for the com-
pany to scale with user demand, which is an advantage for both. A disadvan-
tage for the users is that it becomes more difficult to track their own data, and
what it is used for.
,2 PROBLEM SOLUTIONS FOR CHAPTER 1
5. The LAN model can be grown incrementally. If the LAN is just a long cable,
it cannot be brought down by a single failure (if the servers are replicated). It
is probably cheaper. It provides more computing power and better interactive
interfaces.
6. A transcontinental fiber link might have many gigabits/sec of bandwidth, but
the latency will also be high due to the speed of light propagation over thou-
sands of kilometers. Similarly, a satellite link may run at megabits/sec but have
a high latency to send a signal into orbit and back. In contrast, a 56-kbps
modem calling a computer in the same building has low bandwidth and low
latency. So do low-end local and personal area wireless technologies such as
Zigbee.
7. No. The speed of propagation is 200,000 km/sec or 400 meters/ µ sec. In 20
µ sec, the signal travels 4 km. Thus, each switch adds the equivalent of 4 km
of extra cable. If the client and server are separated by 5000 km, traversing
even 50 switches adds only 200 km to the total path, which is only 4%. Thus,
switching delay is not a major factor under these circumstances.
8. The delay is 1% of the total time, which means
100 µ s × n
= 0. 01
29, 700 km
+ 100 µ s × n
300, 000 km/s
, where n is the number of satellites.
29, 700 km
+ 100 µ s × n = 100 × 100 µ s × n
300, 000 km/s
29, 700 km
= 99 × 100 µ s × n
300, 000 km/s
29, 700 km
= 10 = n
300, 000 km/s × 99 × 100 µ s
This means the signal must pass 10 satellites for the switching delay to be 1%
of the total delay.
9. The request has to go up and down, and the response has to go up and down.
The total path length traversed is thus 160,000 km. The speed of light in air
and vacuum is 300,000 km/sec, so the propagation delay alone is
160,000/300,000 sec or about 533 msec.
10. Traveling at 2/3 the speed of light means 200,000 km/sec. The signal travels
for 100 milliseconds, or 0.1 seconds. This means the signal traversed a dis-
tance of 200, 000 × 0. 1 = 4000 km.
, PROBLEM SOLUTIONS FOR CHAPTER 1 3
11. There is obviously no single correct answer here, but the following points
seem relevant. The present system has a great deal of inertia (checks and bal-
ances) built into it. This inertia may serve to keep the legal, economic, and
social systems from being turned upside down every time a different party
comes to power. Also, many people hold strong opinions on controversial
social issues, without really knowing the facts of the matter. Allowing poorly
reasoned opinions be to written into law may be undesirable. The potential ef-
fects of advertising campaigns by special interest groups of one kind or anoth-
er also have to be considered. Another major issue is security. A lot of people
might worry about some 14-year kid hacking the system and falsifying the re-
sults.
12. Call the routers A, B, C, D, and E. There are ten potential lines: AB, AC, AD,
AE, BC, BD, BE, CD, CE, and DE. Each of these has four possibilities (three
speeds or no line), so the total number of topologies is 410 = 1, 048, 576. At 50
ms each, it takes 52,428.8 sec, or about 14.6 hours to inspect them all.
13. The mean router-router path is twice the mean router-root path. Number the
levels of the tree with the root as 1 and the deepest level as n. The path from
the root to level n requires n 1 hops and 0.50 of the routers are at this level.
The path from the root to level n 1 has 0.25 of the routers and a length of
n 2 hops. Hence, the mean path length, l, is given by
l = 0. 5 × (n 1) + 0. 25 × (n 2) + 0. 125 × (n 3) + . . .
or
infinity infinity
l= n (0. 5)i i(0. 5)i
i=1 i=1
This expression reduces to l = n 2. The mean router-router path is thus
2n 4.
14. Distinguish n + 2 events. Events 1 through n consist of the corresponding host
successfully attempting to use the channel, i.e., without a collision. The
probability of each of these events is p(1 p)n 1 . Event n + 1 is an idle chan-
nel, with probability (1 p)n . Event n + 2 is a collision. Since these n + 2
events are exhaustive, their probabilities must sum to unity. The probability of
a collision, which is equal to the fraction of slots wasted, is then just
1 np(1 p)n 1 (1 p)n .
15. Instead of trying to foresee bad things and avoid them from happening, suc-
cessful networks are fault-tolerant. They allow bad things to happen but iso-
late or hide them from the rest of the system. Examples include error correc-
tion, error detection, and network routing.
SIXTH EDITION
SOLUTIONS MANUAL
ANDREW S. TANENBAUM
NICK FEAMSTER
DAVID WETHEREL
, PROBLEM SOLUTIONS 1
SOLUTIONS TO CHAPTER 1 PROBLEMS
1. Because the raven flies at an average speed of 40 km/h, it needs 160/40 = 4
hours for each one-way trip.
(i) The raven makes only one trip because 1.8 terabytes exactly fits on one
scroll.
1800 GB 1
= GB/s = 1 Gbps
4 h × 3600 8
(ii) To communicate 3.6 TB of data, the raven has to fly back to pick up a
second scroll. This means that it needs to fly a total of 3 × 4 = 12 hours.
3600 GB 1 2
= GB/s = Gbps
12 h × 3600 12 3
(iii) The receiving castle receives 1.8 terabytes of data every 8 hours.
1800 GB 1 1
= GB/s = Gbps
8 h × 3600 16 2
2. There are multiple correct answers. A significant disadvantage is the
increased risk of invading people’s privacy. The increase in the number of net-
worked devices means a larger attack surface for malicious parties trying to
obtain personal information. If the information is not stolen, companies that
process and store data from IoT devices could sell it to third parties such as
advertising companies.
3. Secondly, wireless networks allow people to move around, instead of tying
them to a wall. Secondly, although wireless networks provide lower band-
width than wired networks, their bandwidth has become large enough to sup-
port applications that people find meaningful. Examples include media
streaming and video conferencing. Finally, installing wires in (old) buildings
can be expensive.
4. An advantage for the company is that they do not have to pay the up-front cost
when buying expensive hardware. They lease machines from the data center,
paying only for what they use. A disadvantage for the company is that they
may not know the underlaying infrastructure used by the data center, making it
more difficult to obtain high performance from their applications. The large
amount of resources available in data centers makes it is easier for the com-
pany to scale with user demand, which is an advantage for both. A disadvan-
tage for the users is that it becomes more difficult to track their own data, and
what it is used for.
,2 PROBLEM SOLUTIONS FOR CHAPTER 1
5. The LAN model can be grown incrementally. If the LAN is just a long cable,
it cannot be brought down by a single failure (if the servers are replicated). It
is probably cheaper. It provides more computing power and better interactive
interfaces.
6. A transcontinental fiber link might have many gigabits/sec of bandwidth, but
the latency will also be high due to the speed of light propagation over thou-
sands of kilometers. Similarly, a satellite link may run at megabits/sec but have
a high latency to send a signal into orbit and back. In contrast, a 56-kbps
modem calling a computer in the same building has low bandwidth and low
latency. So do low-end local and personal area wireless technologies such as
Zigbee.
7. No. The speed of propagation is 200,000 km/sec or 400 meters/ µ sec. In 20
µ sec, the signal travels 4 km. Thus, each switch adds the equivalent of 4 km
of extra cable. If the client and server are separated by 5000 km, traversing
even 50 switches adds only 200 km to the total path, which is only 4%. Thus,
switching delay is not a major factor under these circumstances.
8. The delay is 1% of the total time, which means
100 µ s × n
= 0. 01
29, 700 km
+ 100 µ s × n
300, 000 km/s
, where n is the number of satellites.
29, 700 km
+ 100 µ s × n = 100 × 100 µ s × n
300, 000 km/s
29, 700 km
= 99 × 100 µ s × n
300, 000 km/s
29, 700 km
= 10 = n
300, 000 km/s × 99 × 100 µ s
This means the signal must pass 10 satellites for the switching delay to be 1%
of the total delay.
9. The request has to go up and down, and the response has to go up and down.
The total path length traversed is thus 160,000 km. The speed of light in air
and vacuum is 300,000 km/sec, so the propagation delay alone is
160,000/300,000 sec or about 533 msec.
10. Traveling at 2/3 the speed of light means 200,000 km/sec. The signal travels
for 100 milliseconds, or 0.1 seconds. This means the signal traversed a dis-
tance of 200, 000 × 0. 1 = 4000 km.
, PROBLEM SOLUTIONS FOR CHAPTER 1 3
11. There is obviously no single correct answer here, but the following points
seem relevant. The present system has a great deal of inertia (checks and bal-
ances) built into it. This inertia may serve to keep the legal, economic, and
social systems from being turned upside down every time a different party
comes to power. Also, many people hold strong opinions on controversial
social issues, without really knowing the facts of the matter. Allowing poorly
reasoned opinions be to written into law may be undesirable. The potential ef-
fects of advertising campaigns by special interest groups of one kind or anoth-
er also have to be considered. Another major issue is security. A lot of people
might worry about some 14-year kid hacking the system and falsifying the re-
sults.
12. Call the routers A, B, C, D, and E. There are ten potential lines: AB, AC, AD,
AE, BC, BD, BE, CD, CE, and DE. Each of these has four possibilities (three
speeds or no line), so the total number of topologies is 410 = 1, 048, 576. At 50
ms each, it takes 52,428.8 sec, or about 14.6 hours to inspect them all.
13. The mean router-router path is twice the mean router-root path. Number the
levels of the tree with the root as 1 and the deepest level as n. The path from
the root to level n requires n 1 hops and 0.50 of the routers are at this level.
The path from the root to level n 1 has 0.25 of the routers and a length of
n 2 hops. Hence, the mean path length, l, is given by
l = 0. 5 × (n 1) + 0. 25 × (n 2) + 0. 125 × (n 3) + . . .
or
infinity infinity
l= n (0. 5)i i(0. 5)i
i=1 i=1
This expression reduces to l = n 2. The mean router-router path is thus
2n 4.
14. Distinguish n + 2 events. Events 1 through n consist of the corresponding host
successfully attempting to use the channel, i.e., without a collision. The
probability of each of these events is p(1 p)n 1 . Event n + 1 is an idle chan-
nel, with probability (1 p)n . Event n + 2 is a collision. Since these n + 2
events are exhaustive, their probabilities must sum to unity. The probability of
a collision, which is equal to the fraction of slots wasted, is then just
1 np(1 p)n 1 (1 p)n .
15. Instead of trying to foresee bad things and avoid them from happening, suc-
cessful networks are fault-tolerant. They allow bad things to happen but iso-
late or hide them from the rest of the system. Examples include error correc-
tion, error detection, and network routing.
