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Computer Networking(2656) a Top-down Approach 6th Edition Solutions

Sandra Watson
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Computer Networking: A Top-Down Approach,
6th Edition
Solutions to Review Questions and Problems

Chapter 1 Review Questions
1. There is no difference. Throughout this text, the words “host” and “end system” are
used interchangeably. End systems include PCs, workstations, Web servers, mail
servers, PDAs, Internet-connected game consoles, etc.
2. From Wikipedia: Diplomatic protocol is commonly described as a set of international
courtesy 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 always been the
acknowledgment of the hierarchical standing of all present. Protocol rules are based
on the principles of civility.
3. Standards are important for protocols so that people can create networking systems
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, namely, 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. Today, Ethernet most commonly 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 today:

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 typically 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 systems, packets are
transmitted over the same wireless infrastructure used for cellular telephony, with the
base station thus being managed by 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
delay). 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 delay). Thus, the end-to-end delay 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 today (including the
Internet) cannot make any end-to-end guarantees for bandwidth. FDM requires
sophisticated analog hardware to shift signal into appropriate frequency 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
simultaneously, a maximum of 2Mbps will be required. Since the available
bandwidth of the shared link is 2Mbps, there will be no queuing delay before the
link. Whereas, if three users transmit simultaneously, the bandwidth required
will be 3Mbps which is more than the available bandwidth of the shared link. In
this case, there will be queuing delay before the link.
c) Probability that a given user is transmitting = 0.2
d) Probability that all three users are transmitting simultaneously = 3 􀀋 􀀌3 3 1
3 􀀐 􀀐 􀂸 􀂸􀂹
􀂨 􀂨􀂩
p p
= (0.2)3 = 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 probability
that all three users are transmitting simultaneously) is 0.008.
14. If the two ISPs do not peer with each other, then when they send traffic to each other
they have to send the traffic through a provider ISP (intermediary), to which they
have to pay for carrying the traffic. By peering with each other directly, the two ISPs
can reduce their payments to their provider ISPs. An Internet Exchange Points (IXP)
(typically 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 money by charging
each of the the ISPs that connect to the IXP a relatively small fee, which may 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. Many of these data centers are located in, or close to, lower tier ISPs.
Therefore, when Google delivers content to a user, it often can bypass 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
intermediary ISPs. Second, it can save money by sending less traffic into provider

networks. Third, if ISPs decide to charge more money to highly profitable content
providers (in countries where net neutrality doesn’t apply), the content providers can
avoid these extra payments.
16. The delay components are processing delays, transmission delays, propagation
delays, and queuing delays. All of these delays are fixed, except for the queuing
delays, which are variable.
17. a) 1000 km, 1 Mbps, 100 bytes
b) 100 km, 1 Mbps, 100 bytes
18. 10msec; d/s; no; no
19. a) 500 kbps
b) 64 seconds
c) 100kbps; 320 seconds
20. End system A breaks the large file into chunks. It adds header to each chunk, thereby
generating multiple packets from the file. The header in each packet includes the IP
address of the destination (end system B). The packet switch uses the destination IP
address in the packet to determine the outgoing link. Asking which road to take is
analogous to a packet asking which outgoing link it should be forwarded on, given
the packet’s destination address.


Computer Networking(2656) a Top-down Approach 6th Edition Solutions


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