Computer Networks exam 1 – updated
2024/2025 VersioN with 100% CorreCt
aNswers
◉ Suppose you would like to urgently deliver 40 terabytes data from Boston to
Los Angeles. You have available a 100 Mbps dedicated link for data transfer.
Would you prefer to transmit the data via this link or instead use FedEx
overnight delivery? Answer: 40(10^12) x 8 = 320(10^12) bits,
320(10^12)/100(10^6) = 3200000 seconds = 37.037 days. Fedex can deliver
it in one day.
◉ The data link layer is responsible for sending data into packets. Answer:
False
◉ The network layer encapsulates frames into segments. Answer: False
◉ The transport layer is responsible for sending data in frames. Answer: False
◉ Twisted-pair copper wire is no longer present in computer networks.
Answer: False
,◉ A cable TV system has 100 commercial channels, all of them alternating
programs with advertising. Is this more like TDM or like FDM? Answer: Both
◉ A cable company decides to provide Internet access over cable in a
neighborhood consisting of 5000 houses. The company uses a coaxial cable
and spectrum allocation allowing 100 Mbps downstream bandwidth per cable.
To attract customers, the company decides to guarantee at least 2 Mbps
downstream bandwidth to each house at any time. What is the maximum
number of houses that can be serviced per cable? Answer: A 2-Mbps
downstream bandwidth guarantee to each house implies at most 50 houses
per coaxial cable. Thus, the cable company will need to split up the existing
cable into 100 coaxial cables and connect each of them directly to a fiber node.
◉ A cable company decides to provide Internet access over cable in a
neighborhood consisting of 5000 houses. The company uses a coaxial cable
and spectrum allocation allowing 100 Mbps downstream bandwidth per cable.
To attract customers, the company decides to guarantee at least 2 Mbps
downstream bandwidth to each house at any time. What is the minimum
number of cables needed? Answer: A 2-Mbps downstream bandwidth
guarantee to each house implies at most 50 houses per coaxial cable. Thus, the
cable company will need to split up the existing cable into 100 coaxial cables
and connect each of them directly to a fiber node.
,◉ A channel using 4B/5B encoding sends data at a rate of 64 Mbps. What is
the minimum bandwidth used by this channel (MHz)? Answer: For every four
data bits, 5 bits are sent. The channel sends 80 million bits per second, which
can be done using a 40 MHz channel.
◉ At the low end, the telephone system is star shaped, with all the local loops
In a neighborhood converging on an end office. In contrast, cable television
consists of a single long cable snaking its way past all the houses in the same
neighborhood. Suppose that a future TV cable were 10-Gbps fiber Instead of
copper. If it was being used to simulate the telephone model of everybody
having their own private line to the end office (assume 64kbps), how many
one-telephone houses could be hooked up to a single fiber? Answer: Ignoring
speech compression, a digital PCM telephone needs 64 kbps. If we divide 10
Gbps by 64 kbps we get 156,250 houses per cable. Current systems have
hundreds of houses per cable.
◉ Can a station using QAM-16 send 3 bits per symbol? Answer: True. QAM-16
uses 16 distinct symbols. Each of these symbols can be assign-ed to a bit
sequence, in which case it can send 4 bits per symbol. Alternatively, some of
the symbols can be used as control signals, leaving fewer signals to send bit
sequences.
◉ Which of the following descriptions below correspond to a "services" view
of the Internet? Answer: A platform for building network applications.
, A place I go for information, entertainment, and to communicate with people.
A collection of billions of computing devices, and packet switches
interconnected by links.
A collection of hardware and software components executing protocols that
define the format and the order of messages exchanged between two or more
communicating entities, as well as the actions taken on the transmission
and/or receipt of a message or other event.
◉ Which of the following human scenarios involve a protocol? Answer: One
person asking, and getting, the time to/from another person.
A person sleeping.
A person reading a book.
A student raising her/his hand to ask a really insightful question, followed by
the teaching acknowledging the student, listening carefully to the question,
and responding with a clear, insightful answer. And then thanking the student
for the question, since teachers love to get questions.
Two people introducing themselves to each other.
2024/2025 VersioN with 100% CorreCt
aNswers
◉ Suppose you would like to urgently deliver 40 terabytes data from Boston to
Los Angeles. You have available a 100 Mbps dedicated link for data transfer.
Would you prefer to transmit the data via this link or instead use FedEx
overnight delivery? Answer: 40(10^12) x 8 = 320(10^12) bits,
320(10^12)/100(10^6) = 3200000 seconds = 37.037 days. Fedex can deliver
it in one day.
◉ The data link layer is responsible for sending data into packets. Answer:
False
◉ The network layer encapsulates frames into segments. Answer: False
◉ The transport layer is responsible for sending data in frames. Answer: False
◉ Twisted-pair copper wire is no longer present in computer networks.
Answer: False
,◉ A cable TV system has 100 commercial channels, all of them alternating
programs with advertising. Is this more like TDM or like FDM? Answer: Both
◉ A cable company decides to provide Internet access over cable in a
neighborhood consisting of 5000 houses. The company uses a coaxial cable
and spectrum allocation allowing 100 Mbps downstream bandwidth per cable.
To attract customers, the company decides to guarantee at least 2 Mbps
downstream bandwidth to each house at any time. What is the maximum
number of houses that can be serviced per cable? Answer: A 2-Mbps
downstream bandwidth guarantee to each house implies at most 50 houses
per coaxial cable. Thus, the cable company will need to split up the existing
cable into 100 coaxial cables and connect each of them directly to a fiber node.
◉ A cable company decides to provide Internet access over cable in a
neighborhood consisting of 5000 houses. The company uses a coaxial cable
and spectrum allocation allowing 100 Mbps downstream bandwidth per cable.
To attract customers, the company decides to guarantee at least 2 Mbps
downstream bandwidth to each house at any time. What is the minimum
number of cables needed? Answer: A 2-Mbps downstream bandwidth
guarantee to each house implies at most 50 houses per coaxial cable. Thus, the
cable company will need to split up the existing cable into 100 coaxial cables
and connect each of them directly to a fiber node.
,◉ A channel using 4B/5B encoding sends data at a rate of 64 Mbps. What is
the minimum bandwidth used by this channel (MHz)? Answer: For every four
data bits, 5 bits are sent. The channel sends 80 million bits per second, which
can be done using a 40 MHz channel.
◉ At the low end, the telephone system is star shaped, with all the local loops
In a neighborhood converging on an end office. In contrast, cable television
consists of a single long cable snaking its way past all the houses in the same
neighborhood. Suppose that a future TV cable were 10-Gbps fiber Instead of
copper. If it was being used to simulate the telephone model of everybody
having their own private line to the end office (assume 64kbps), how many
one-telephone houses could be hooked up to a single fiber? Answer: Ignoring
speech compression, a digital PCM telephone needs 64 kbps. If we divide 10
Gbps by 64 kbps we get 156,250 houses per cable. Current systems have
hundreds of houses per cable.
◉ Can a station using QAM-16 send 3 bits per symbol? Answer: True. QAM-16
uses 16 distinct symbols. Each of these symbols can be assign-ed to a bit
sequence, in which case it can send 4 bits per symbol. Alternatively, some of
the symbols can be used as control signals, leaving fewer signals to send bit
sequences.
◉ Which of the following descriptions below correspond to a "services" view
of the Internet? Answer: A platform for building network applications.
, A place I go for information, entertainment, and to communicate with people.
A collection of billions of computing devices, and packet switches
interconnected by links.
A collection of hardware and software components executing protocols that
define the format and the order of messages exchanged between two or more
communicating entities, as well as the actions taken on the transmission
and/or receipt of a message or other event.
◉ Which of the following human scenarios involve a protocol? Answer: One
person asking, and getting, the time to/from another person.
A person sleeping.
A person reading a book.
A student raising her/his hand to ask a really insightful question, followed by
the teaching acknowledging the student, listening carefully to the question,
and responding with a clear, insightful answer. And then thanking the student
for the question, since teachers love to get questions.
Two people introducing themselves to each other.
