IST 220 Final Exam 2026 Questions and
Answers
CRC goal - Correct answer-choose r CRC bits, R, such that
•<D,R> exactly divisible by G (modulo 2)
•receiver knows G, divides <D,R> by G. If non-zero remainder: error detected!
•can detect all burst errors less than r+1 bits
Link-layer protocols run at which numbered network layer? - Correct answer-
Layer 2
Which term may be used synonymously with "network adapter"? - Correct answer-
Network interface card
Assume we are using a one-bit even parity scheme. True or false: When an even
number of bit errors occurs, the result is an undetected error. T/F? - Correct
answer-TRUE
The link layer is a combination of hardware and software. T/F - Correct answer-
TRUE
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,CRC (cyclic redundancy check) error-detection techniques provide weaker
protection against errors as compared with checksumming methods like the IP
checksum. T/F - Correct answer-FALSE
hosts and routers: - Correct answer-nodes
Consider the transportation analogy in Section 6.1.1 . If the passenger is analagous
to a datagram, what is analogous to the link layer frame? - Correct answer-The
transportation mode, e.g., car, bus, train, car.
If all the links in the Internet were to provide reliable delivery service, would the
TCP reliable delivery service be redundant? Why or why not? - Correct answer-
each link guarantees that an IP datagram sent over the link will be received at the
other end of the link without errors, it is not guaranteed that IP datagrams will
arrive at the ultimate destination in the proper order. With IP, datagrams in the
same TCP connection can take different routes in the network, and therefore arrive
out of order. TCP is still needed to provide the receiving end of the application the
byte stream in the correct order. Also, IP can lose packets due to routing loops or
equipment failures.
What are some of the possible services that a link-layer protocol can offer to the
network layer? Which of these link-layer services have corresponding services in
IP? In TCP? - Correct answer-Framing: there is also framing in IP and TCP; link
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,access; reliable delivery: there is also reliable delivery in TCP; flow control: there
is also flow control in TCP; error detection: there is also error detection in IP and
TCP; error correction; full duplex: TCP is also full duplex.
Suppose two nodes start to transmit at the same time a packet of length L over a
broadcast channel of rate R. Denote the propagation delay between the two nodes
as dprop. Will there be a collision if ? Why or why not? - Correct answer-There
will be a collision in the sense that while a node is transmitting it will start to
receive a packet from the other node.
In Section 6.3, we listed four desirable characteristics of a broadcast channel.
Which of these characteristics does slotted ALOHA have? Which of these
characteristics does token passing have? - Correct answer-Slotted Aloha: 1, 2 and 4
(slotted ALOHA is only partially decentralized, since it requires the clocks in all
nodes to be synchronized). Token ring: 1, 2, 3, 4.
In CSMA/CD, after the fifth collision, what is the probability that a node chooses ?
The result corresponds to a delay of how many seconds on a 10 Mbps Ethernet? -
Correct answer-After the fifth collision, the adapter chooses from {0, 1, 2,..., 31}.
The probability that it chooses 4 is 1/32. It waits 204.8 microseconds.
Describe polling and token-passing protocols using the analogy of cocktail party
interactions. - Correct answer-In polling, a discussion leader allows only one
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, participant to talk at a time, with each participant getting a chance to talk in a
round-robin fashion. For token ring, there isn't a discussion leader, but there is
wine glass that the participants take turns holding. A participant is only allowed to
talk if the participant is holding the wine glass.
Why would the token-ring protocol be inefficient if a LAN had a very large
perimeter? - Correct answer-When a node transmits a frame, the node has to wait
for the frame to propagate around the entire ring before the node can release the
token. Thus, if L/R is small as compared to tprop, then the protocol will be
inefficient.
How big is the MAC address space? The IPv4 address space? The IPv6 address
space? - Correct answer-248 MAC addresses; 232 IPv4 addresses; 2128 IPv6
addresses.
Suppose nodes A, B, and C each attach to the same broadcast LAN (through their
adapters). If A sends thousands of IP datagrams to B with each encapsulating frame
addressed to the MAC address of B, will C's adapter process these frames? If so,
will C's adapter pass the IP datagrams in these frames to the network layer C? How
would your answers change if A sends frames with the MAC broadcast address? -
Correct answer-C's adapter will process the frames, but the adapter will not pass
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Answers
CRC goal - Correct answer-choose r CRC bits, R, such that
•<D,R> exactly divisible by G (modulo 2)
•receiver knows G, divides <D,R> by G. If non-zero remainder: error detected!
•can detect all burst errors less than r+1 bits
Link-layer protocols run at which numbered network layer? - Correct answer-
Layer 2
Which term may be used synonymously with "network adapter"? - Correct answer-
Network interface card
Assume we are using a one-bit even parity scheme. True or false: When an even
number of bit errors occurs, the result is an undetected error. T/F? - Correct
answer-TRUE
The link layer is a combination of hardware and software. T/F - Correct answer-
TRUE
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,CRC (cyclic redundancy check) error-detection techniques provide weaker
protection against errors as compared with checksumming methods like the IP
checksum. T/F - Correct answer-FALSE
hosts and routers: - Correct answer-nodes
Consider the transportation analogy in Section 6.1.1 . If the passenger is analagous
to a datagram, what is analogous to the link layer frame? - Correct answer-The
transportation mode, e.g., car, bus, train, car.
If all the links in the Internet were to provide reliable delivery service, would the
TCP reliable delivery service be redundant? Why or why not? - Correct answer-
each link guarantees that an IP datagram sent over the link will be received at the
other end of the link without errors, it is not guaranteed that IP datagrams will
arrive at the ultimate destination in the proper order. With IP, datagrams in the
same TCP connection can take different routes in the network, and therefore arrive
out of order. TCP is still needed to provide the receiving end of the application the
byte stream in the correct order. Also, IP can lose packets due to routing loops or
equipment failures.
What are some of the possible services that a link-layer protocol can offer to the
network layer? Which of these link-layer services have corresponding services in
IP? In TCP? - Correct answer-Framing: there is also framing in IP and TCP; link
©COPYRIGHT 2025, ALL RIGHTS RESERVED 2
,access; reliable delivery: there is also reliable delivery in TCP; flow control: there
is also flow control in TCP; error detection: there is also error detection in IP and
TCP; error correction; full duplex: TCP is also full duplex.
Suppose two nodes start to transmit at the same time a packet of length L over a
broadcast channel of rate R. Denote the propagation delay between the two nodes
as dprop. Will there be a collision if ? Why or why not? - Correct answer-There
will be a collision in the sense that while a node is transmitting it will start to
receive a packet from the other node.
In Section 6.3, we listed four desirable characteristics of a broadcast channel.
Which of these characteristics does slotted ALOHA have? Which of these
characteristics does token passing have? - Correct answer-Slotted Aloha: 1, 2 and 4
(slotted ALOHA is only partially decentralized, since it requires the clocks in all
nodes to be synchronized). Token ring: 1, 2, 3, 4.
In CSMA/CD, after the fifth collision, what is the probability that a node chooses ?
The result corresponds to a delay of how many seconds on a 10 Mbps Ethernet? -
Correct answer-After the fifth collision, the adapter chooses from {0, 1, 2,..., 31}.
The probability that it chooses 4 is 1/32. It waits 204.8 microseconds.
Describe polling and token-passing protocols using the analogy of cocktail party
interactions. - Correct answer-In polling, a discussion leader allows only one
©COPYRIGHT 2025, ALL RIGHTS RESERVED 3
, participant to talk at a time, with each participant getting a chance to talk in a
round-robin fashion. For token ring, there isn't a discussion leader, but there is
wine glass that the participants take turns holding. A participant is only allowed to
talk if the participant is holding the wine glass.
Why would the token-ring protocol be inefficient if a LAN had a very large
perimeter? - Correct answer-When a node transmits a frame, the node has to wait
for the frame to propagate around the entire ring before the node can release the
token. Thus, if L/R is small as compared to tprop, then the protocol will be
inefficient.
How big is the MAC address space? The IPv4 address space? The IPv6 address
space? - Correct answer-248 MAC addresses; 232 IPv4 addresses; 2128 IPv6
addresses.
Suppose nodes A, B, and C each attach to the same broadcast LAN (through their
adapters). If A sends thousands of IP datagrams to B with each encapsulating frame
addressed to the MAC address of B, will C's adapter process these frames? If so,
will C's adapter pass the IP datagrams in these frames to the network layer C? How
would your answers change if A sends frames with the MAC broadcast address? -
Correct answer-C's adapter will process the frames, but the adapter will not pass
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