COMP3234 Computer and Communication Networks
Assignment 4 (10%)
Due by: 23:59 Friday May 15, 2020
Total mark is 100.
1. (11 marks) [Parity checks (Learning Outcome 3)]
Suppose the data in a packet, D=1010011010011101001111010, is to be protected
by a two-dimensional parity scheme and an odd parity scheme is being used. What
would the parity bits be such that the EDC field in the packet has minimum-length?
Refer to pages 9 and 11 of 14_Link_I_COMP3234_s2020.pdf. Give the answer in the
form like the following:

2. (12 marks) [Checksum (Learning Outcome 3)]
Compute the Internet checksum for a 12-byte data chunk containing the following
in sequence: (i) IP address 192.168.0.11; (ii) IP address 128.119.245.12; (iii) 8 bits
of zeros; (iv) 1 byte representing value 6; (v) 2 bytes representing value 1480.
3. (9 marks) [CRC (Learning Outcome 3)]
Consider a 5-bit generator G = 10011. Calculate the CRC code R given the following
values of D:
(1) 10101100
(2) 111000111001
(3) 1010011101011010
4. (10 marks) [Multiple Access (Learning Outcomes 2, 3)]
Consider a 1000Mbps CSMA/CD channel over a 2km coaxial cable. The speed of signal
propagation in the coaxial cable is 2.3 × 10^8m/s. A few nodes are sharing the channel
for frame transmission.
(1) Consider the worst case. What is the maximum elapsed time before a sending node
detects a collision, after it has started a frame transmission?
(2) What is the minimum frame size in this CSMA/CD network, to allow a sending
station to detect any possible collision during its frame transmission?
5. (20 marks) [Multiple Access (Learning Outcomes 2, 3)]
Consider two nodes A and B on a shared 15 Mbps CSMA/CD channel. A and B each
have two 512-byte frames to send. Ignore propagation delay and assume that any
existing transmission from one node can be immediately detected by the other.
Suppose both nodes begin transmitting their first frame at time t = 0, and then
collision occurs and both nodes back off. The CSMA/CD protocol at A and B follows
the following conventions: A (or B) will immediately abort its transmission as soon
as a collision is detected. After failure of an attempt to send a frame, A will
attempt to retransmit after waiting 512K bit times, where K is chosen each time by
exactly following the sequence of {1, 2, 3, 4, 5, . . .}, i.e., K = 1 is used when the
first attempt to send frame x (x=1, 2) fails, K = 2 is used when the second attempt
to send frame x (x=1, 2) fails, and so on. Similarly, B will attempt to retransmit a
frame after waiting 396K bit times, where K is chosen each time by exactly
following the sequence of {1, 2, 3, 4, 5, . . .}. A (or B) will attempt to send the
second frame 768 bit time after the first frame is successfully sent.
(1) When will A start the successful transmission of its first frame?
(2) When will B start the successful transmission of its second frame?
(3) Let T be the time when both A and B have completed the successful
transmission of their respective two frames. Calculate the efficiency of the CSMA/
CD channel during the time interval [0, T].
6 (20 marks) [Multiple Access (Learning Outcomes 2, 3)]
Consider an Ethernet LAN with two nodes, A and B, attempting to transmit frames.
Both A and B have a steady queue of frames ready to send: A's frames are
numbered A1, A2, A3, …, while B's frames are numbered B1, B2, B3…. Let
be the exponential backoff base unit. The transmission time of each
frame is as well. Ignore propagation delay between A and B and jam signals.
Suppose A and B simultaneously attempt to send frame 1 (A1 and B1), collide, and
happen to choose backoff times of and , respectively. As a result, A
transmits A1, while B waits. After this transmission, B attempts to retransmit B1
while A attempts to transmit A2. These two attempts collide, but now A backs off
for either or (first collision for A2), while B backs off for time equal to
one of , , , and (second collision for B1).
(1) Calculate the probability that A wins this second backoff race in its second
attempt to transmit A2.
(2) Suppose A wins this second backoff race. A transmits A2, and when it is finished,
A and B collide again as A tries to transmit A3 while B tries again to transmit B1.
Calculate the probability that A wins this third backoff race in its second attempt
to transmit A3.
7. (18 marks) [Wireless Network (Learning Outcomes 2, 3)]
Consider the scenario shown in the following figure, in which there are four
wireless hosts, A, B, C, and D. The radio coverage of the four hosts is shown via the
shaded ovals; all hosts share the same frequency band. Suppose that each host has
an infinite supply of messages that it wants to send to other hosts. If a message’s
destination is not an immediate neighbor (i.e., the destination is not in radio
coverage of the sender), then the message must be relayed. For example, if A
wants to send to D, a message from A must first be sent to B, which then sends the
message to C, which then sends the message to D. Time is slotted, with a message
transmission time taking exactly one time slot. As always, if a host hears two or
more simultaneous transmissions, a collision occurs and none of the transmitted
messages are received successfully. Assume that there are no bit-level errors, and
thus if exactly one message is sent, it will be received correctly by those within the
transmission radius of the sender. Give the answers in message(s)/time slot in the
following questions, and briefly explain how you come up with the numbers.
€
T = 51.2µs
€
T
€
0 × T
€
1× T
€
0 × T
€
1× T
€
0 × T
€
1× T
€
2 × T
€
3 × T

(1) What is the maximum rate at which data messages can be transferred from D to
B, given that there are no other messages between any other source/destination
pairs?
(2) Suppose now that A sends messages to B, and D sends messages to C. What is
the combined maximum rate at which data messages can flow from A to B and from
D to C?
(3) Suppose now that A sends messages to B, and C sends messages to D. What is
the combined maximum rate at which data messages can flow from A to B and from
C to D?
(4) Suppose now that the wireless links are replaced by wired links. Repeat
questions (1) through (3) again in this wired scenario.
(5) Now suppose we are again in the wireless scenario, and that for every data
message sent from source to destination, the destination will send an ACK message
back to the source. Also suppose that each ACK message’s transmission takes up
one time slot. Repeat questions (1)–(3) above for this scenario.
Submission:
You can write your answers in a word document or other document at your choice.
Please convert your answer document to a a4-yourstudentid.pdf file and submit
the PDF file on Moodle before 23:59 Friday May 15, 2020:
(1) Login Moodle.
(2) Find “Assignments” in the left column and click “Assignment 4”.
(3) Click “Add submission”, browse your .pdf file and save it. Done.
(4) You will receive an automatic confirmation email, if the submission was
successful.
(5) You can “Edit submission” to your already submitted file, but ONLY before the
deadline.