University of Bahrain
College of Information Technology
Department of Computer Engineering
Computer Networks II– ITCE 416/ITNE 241
Academic year: 2022– 2023
Semester II
Midterm Exam
Student Name:
Student ID:
Course Code
ITNE 241
Section Number
1
ITCE 416
2
3
4
Se
Dr.Reham Almesaeed Section 1,2,3
Dr.Amal Abu Hassan
Section 4,5
Duration:
1 Hour: 15mins
Date
30th April, 2023
Question
Q1: True/ False [2.5 Marks]
Q2: Multiple choice [2.5 Marks]
Q3: Multiple access & Error correction [9 Marks]
Q4: Switching and LAN [13 Marks]
Q5: Wireless Networks [3 Marks]
Total [ 30 Marks]
Gained points
5
Q1: Mark the following statements True or False: [2.5 Marks]
1. 2D-parity check can detect and correct any case of two bits flips in the payload.
F
2. In channel partitioning protocols, there can be simultaneous transmission resulting in F
collisions.
3. In pure Aloha, if only one node is active it transmits at the full channel transmission T
rate.
4. In CSMA/CD, when a node detects collision, it aborts the transmission and retransmit F
immediately.
5. A switch is a transparent device, it doesn’t have a MAC but it does have an IP address. F
Q2: Select one or more correct answers: [2.5 Marks]
Note: If you choose more than one answer, which some right and some
wrong, you will get half of the grade.
1. If single bit, even parity check is used, which of the following received
data will be accepted by the receiver:
a) 101011001
b) 110010101
c) 100100101
d) 100000100
2. In which of the following multiple access protocols, a collision will
never happen:
a) Aloha
b) CSMA and CSMA/CD
c) FDMA/TDMA
d) Polling
2
3. Which of the following multiple access protocols is fully decentralized:
a) Slotted Aloha
b) CSMA/CD
c) FDMA/TDMA
d) Polling
4. Which of the following is not true about switches (select one):
a) Switches forward and store MAC frames
b) Switches are plug and play devices
c) Switches use ARP to map devices to their different ports
d) Switches supporting VLAN capability provide virtual LANs over
single physical LAN
5. In MPLS (Multiprotocol Label Switching), which of the following is
not true (select one):
a) The datagram forwarding is based on fixed length label
b) In label and out label are added to the routing table
c) A device can have more than one out label
d) The IP addresses are removed from the datagram
Q3: Answer the following questions in relation to multiple access & error correction
[9 Marks]
a. Explain the role of the binary backoff timer in the CSMA/CD protocol. (1.5 Marks)
Answer: The backoff timer force devices to wait random times to randomize their access to the shared
medium so this reduce possibility of collision
Note: The following answers not accepted:
1. You only explain CSMA/CD and you mention the timer and how it is calculated !
2. Or you just say it is waiting time.
3
b. At the sender side the data link layer of a device uses CRC for error detection, the data to be
sent is 101010 and the generator G = 10011, what will be the full bit sequence to be sent?
(3 Marks)
Answer:
1 mark for adding FOUR zeros to the data , if you added different number of zeros, you will get 0.5
1 Mark for showing right division , if you didn’t get the right answer, you will get 0.5
1 Mark for saying the full sequence is 101010 0111, so CRC bits are 0111, if you didn’t mention the full
sequence, you will get ZERO in this part.
c. Assume a wired LAN network with bus topology operating with CSMA/CD, and a total of 50
devices connected to the bus. Each node transmits a large number of frames, and the capacity
of the link is 100 Mbps. The data link layer generates a maximum frame size of 536 bytes, with
the signal propagating at speed of 2X108 m/s.
( 3 Marks)
i.
Calculate the maximum propagation delay to achieve an efficiency of 80% in
CSMSA/CD protocol. (1.5 Marks)
Answer:
Part 1: 0.5 Mark for showing the right efficiency formula.
Part 2: 0.5 mark for calculating the transmission time which is Frame size/ data rate= 536 byte*8/100 Mbps
Part 3: 0.5 for finding out the value of Propagation from the formula (Note that the formula has one
unknown value which (Tprop)). You have to mention the unit in second or ms,etc…
Note:
1. If you calculated cable length or propagation delay you will get 0 in this part 3.
2. If you didn’t mention unit in propagation delay, you will get 0.25 in part 3.
4
ii.
Explain the impact of network cable length on CSMA/CD efficiency. Justify your
answer. (1.5 Marks)
Answer: Longer cable length results in higher propagation delays, which increases probability of collision,
therefore reduce CSMA efficiency.
OR
You explain how longer cables lead to higher propagation delays and lower efficiency according to the
formula used in previous question.
Note:
1. If you just say longer cable, less efficiency, you will get 1 Mark only.
d. Discuss an advantage of CDMA over FDMA access, then discuss one limitation in CDMA
(1.5 Marks)
Answer:
Part 1: 1 Mark for advantage, which is mainly CDMA utilize the full bandwidth/spectrum while FDAM
the spectrum is divided between users, so CDMA can transmit at full rate.
Part 2: 0.5 Mark for discussing limitation for CDMA which could be , the limited number of orthogonal
codes, or interference between codes.
Note:
1. If you explain how FDMA and CDMA work, this is not accepted as answer.
2. If you say CDMA is cheaper or has less interference, this is not right !!
5
Q4: Answer the following questions in relation to switching and LAN.
[13 Marks]
a. Compare routers and switches in the way they isolate the traffic. (1 Marks)
Answer: Router can configure routing tables to create subnet and allow the traffic to be forwarded
in certain subnet, while switch has to use port-based VLAN to provide traffic isolation
0.5 Mark for router + 0.5 Mark for the switch
Note:
1. If you compare between router and switch in how they build their routing/switching table, this
not accepted as it is not relevant to the question!
2. If you say router uses MPLS to isolate traffic, this is not right, as we talk about traffic isolation
here not virtualization.
b. Consider the following multi-switch LAN, and the sequence of events that happened in
the network. Then fill the switching tables for S1, S2, S3, S4. (Show the switch table
for every switch individually). (5 Marks)
Assume the following events occurred in the network and the switching tables initially
have the following content:
Note: S3 table is empty.
B
2
▪
Host C sent frame to B.
C
3
▪
Host B replied to C.
H
4
E
2
B
4
▪
Host B sent frame to E.
▪
H sent frame to B
60
S1
60
S2
6
F
3
4
B
1
2
H
2
F
3
B
H
60
60
S4
S3
Answer:
1. 1 Mark for adding TTL to all tables.
2. Each table is 1 mark, if the table has mixed of right and wrong entries, you will get 0.5.
c. Consider the figure below, where the IP and MAC addresses of the different devices
(A, B, C, D and routers interfaces) are shown. Assume an IP datagram being sent
from node B to node A. (4 Marks)
I.
What are the source and destination MAC addresses at point 3?
Source (0.5 Mark):55-CE-A1-CD….
Destination (0.5 Mark): F7-28-63….
II.
What are the source and destination IP addresses at point 3?
Source (0.5 Mark):128-119-163-13…
Destination (0.5 Mark): 128-119-128..
III.
What are the source and destination MAC addresses at point 2?
Source (0.5 Mark):A4-B2-D9….
Destination (0.5 Mark): 1D-64-1C…
IV.
Does the source and destination MAC addresses change at point 1 compared
to point 2?
NO
Note: If the you switch destination and source, you will get Zero.
7
d. In gigabit ethernet operating at full duplex, no collision occurs and hence no
CSMA/CD is required, Justify.
(1 Marks)
Answer: In gigabit ethernet, the topology is star, so each switch has local buffer, and each interface has
it own local interface, so no collision expected.
e. Assume there are two physical switches that each connect two VLANs representing
two different institution departments, as shown in the tables below. (2 Marks)
Assume each switch has a total of 8 ports
Switch 1:
Switch 2:
Department 1 Ports:1,2,4,5
Department 1 Ports: 3,4,5,6
Department 2 Ports: 3,6,7
Department 2 Ports: 1,2
I.
Show through a sketch how to connect the switches, and state what is required?
You need to draw two switches and show they are connected with 1 wire through
trunk port.
8
II.
Can users move between switches while maintaining the same VLAN connectivity?
How?
Answer: Yes (0.5 Mark) , by configuring the new port which is connected to the device to assign it to the
same VLAN ID (0.5 Mark)
Note: If you just say use VLAN, or trunk port, this is wrong!
Q5: Answer the following questions in relation to Wireless Networks.
[ 3 Marks]
a) What is the difference between infrastructure and no infrastructure networks, then give an
example on each. (1.5 Marks)
Answer: 0.5 Mark for difference: Infrastructure: has access point (base station ),etc…, while no infrastructure
no access point.
0.5 Mark for example on infrastructure: Wi-Fi, 4G,etc…. (Note: Internet is not accepted)
0.5 Mark for example on no infrastructure: Ad-hoc, Bluetooth,etc…
b) List three impairments of the wireless signals when it propagates in the wireless channel.
(1.5 Marks)
Answer: 0.5 Mark for each impairment such as:
1.
2.
3.
4.
5.
Frequency
Distance
Multipath propagation
Interference
Environment , noises….
-End of Exam-
9