COMP 3270 Computer Networks
Final Test, Fall 2012
(Note that you should use all the correct data units.)
Student Number:
Name:
1.
(2 marks) List the three network performance criteria.
2.
(2 marks) Define what a protocol is.
3.
(2 marks) List the main responsibilities of the layers, except the application layer, in the Internet network model.
4.
(2 marks) List the four address types in the Internet network model, with the corresponding layer in which each address type is
mainly used.
5.
(2 marks) The IP protocol entity running on a router received an IP packet that goes to another host computer system. List the
four steps used by the IP protocol entity to forward the IP packet to the next node.
6.
(2 marks) Assume a system uses the Internet network model. If the application program creates a message of 500 bytes and each
layer (including all the layers) adds a head of 20 bytes to the data unit, what is the efficiency (the ratio of the number of bytes in
the application message to the number of bytes transmitted) of the systems?
1(6)
7.
(2 marks) The attenuation of a signal is -20 dB. What is the final signal power if the original signal power is 500 mW? (Note that
dB = 10 log10(P2/P1).)
8.
(2 marks) A file contains 100 images and each image consists of 400 × 500 pixels. Each pixel uses 4096 colours. If the
bandwidth of a channel is 20 Kbps, how long does it take to send this file?
9.
(2 marks) How many bits can fit on a link with 8 ms delay if the bandwidth of the link is 800 Kbps?
10. (4 marks) We have a low-pass signal with a bandwidth of 40 KHz. We use 256 quantization levels.
a) Calculate the sampling rate using the Nyquist theorem, not the Nyquist Bit Rate formula.
b) Calculate the bit rate of the digitized signal.
c) We need to send the above digitized signal using NRZ-L. What is the minimum required bandwidth?
d) What is the minimum required bandwidth to send the digitized signal in b), using the combination of 4B/5B and NRZ-L
coding?
11. (2 marks) A corporation has a medium with a 1 MHz bandwidth (lowpass). The corporation needs to create 10 separate
independent channels each capable of sending at least 10 Mbps. The company decided to use QAM technology. What is the
minimum number of bits per baud for each channel? What QAM should be used?
12. (2 marks) Assume that a voice channel requires a bandwidth of 4 KHz. We need to multiplex 20 voice channels with guard
bands of 500 Hz using FDM. Calculate the required bandwidth.
2(6)
13. (3 marks) We need to use synchronous TDM and combine 8 digital sources, each of 120 Kbps. Each output slot carries 4 bits
from each digital source, but one extra bit is added to each frame for synchronization.
a) What is the output frame rate?
b) What is the size of an output frame?
c) What is the output data rate?
14. (2 marks) Assuming odd parity, find the parity bit for each of the following data units.
a) 1001011
b) 1110111
15. (3 marks) Compute the following bitwise operations.
a) 10001 XOR 10001
b) 11100 XOR 00000
c) 10011 XOR 11111
d) 10001 AND 10001
e) 11100 AND 00000
f)
10011 AND 11111
g) 10001 OR 10001
h) 11100 OR 00000
i)
10011 OR 11111
16. (2 marks) Assume a packet is made only of 3 8-bit words 0x2B, 0x3F, and 0xAF. Find the 8-bit checksum.
3(6)
17. (2 marks) Given the dataword 101101 and the divisor 1101, show the generation of the CRC codeword at the sender site.
18. (2 marks) Briefly explain the two strategies CS and CD in CSMA/CD, which are used to avoid collision.
19. (2 marks) How does a VLAN reduce network traffic?
20. (3 marks) In the Stop-and-Wait ARQ algorithm, what are the receiver’s correct actions in the following cases?
a) The data frame is corrupted.
b) The data frame is not corrupted, but the sequence number in the data frame is wrong.
c) The data frame is not corrupted, and the sequence number in the data frame is correct.
21. (2 marks) Rewrite the following IP address using binary notation: 206.123.168.46.
22. (2 marks) Rewrite the following IP addresses using dotted-decimal notation:
11001100 00101110 01100011 00110111
4(6)
23. (2 marks) Combine the following three blocks of addresses into a single address block.
198.162.32.0/24
198.162.33.0/24
198.162.43.0/23
24. (3 marks) An ISP is granted the block 198.162.32.0/20. The ISP needs to allocate addresses for two organizations: organization A
with 900 addresses, and organization B with 480 addresses. Find the addresses of the address blocks allocated to the two
organizations. The unsigned integer values of the addresses allocated to organization B must be smaller than the addresses
allocated to organization A.
25. (2 marks) Here is the routing table at a router.
Destination Network
Mask
Next hop
180.70.65.192
/26
A
180.70.65.128
/25
B
201.4.22.0
/24
C
201.4.16.0
/22
D
0.0.0.0
/0
E
Decide the next hops when the router receives IP packets with the destination IP addresses – 180.70.65.140, 180.70.65.221,
201.4.32.78, and 201.4.17.35. (Assume there is no source routing option in packets.)
26. (2 marks) In a LAN, two computers are assigned the same IP address mistakenly. Can they communicate with other computers
outside the LAN? Justify your answer.
20. (2 marks) List all the fields in an IPv4 packet, which are changed by all the routers on the path from the sender to the receiver.
5(6)
28. (2 marks) Here is a port address translation table in a NAT gateway for a private network of 172.18.0.0/16.
Private
Private
External
Internal
External
Transport
address
port
address
port
port
protocol
172.18.3.1
2500
198.162.3.2
2345
80
TCP
172.18.4.2
2500
198.162.3.3
4567
20
UDP
172.18.5.3
2500
198.162.3.3
4567
20
TCP
172.18.6.4
3600
198.162.3.3
5678
22
TCP
After the NAT gateway receives an IP packet from inside with the destination IP address 198.162.3.3 and the destination TCP
port 22, the NAT gateway sends the packet to the destination host outside after the gateway translates the packet. List the
destination IP address, the source IP address, the destination port and the source port in the packet that is forwarded to the
destination host from the gateway.
29. (2 marks) The distance vector routing algorithm is running on a network. On the network, a router A contains the following
routing information.
Network
cost
next
N1
8
E
N2
4
C
N3
3
D
N4
3
B
Assuming the cost to C from A is 2, draw the updated routing information on A after it receives the following routing information
from its neighbor router C.
Network
cost
N2
1
N3
3
N5
2
30. (2marks) List the two advantages of IPv6 when compared to IPv4.
6(6)