ECE 358 Final Exam, S’2012
© 2012 University of Waterloo
Dept. of Electrical and Computer Engineering
ECE 358 – Computer Networks
Instructor:
... [Show More] Sagar Naik
Exam. Date and Time: August 07, 2012, 9:00 AM—11:30 AM
Room: PAC 6
Instructions
You have two hours and 30 minutes to complete the exam.
This is a closed book exam. You may use a calculator.
Answer all questions.
All the acronyms are common in computer networks, and those have been explained in class.
Should there be a need to make an assumption to answer a question, state your assumption and
proceed.
Student ID Student Name Signature
Total = /55
All the best…
Question Marks Question Marks
Q1 /10/ /10 Q4 /12
Q2 /10 Q5 /05
Q3
/08 Q6 /10
ECE 358/ Final Exam/ Spring 2012 Page 2 of 11
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Q1. General [10 marks]
1. What is a data communication protocol?
2. Identify two reasons for packets (data or control) without bit errors to be dropped in computer
networks.
3. Identify two reasons for different packets (data or control) to encounter different delays between
the same pair of source and destination in a multi-hop computer network.
4. The retransmission timers in TCP are estimated based on actual measurements of round-trip
segment delays, whereas no measurement is performed to estimate retransmission timers in
CSMA/CD and CSMA/CA protocols. What is the reason for such a philosophical difference in the
estimation of timers?
5. Identify two concepts that make the internet scalable.
6. What is the advantage of using a random initial sequence number (ISN) in a TCP connection?
7. Explain how TCP level piggybacking enhances router performance.
8. What are three communication parameters sent in the option field of a TCP segment?
9. Write the four sources of delay encountered by an IP packet while moving from one router to
another, and attribute them to routers and links.
10. What are the two general principles for correcting bit errors in computer networks?
11. Why does the slotted Aloha protocol give better throughput performance than the pure Aloha
protocol?
ECE 358/ Final Exam/ Spring 2012 Page 3 of 11
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12. Explain the need for an ACK mechanism in the CSMA/CA MAC protocol and no need for an ACK
mechanism in the CSMA/CD protocol.
13. What is head-of-line (HOL) blocking in the network layer? Why does it not occur in the CSMA/CD
MAC protocol?
14. Identify two main advantages of the layered communication architecture used on the internet.
15. What is the difference between flow control and congestion control in TCP operation?
16. State one advantage and one disadvantage of considering hop count as a cost metric in intra-AS
routing.
17. Why does the two-node instability problem not occur in OSPF?
18. Given that routers forward packets on a hop-by-hop basis in the network layer, why do some
routers run transport layer and application layer protocols to support packet routing?
19. What is the sum of the lengths of TCP and IPv4 headers without the option fields?
20. A TCP entity’s RTT measurements fall in the range of [300 ms – 302 ms]. Estimate of the range of
values of the TCP entity’s RTO.
ECE 358/ Final Exam/ Spring 2012 Page 4 of 11
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Q2 Link Layer [10 marks]
1. [5] Describe the CSMA/CD protocol discussed in class by means of a flowchart. Discuss the
impact of replacing the binary exponential backoff scheme with a fixed-length backoff scheme.
2. [5] Consider a 120 Mbps (megabits per second) Ethernet switch-based local area network (LAN)
comprising 100 desktop computers and one router. The router on the LAN is connected to
another router via a 500 Mbps link for internet connectivity. All data frames exchanged
between the desktops and the first router are 1500 byte long.
a. [1] What are the three different kinds of delays that a MAC frame encounters while moving
between a desktop computer and the first hop router?
b. [1] What kind of multiplexing is used between the two routers?
c. [1] If all the desktops are equally very busy downloading data, at what rate can one
desktop download data from the internet?
d. [1] Calculate the transmission time of a MAC frame from a desktop.
e. [1] If the 500 Mbps link is replaced with a 1Mbps link, your answers to what parts above –
(a) through (d) – will change? [Show Less]