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CS 447 – Networks and Data Communications
Final Exam, Spring 2002
April 30, 6:30-8:10 P.M.
This question is a closed-book exam. However, you can use one 11”  8.5” cheat sheet created by
your own hands (no photocopied or pasted cheat sheet is allowed). Your cheat sheet must be
strictly within 11”  8.5” (no “balcony” please). There are 8 questions in this exam. You have
100 minutes to finish the questions.
Please write your answers on separated pieces of papers.
To avoid grading problems, please put your papers in the ascending order in the question number.
You can use a calculator during this exam (no sharing calculator, please).
Name: ________________ ID (Optional): ____________
QUESTION #1 (7.5 Minutes – 10 points):
(1)
Sketch the OSI 7-layer protocol model. Describe the purpose of each layer by some
keywords
(2) What is the “synchronous transmission”?
Give the most significant definition of the
synchronous transmission (one most significant definition). What are the advantage and
disadvantage (one for each)?
(3) For parity error detection, what is the probability of non-detectable errors? Give the
formula (you need to define any variable you are going to use within the formula).
Assume n = # of bits in every word (byte) and p = the probability of a bit error.
(4)
What are the packet-switching networks?
QUESTION #2 (7.5 Minutes – 10 points):
(1) Give the five major measurements to be used for routing (make sure to include at least the
one being used in the Internet and the one that will be used in the future Internet).
(2) Which address class does an IP address, 191.117.8.1, belong to?
(3) Suppose we have three networks connected by two routers (as shown below). Show the
Source IP address, source MAC address, destination IP address and destination MAC
address of packets at “This Point” if packetsLAN
are transmitting
from host A to F.
2
LAN 1
131.247.XX.XX
143.163.XX.XX
C
D
LAN 3
28.181.XX.XX
Router #1
B
Router #2
E
This Point
F
A
Assume the following network setting:
Host A: 28.181.1.1 (MAC address: 1050)
Host B: 28.181.1.2 (MAC address: 1ABC)
Router #2 (LAN 3 side): 28.181.1.100 (MAC address: 2019)
Router #2 (LAN 2 side): 143.163.32.99 (MAC address: 1021)
Host C: 143.163.32.1 (MAC address: 3082)
Router #1: (LAN 2 side): 143.163.32.98 (MAC address: 7547)
Router #1: (LAN 1 side): 131.247.68.99 (MAC address: 7546)
Host D: 131.247.68.1 (MAC address: ABCD)
Host E: 131.247.68.2 (MAC address: 9356)
Host F: 131.247.68.3 (MAC address: 5466)
(4) Suppose we use CSMA/CD for MAC. Given: network distance = 1500m, the signal
propagation speed = 200m/s and transmission speed = 500Mbps, what is the minimum
frame size to guarantee collision detection?
QUESTION #3 (10 Minutes – 10 points):
Suppose there is a ring network where there are 100 hosts.
Each host is connected to the ring
network by a tapping repeater. Each tapping repeater needs to insert one bit delay. Show all
your work.
Assume the following rules:
1. Whichever host that has "a token" can transmit data (data as a packet).
2. If a host finishes transmitting a packet, the host has to wait for the transmitted packet to
come back on the ring.
3. Then the host removes the transmitted packet.
4. When a transmitting host completely receives the packet (and the host removes all the
bits in the packet from the ring), the host sends its token to its next neighbor.
Given: every host always has data to transmit, each packet contains 200 bits, all links are 100m
long, transmission rate = 1 Mbps and signal propagation delay is 200m/s, what will be
the utilization of this ring network?
Note: the size of token message is small enough to
be ignored.
QUESTION #4 (15 Minutes – 10 points):
Derive the formula for the link utilization for Go-Back-N ARQ, given P is the probability of a
frame error.
You can first show the average number of actually transmitted frames for
successfully transmitting one frame.
Decent partial credit will be given for this question.
Show as much as you can in 15 minutes (or you have extra time).
QUESTION #5 (10 Minutes – 10 points):
A transport layer message consisting of 1600 bits of data and 150 bits of header is sent to an
internet layer, which appends another 150 bits of header. This is then transmitted through two
networks, each of which uses a 24-bit packet header. The destination network has a maximum
packet size of 400 bits.
How many bits, including headers, are delivered to the network-layer
protocol at the destination?
QUESTION #6 (10 Minutes – 10 points):
A disadvantage of the contention approach for LANs is the capacity wasted due to multiple
stations attempting to access the channel at the same time.
Suppose that time is divided into
discrete slots with each of N stations attempting to transmit with probability p. What fraction
of slots is wasted due to multiple simultaneous transmission attempts for p = 0.03 and N = 15?
Assume that stations will start transmission only at the beginning of a slot. Show all your
work (2 points for a correct solution and 8 points for showing your work for the correct
solution).
QUESTION #7 (15 Minutes – 10 points):
(1) Why “hierarchical routing” is important?
Give the two reasons you think the most
important.
(2) The following is the figure that shows various transmission methods. We can see that
circuit-switching transmission is the best in terms of both simplicity and throughput,
especially compare to datagram transmission. If it is so, why datagram transmission is
used for the Internet? Give two most important reasons.
Simplest
Circuit Switching
Operation at a router
Multirate
Circuit
Switching
Cell Relay
(ATM)
Most Complex
Frame Relay Packet Switching
(IP Network)
(3) Mention the four different types of Internet traffic the IETF’s ISA architecture tries to
efficiently transmit.
Name the four types of Internet traffic and give one network
application to each of the four traffic types.
QUESTION #8 (15 Minutes – 15 points):
Suppose that a sender is sending data to a destination using frame-relay transmission. There
are 15 intermediate routers between the sender and the receiver on the path between the sender
and the destination. Assuming that sliding window flow control is used for the frame-relay
transmission, calculate the open window size to achieve utilization = 100%. Show all your
work.
Assumptions:
(1)
Each link is 50 Km (1Km = 1000m)
(2)
Frame size is 125 bytes (including the frame header)
(3)
Frame size is 125 bytes for all the 15 routers
(4)
Transmission rate = 250 Mbps (M = 106) for all the 15 routers
(5)
Propagation delay = 200m/s
(6)
Ignore any overhead except frame transmission and propagation delay
(7)
No error during frame transmission and frame propagation