CS 447-Network and Data Communication
Midterm Exam No.2
Spring, 2005
11:00-12:15 P.M.
March 29, 2005
This exam is a closed-book and closed note exam. There are 8 questions in this exam. You
have 75 minutes to finish the questions. Please write your answers on separated piece of
papers (unless specified to write a solution in your question paper). To avoid grading problems,
please staple your papers in the ascending order in the question number. Calculator can be
used during this exam, but you can not share a calculator with anyone else.
Name: _______________________________
Student ID (optional): ___________________
QUESTION #1 (10 minutes)
#1: Which layers does the internal operation consist of?
#2: Give an example of real networks for each of the following three switching networks (you
can answer your solutions in the underlined blanks below):
(1) Circuit switching networks: _______________ (one example)
(2) Datagram packet switching networks: _______________ (one example)
(3) Virtual circuit packet switching networks: _______________ (one example)
#3: Show which layers in the TCP/IP model correspond to which layers in the OSI 7-layer
model.
#4: Show the organization of the Internet structure (how end hosts (i.e., your PCs) are connected
and how the Internet is organized - as shown in the classroom).
#5: What are the major advantages and disadvantages in the datagram networks?
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QUESTION #2 (10 minutes)
#1: Given the following assumptions, what is the condition to be satisfied fore a TDM switch
(i.e., Statistical TDM) to be a non-blocking switch (show the condition)?
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Number of input lines is N lines.
Slot payload size is L bits.
The peak transmission rate for each input is R bps (the peak transmission rate is
same for all input lines).
The average utilization is U for all the input lines.
#2: What is the primary bottle-neck for scalability in each of the three different types of
switches? Mention only one (but most significant one). If more than one is answered,
whichever appeared first in your solution will be graded (and all the others will be ignored
even if they are correct). You can answer your solutions in the underlined blanks
below.
(1) Cross-bar switches: _______________________
(2) TDM switches: _______________________
(3) Bus switches: _______________________
#3: Define non-blocking and blocking switches (You can answer your solutions in the
underlined blanks below).
(1) Non-blocking switches are: ___________________________________________
_________________________________________________________________
_________________________________________________________________
(2) Blocking switches are: ___________________________________________
_________________________________________________________________
_________________________________________________________________
#4: What are the two factors for decision making (how are routing decisions made?) of routing
algorithms (those we discussed)?
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QUESTION #3 (5 minutes)
Complete the following table for packet switching networks. Answer by either YES or NO,
except for the entry with * symbol. For “overhead bits, answer by largest (overhead or delay),
middle and smallest.
Factors
Fixed bandwidth?
Packets?
Stored messages?
Path setup?
Overhead bits (*)?
Same path?
Delay at a switch (*)
Circuit Switching
Datagram
Virtual Circuit
QUESTION #4 (10 minutes)
Assume that you are at node (router) A in the figure below. Find (1) minimum-cost path (a
sequence of nodes to reach a destination) for each destination and (2) shortest distance (the cost
for the minimum path) to each destination in the network using Bellman-Ford network. Show
all the work.
1,043
6,032
B
D
5,025
1,019
A
1,015
2,017
F
5,011
2,018
10,031
C
1,037
E
QUESTION #5 (8 minutes)
Forty-eight 14,400-bps lines are to be multiplexed using TDM. Ignoring overhead bits in the
TDM frame, what is the total (link) capacity required for synchronous TDM? Assuming that we
wish to limit the average link utilization of 0.85, and assuming that each link is busy 72% of the
time, what is the capacity required for statistical TDM? Show all your work.
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QUESTION #6 (10 minutes)
For the following questions are from the required exercise questions. There was no change
added to the question (i.e., the questions asked here are all exactly as given in the textbook).
#1: (This is Exercise Question #8.4 in the textbook): In synchronous TDM, it is possible to
interleave bits, one bit from each channel participating in a cycle. If the channel is using a
self-clocking code to assist synchronization, might this bit interleaving introduce problems
because there is not a continuous stream of bits from one source?
(1) Answer by YES or NO. (2) Attach a very short (up to two lines of a sentence)
explanation (or description) to each of your solutions.
#2: (This is Exercise Question #10.3 in the textbook): Explain the flaw in the following
reasoning: Packet switching requires control and address bits to be added to each packet.
This introduces considerable overhead in packet switching. In circuit switching, a
transparent circuit is established. No extra bits are needed. Therefore, there is no
overhead in circuit switching. Because there is no overhead in circuit switching, line
utilization must be more efficient than in packet switching.
#3: (This is Exercise Question #10.6 in the textbook): Assuming no malfunction in any of the
stations or a network, is it possible for a packet to be delivered to a wrong destination?
(1) Answer by YES or NO. (2) Attach a very short (up to two lines of a sentence)
explanation (or description) to each of your solutions.
#4: (This is Exercise Question #12.11 in the textbook): Will Dijkstra’s algorithm and the
Bellman-Ford algorithm always yield the same solutions?
(1) Answer by YES or NO. (2) Attach a very short (up to two lines of a sentence)
explanation (or description) to each of your solutions.
QUESTION #7 (5 minutes)
Here are some questions from the textbook. All the questions are very simple and easy (and I
guarantee that you can answer these questions without any problem as long as you read the
textbook). Your solution should be concise (up to three lines of a sentence, at most).
#1: Give one example of “space-division switches”. Make sure to answer only one example.
If more than one example is answered, whichever appears first in your solution will be
graded (if “which one is the first” is difficult to determine, Fujinoki will make a decision).
You can answer in the blank below:
Example of “space-division switches”: ________________________________________
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#2: Describe the effect of packet size on transmission frames.
#3: What is “adaptive routing”?
Adaptive routing is: _______________________________________________________
________________________________________________________________________
QUESTION #8 (12 minutes)
Question (1): Calculate the data transmission time for circuit switching and datagram packet
switching for the following assumptions (a through i).
(a) Signal transmission rate is 100Kbps (K = 103)
(b) Amount of payload to be transmitted = 3,000 bytes
(c) There are three intermediate switches between a sender and a destination (total of four
hops)
(d) The link distance for the four hops is all same and the signal propagation delay for
each link is 20ms.
(e) For datagram transmission, there is 10ms switching delay at each switch (there is no
switch delay for circuit switching).
(f) “the data transmission time” is defined in the attached figure.
(g) The frame (packet) size is 1,000 bits
(h) Assume that there is no header in frames (this is not true in the real situation, but just
to simplify this question).
(i) There is no signal processing delay for circuit switching.
 Provide your solution in ms.
 Show all your work.
Question (2): Which one results in the shorter data transmission time?
Question (3): In what situation(s) circuit switching is expected to result in shorter data
transmission time than datagram in the above case (the case shown in (1))?
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CS447, Network and Data Communication, Midterm #2, March 29, 2005
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