CS 447 Network and Data Communication
Midterm Exam No.2
Spring, 2003
7:30-8:45 P.M.
April 8, 2003
This question is a closed book and closed note exam. There are 6 questions in this exam.
You have 75 minutes to finish the questions. Please write your answers on separate
pieces of papers. To avoid grading problems, please staple your papers in the ascending
order in the question number. Please bring a calculator (no calculator sharing, please).
Name: _______________________________
Student ID (optional): ___________________
Question #1 (10 minutes - 10 points)
(1) What are the two potential problems for throughput in “buffered-statistical” TDM?
Mention two possible problems (2 minutes) – 2 points
(2) What are the two types of switches used in a circuit-switching network? (1 minutes) –
2 points
(3) What are the internal and external operations in the Internet? (1 minutes) – 2 points
(4) What are the two types of switching networks? (1 minutes) – 2 points
(5) Compare, circuit switching, virtual circuit packet switching and datagram packet
switching networks in terms of  bandwidth sharing of links and  message storing
at each router (you can answer by just mentioning the keywords, you do NOT have
describe your solution) (5 minutes) – 2 points
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Question #2 (15 minutes - 20 points)
(1) What is the minimum requirement (i.e., how much transmission rate is required) for
the shared transmission cable for a statistical TDM shown below (make sure this
TDM is not a blocking statistical TDM)? – (10 minutes) 10 points

N input lines

Multiplexer
N output lines
De-Multiplexer
Shared Transmission Cable
Assumptions are:




Each slot now has a slot header (the slot header includes only the information
that indicates the destination output line – nothing else).
Assuming there are 128 input lines.
All the input lines have the same transmission rate of 10Mbps.
The slot includes 35 bits as user payload for each input line (excluding the
slot header)
(2) Assume that the buffered-statistical TDM will be used. If the link bandwidth of the
shared transmission cable is 900 Mbps, and each input line always transmits data at
120 Mbps, what is the requirement for the throughput of the memory buffer (in the
number of bits that have to go through the memory unit per second), if N = 10? – (5
minutes) 10 points
Shared Transmission Cable

N input lines

Multiplexer
De-Multiplexer
Memory Buffer
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N output lines
Question #3 (10 minutes - 10 points)
(Exercise #8.13) Ten 57.6-Kbps lines are to be multiplexed using TDM. Ignoring
overhead bits, what is the total capacity required for synchronous TDM? Assuming that
we wish to limit average line utilization of 75%, and assuming that each line is busy 45
percent of the time, what is the capacity required for statistical TDM?
Question #4 (10 minutes - 10 points)
(Exercise #9.2) Consider a simple telephone network consisting of two end offices and
the intermediate switch with a 1-MHz full-duplex trunk (trunk = a shared transmission
cable) between each end office and the intermediate switch. The average telephone is
used to make four calls per 8-hour workday, with a mean call duration of six minutes.
Ten percent of the calls are long distance (i.e., not local – they go through the
intermediate switch). What is the maximum number of telephones and end office can
support?
Question #5 (10 minutes - 15 points)
(Exercise #10.2) Define the following parameters:
N: Number of hops between two given end hosts
L: Message length in bits
B: Data rate in bps on all links
P: Fixed packet size in bits
H: Overhead bits per packet
D: Signal propagation delay
Assume that the signaling message to set up a path and the ACK message for path set up
so small that the packet transmission delay is negligible for both of them. Also assume
that there is no overhead to process the signaling message.
N = 4, L = 3200, B = 9600, P = 1024, H = 16, D = 0.001, compute the estimated time
required to complete the data transmission.
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Question #6 (20 minutes - 10 points)
This question is about TDM bus switch. The figure below shows the internal structure of
typical 33 TDM bus switch. The transmission rate of the shared bus is usually R  n
Mbps where R is the transmission rate (in Mbps) of each input (assume that all input lines
have the same transmission rate and they are always transmitting at their full transmission
rate and there are n input lines).
TDM Bus switch
Input Buffer
Input lines
Switch Control Unit
A
B
Output switch
C
Input switch
X
Y
Z
Output lines
Shared Bus
The length of the TDM bus (i.e., the shared cable within a TDM bus switch) will be a
significant problem if transmission rate of each input line increases. The problem is the
signal propagation delay in the bus. Even after a bit has been transmitted at an input
switch, the output switch must hold a connection until the bit signal propagates in the bus
cable to the target output switch (this means that there must be some waiting time as
much as the signal propagation delay for the bus cable). Only after this, the switch of the
next input line can start transmit its data.
Question (a): In order to demonstrate this problem, calculate the utilization of the
transmission cable if R = 1,000 Mbps (= 1 Gbps) and n = 3. Assume that bit interleave is
used (i.e., the bus transmits a bit for each input at a time) and the transmission rate of the
bus cable is 3 Gbps. Assume that the distance of the bus cable is 10 meters and the signal
propagation speed is 300,000Km/second. Assume that the bus length is same for any
input-output port pair. (15 minutes)
Hints: 1 bit at 1Gbps = 1ns, which is 10-9 second.
Question (b): If two-bit interleaving (the bus transmits two bits for an input line at a time)
is used, what will happen to the bus cable utilization (will it increase or decrease)? (5
minutes)
Hint: this question is really easy, if you understand Question (a) above. You could
answer this question within two minutes without calculator.
CS 447-Network and Data Communication, Midterm Exam #2, April 8, 2003
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