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Computer Communication & Networks MID Term Exam (Spring 2009) Page 1 / 11
Computer Communication
& Networks
Spring 2009
MID Term Exam
Friday, 3rd
April 2009
Total Time: 90 Minutes
Total Marks: 46
Roll Number _____________________
Name ___________________________
Signature: __________
________________________________
Signature of Invigilator
Course Instructors: Engr. Waleed Ejaz
Q 1 2 3 4 5 Total
Marks 9 10 12 9 6 46
Obtained
Marks
You are advised to READ these notes:
1. Attempt the paper on the question paper. NO EXTRA SHEETS will be provided. Use the
back of the page if more space is required. However, no extra sheet will be checked.
2. After asked to commence the exam, please verify that you have nine (9) different printed pages
including this title page.
3. There are 5 questions. Attempt all of them. It is advisable to go through the paper once before
starting with the first question.
4. All questions don’t carry equal marks. Marks for subparts are indicated.
5. If part of a problem depends on a previous part that you are unable to solve, explain the method
for doing the current part, and, if possible, give the answer in terms of the quantities of the
previous part that you are unable to obtain.
6. Exam is closed books, closed notes. Please see that the area in your threshold is clean. You will be
charged for any material which can be classified as ‘helping in the paper’ found near you.
7. Calculator sharing is strictly prohibited.
8. Students who attempt the paper with lead pencils loose the right to get them rechecked.
9. The invigilator present is not supposed to answer any questions. No one may come to your room
for corrections and you are not supposed to request to call anyone. Make assumptions wherever
required and clearly mark them.

Question 1 [3+6]
1. The following diagram shows the bottom three layers of OSI model on two machines.
a. Which layers on which machines would be affected if the protocol defined for layer 2 on machine
A was changed?
Solution:
Layer 2 on machine B
b. Which layers on which machines would be affected if the service defined for layer 1 on machine
A was changed?
Solution:
Layer 2 on machine A
2. Consider the sliding window ARQ protocol. Suppose that the protocol uses a very large sequence
number space that would not wraparound, and it starts from 0. And the sender always has frames to
send. The receiver always sends an ACK when receiving a CRC-validated data frame.
Let SWS = 2 and RWS = 1. Moreover, we introduce a new retransmission scheme to the sender. In
this scheme, the sender will immediately send the oldest unacknowledged frame if it receives an old
ACK. An ACK is considered old if it has been received before or the value of the ACK is 0. Assume
that this retransmission scheme always retransmits earlier than the case with timeout mechanism.
a. Draw a time-line diagram for the case where the first data frame (SN = 0) has bit errors and is
dropped by the sender. All subsequent transmissions (data and ACK) are assumed to be error-free.
Draw the diagram until the receiver’s NFE (next frame expected) = 3.

Part a: A B
S=0 F0 R=0
F1
Ack F0 discards F1
S=0
F0
R=0
Ack F1 R=1
S=1
F1
R=1
Ack F2 R=2
S=1
F2
R=3
Ack F3

b. Draw a time-line diagram for the case where the transmissions of the first data frame (SN = 0) and
its ACK are error-free, but the second data frame (SN = 1) has bit errors and is dropped by the
sender. All subsequent transmissions (data and ACK) are assumed to be error-free. Draw the
diagram until the receiver’s NFE (next frame expected) = 3.
Part b: A B
S=0
F0
R=0
Ack F1 R=1
F1
S=0
F2
Discards F1
Ack F1
S=1
F1
R=1
Ack F2 R=2
S=1
F2
R=2
Ack F3 R=3
S=3
R=3
R=4

Question 2
[2+2+2+4]
1. What is the channel capacity for a teleprinter channel with a 300 Hz bandwidth and a signal-to-noise
ratio of 30 dB, where the noise is white thermal noise?
2. Use PCM to translate voice signal. How much data rate is required?
3. A digital signaling system is required to 9600bps and the channel bandwidth is 1.2 KHz. What is the
maximum numbers of discrete voltage levels?
4. A telephone line is known to have a loss of 30 dB. The input signal power is measured as 0.1 W, and
the output noise level is measured as 2.5μW. calculate the output signal-to-noise ratio in dB.
(log2=0.301)

Question 3 [6+6]
1. The mapping between messages a1a2a3a4 and codewords a1a2a3r3a4r2r1 of a (7, 4) Block code is given
as follows. We use Hamming Code to add even-parity-checks bits, where r1=a1+a3+a4, r2=a1+a2+a4,
and r3=a1+a2+a3
a. Given two messages m1=1000 and m2=0101, find the codewords w1 and w2.
Solution:
w1=1001011, w2=0101101
b. What is the Hamming distance of codewords w1 and w2? What is the code rate?
c. Determine the Minimum distance and error-correction capacity this code
Message Codeword Message Codeword
0000 0000000 1000 ?
0001 0000111 1001 1001100
0010 0011001 1010 1010010
0011 0011110 1011 1010101
0100 0101010 1100 1100001
0101 ? 1101 1100110
0110 0110011 1110 1111000
0111 0110100 1111 1111111

2. The following block of bits is received at the input to a channel decoder:
11001000101001
Determine if this block of bits contains any bit errors (and, if possible, the locations of any such
errors) if the channel coding scheme is based on:
a. Even Parity
b. Odd parity based hamming Coding
c. CRC-4 coding with a generator polynomial of G(x)= x4
+x3
+1
Comment on the limitations of each scheme in terms of the maximum number of bit errors that
can be reliably processed in a single block.
(a) 11001000101001
No Error is detected it can detect 1 bit or odd numbers of errors.
(b)
1 1 0 0 1 0 0 0 1 0 1 0 0 1
14 13 12 11 10 9 8 7 6 5 4 3 2 1
r8 r4 r2 r1
r1 1 3 5 7 9 11 13 1
r2 2 3 6 7 10 11 14 0
r4 4 5 6 7 12 13 14 1
r8 8 9 10 11 12 13 14 0
Hamming distance limitizes error detection and correction capability.
(c) 1000000011

11001 11001000101001
11001
000000 Limitations:
000000
000000 It can only detect not correct the
000000 errors
0000
0000
0001
0000
0010
0000
0101
0000
1010
0000
10100
11001
11011
11001
010 Remainder
Question 4 [1+1+1+2+2+2]
We are sending a 30 Mb file from a source host to a destination host. Suppose the message sent consists
of the entire file. All links in the path between source and destination have a transmission rate of 10
Mbps. Assume that the propagation speed of each link is 2 * 108
meters/sec, and the distance between
source and destination is 10,000 km.
a. Initially, suppose there is only one link between source and destination. What is transmission
delay?
transmission delay is
dtransmission = message size = 30 Mbits = 3 sec
transmission rate of outbound link 10 Mbps
b. Referring to the above question, what is the end-to-end delay (transmission delay plus propagation
delay)?
dprop = distance = 107 meters = 0.05 sec
speed 2 X 108 meters/sec
dend-to-end= dtransmission + dprop = 3 + 0.05 = 3.05 sec

c. Referring to the above question, how many bits will the source have transmitted when the first bit
arrives at the destination?
At time t=0, the first bit will be pushed out from the source and it will incur a
propagation delay = 0.05 sec. So, it will arrive at B at t = 0.05 sec.
The transmission rate of the link is 10 Mbps. So, when the first bit arrives at B
in 0.05 sec, the source will have transmitted:
10 Mbps X 0.05 sec = 0.5 Mbits = 0.5 X 106 = 500,000 bits
d. Now, suppose there are two links between source and destination, with one router connecting the
two links. Each link is 5,000 km long. Suppose processing delay is negligible and there is no
congestion, so that the message is transmitted onto the second link as soon as the router receives
the entire message. What is the end-to-end delay?
processing delay is negligible and there is no congestion, so that the message
is transmitted onto the second link as soon as the router receives the entire
message. The end-to-end delay is
dtransmission (Source-Router) = message size = 30 Mbits = 3 sec
dprop (Source-Router) = distance = 5 X 106 meters = 0.025 sec

The transmission and propagation delays from Router to Destination are the
same.
dend-to-end= (dtransmission + dprop ) X 2 = (3 + 0.025) X 2 = 6.05 sec
e. Now, suppose there is only one link between source and destination, and there are 10 TDM
channels in the link. The file is sent over one of the channels. What is the end-to-end delay?
Transmission rate of one channel= link transmission rate=10 Mbps = 1 Mbps
no. of channels 10 channels
f. Now, suppose there is only one link between source and destination, and there are 10 FDM
channels in the link. The file is sent over one of the channels. What is the end-to-end delay?
Transmission rate of one channel = 10 Mbps = 1 Mbps
10 channels

Question 5 [3+3]
a. Suppose that a synchronous serial data transmission is clocked by two clocks (one at the sender
and one at the receiver) that drift one minute per day in opposite direction. Assume that a bit
waveform will be good if it is sample within 40 percent of its center. How long a sequence of bits
can be sent before possible clock drift could cause a problem?
b. A data source produces 15-bit characters. We use synchronous transmission, with a frame
consisting of 176 control bits and 1024 information bits. The information field contains 16-bit
(parity included) characters. Derive as expression of the maximum effective data rate over a 2
kbps line.

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