CUSTOMER_CODE
SMUDE
DIVISION_CODE
SMUDE
EVENT_CODE
SMUAPR15
ASSESSMENT_CODE BCA3040_SMUAPR15
QUESTION_TYPE DESCRIPTIVE_QUESTION
QUESTION_ID
QUESTION_TEXT
73512
Explain the Digital Pulse Code and Adaptive Differential Pulse Code
modulation techniques.
(1 mark for each point)
SCHEME OF
EVALUATION

During the PCM process, the difference between successive
input sample signals is minimal. A common technique used
in speech coding is to predict the value of the next sample
from that of the preceding samples.

This is possible because of the correlations in speech
samples due to the effect of vocal tract and the vibrations of
the vocal chords.

DPCM schemes quantize the difference between the original
and the predicted signals. This means a reduction in the
number of bits used per sample over that used for PCM.
Using DPCM can reduce the bit rate of voice transmission
down to 48Kbps. DPCM can be described as predictive
coding scheme.

The first part of DPCM works like PCM in that input signal is
sampled at a constant sampling frequency, and the samples
are modulated using amplitude modulation.

The sampled input signal is then stored in a predictor. The
predictors send the stored sample signal through a
differentiator. The differentiator compares he current
sample signal with the previous sampling signal and sends it
to the quantizing and coding phase of PCM.

After quantizing and coding, the difference signal is
transmitted. At the receiver, the difference signal is dequantized, added to a sample signal stored in a predictor
and sent to a low-pass filter that reconstructs the original
input signal.

Although DPCM reduce the bits rate for voice transmission,
the uniform quantization used means that large sample
signals have higher signal-to-noise ratio than small sample
signals, so voice quality is better at higher signals.
Adaptive Differential Pulse Code
modulation:
(2+1)marks
o
In ADPCM the predictor and quantizer are adaptive.
It adapts the quantization levels of different signals
that are generated during the DPCM process. If the
difference signal is low, ADPCM reduces the size of
the quantization levels. If the difference signal is high
ADPCM increases the size of quantization levels.
o
The quantization level is adapted to the size of the
input difference signal, generating a uniform signalto-noise ratio throughout the dynamic range of the
difference signal.
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
73514
Discuss the important requirements for wireless LAN.
QUESTION_TEXT
SCHEME OF
EVALUATION
The important requirements of wireless LAN
are:
(1x10)M
Throughput: The medium access control protocol should make
efficient use of wireless medium to maximize capacity.
Number of nodes: Wireless LANs may need to support hundreds of
nodes across multiple cells.
Connection to backbone LAN: IN most cases, interconnection with
stations on a wired backbone LAN is required. There may also need
to be accommodation for mobile users and ad hoc wireless network.
Service area: A typical coverage area for a wireless LAN has a
diameter of 100m to 300m.
Battery power consumption: Mobile workers use battery-powered
workstations that need to have a long battery life when used with
wireless adapters.
Transmission robustness and security: Wireless LAN must permit
reliable transmission even in a noisy environment and should provide
some level of security from eavesdropping.
Collocated network operation: Interference between the LANs is
possible when two or more wireless LANs operate in the same area
or in some area. Such interference may thwart the normal operation
of a MAC algorithm and may allow unauthorized access to a
particular LAN.
License Free operation: Users would prefer to buy and operate
wireless LAN products without having to secure a license for the
frequency band used by the LAN.
Handoff/roaming: The MAC protocol used in the wireless LAN
should enable mobile stations to move from one cell to another
without any problems.
Dynamic configuration: The MAC addressing and network
management aspects of the LAN should permit dynamic and
automated addition, deletion, and relocation of end systems without
disruption to other users.
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
73516
QUESTION_TEXT What is congestion control? Explain one congestion control technique.
SCHEME OF
EVALUATION
Congestion control is concerned with controlling traffic entry into a
network so as to avoid congestive collapse by attempting to avoid
oversubscription of any of the processing or link capabilities of the
intermediate nodes and networks and taking resource reducing steps,
such as reducing the rate of sending packets. (2 marks)
Explain any one of the following techniques:
Backpressure
Choke packet
Implicit congestion signalling
Explicit congestion signaling
(8 marks)
QUESTION_TYPE DESCRIPTIVE_QUESTION
QUESTION_ID
QUESTION_TEXT
125759
Explain the different types of errors and the error detecting methods.
Different types of errors are:
Single bit error: It means only 1 bit of a given data unit is changed from 1 to
0 or from 0 to 1. Single bit errors are the least likely type of error in serial
data transmission. For example of 00000010 was sent, but 00001010 was
received, meaning single bit error resulted 2M
Burst Error: Means 2 or more bits in the data unit have changed from 1 to 0
or from 0 to 1. Suppose that the data 0100010001000011 was sent, but
0101110101100011 was received. Note that the burst error results. More
bits in the data unit have changed. Error does not mean that the errors
occur in consecutive bits. The length of the burst is measured from the first
corrupted bit to the last corrupted bit. Some bits in between may not have
been corrupted. A burst error is more likely to occur than the single-bit
errors.
3M
Error Detection methods:
The different error detection methods are:
SCHEME OF
EVALUATION
1. Parity checking method
2. Cyclic Redundancy Check (CRC)
Parity Checking is a simple error detection method used with character
oriented protocols. One bit in every character bit sequence is reserved for
parity. This bit is set by the transmitter to 0 or 1 so that the total number of
one bit in the character is always even or always odd. The receiver checks
for parity bit of each character to see if it as expected. Parity checking is
effective in detecting an odd number of bit errors in an character. If even
numbers of bits in a character are corrupted then this will go
unnoticed.
2.5M
CRC: The CRC is a technique for detecting errors in digital data, but not for
making correction when errors are detected, It is used in data transmission.
In the CRC, a certain number of check bits, often called a checksum, are
appended to the message being transmitted. The receiver can determine
whether or not to check bits agree with the data, to as certain with a
certain degree of probability whether or not an error occurred in
transmission. If an error occurred, the receiver sends a “negative
acknowledgement” back to the sender, requesting that the message be
retransmitted.
2.5M
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
125760
QUESTION_TEXT Explain the following:
a. Pure ALOHA
b. Slotted ALOHA
c. CSMA
d. CSMA/CD
a.
Pure ALOHA: The original ALOHA protocol is called pure
ALOHA. This is a simple, but elegant protocol. The basic idea is that
each station sends a frame whenever it has a frame to send. since
there is only one channel to share, there is the possibility of collision
between frames from different stations.
b.
Slotted ALOHA: Slotted ALOHA was invented to improve the
efficiency of pure ALOHA. In slotted ALOHA the time is divided
into slots of the average time required to send out a frame, and force
the station to send only at the beginning of the time slot.
SCHEME OF
EVALUATION
c.
Carrier Sense Multiple Access (CSMA): If a station senses the
medium before trying to use it. then collision can be reduced. Carrier
sense multiple access requires that each station first listen to the
medium before sending. When a station sends a frame, and any other
station tries to send a frame during this time, a collision will result.
But if the first bit of the frame reaches the end of the medium, every
station will already have heard the bit and will not send its own
frame.
d.
Carrier Sense Multiple Access with Collision Detection
(CSMA/CD): The CSMA method does not specify the procedure
following a collision. But the carrier sense multiple access with
collision detection (CSMA/CD) method, a station monitors the
medium after it sends a frame to see if the transmission was
successful. If, so the station is finished. If there is any collision, the
frame is sent again.
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
125761
QUESTION_TEXT
What are advantages and disadvantages of digital communication
over analog communication?
Advantages
SCHEME OF
EVALUATION
1.
Immunity to noise is relatively high.
2.
Efficient use of communication bandwidth
3.
Data encryption
4.
The ability to detect errors and correct them.
5.
Designing and manufacturing is cheaper.
6.
Much data communication is computer to computer.
Disadvantages
1.
Requires more bandwidth
2.
Additional encoding and decoding circuitry required.