CUSTOMER_CODE
SMUDE
DIVISION_CODE
SMUDE
EVENT_CODE
OCTOBER15
ASSESSMENT_CODE BCA3040_OCTOBER15
QUESTION_TYPE DESCRIPTIVE_QUESTION
QUESTION_ID
73511
Explain the following error correcting methods.
QUESTION_TEXT a. Automatic Repeat Request(ARQ)
b. Hamming Code
a.
Automatic Repeat Request (ARQ):

Protocols in which the sender waits for a positive
acknowledgement before advancing to the next data item are
called ARQ. ARQ also transmit data only in one direction.
Although, it can handle lost frames by timing out, it requires the
timeout interval to be long enough to prevent premature time
outs.
2M

If the sender times out too early, while the acknowledgement is
still on the way, it will send duplicates. When the previous
acknowledgment finally arrives the sender will mistakenly think
that the just sent frame is the one being acknowledged and will
not realize that there is acknowledgment frame somewhere in
the pipe.
2M

If the next frame sent is lost completely but the extra
acknowledgment arrives correctly, the sender will not attempt to
retransmit the lost frame and the protocol will fail. 1M
SCHEME OF
EVALUATION
b.
Hamming Code:
o
The error-detecting and error-correcting properties of a
code depend on its hamming distance. The number of bit
positions in which two code words differ is called
hamming distance. An n-bit unit containing data and
check-bits is often referred to as n-bit code
word.
2M
o
To detect errors, we need a distance d+1 code because
with such a code there is no way that d-single bit error can
change a valid code word into another valid code word.
When the receiver sees an invalid code-word, it can tell
that a transmission error has
occurred.
2M
o
Similarly, to correct errors, we need a distance 2d+1 code
because that way the legal code words are so far apart
that even with d change, the original code word is still
closer than an any other code word. So it can be uniquely
determined.
1M
QUESTION_TYPE DESCRIPTIVE_QUESTION
QUESTION_ID
QUESTION_TEXT
SCHEME OF
EVALUATION
73513
Explain the following digital signal encoding formats.
a. Bipolar-AMI
b. Pseudoternary
c. Manchester code
d. Differential Manchester and biphase technique
a. Bipolar-AMI: In this case, a binary 0 is represented by no line
signal, and binary 1 is represented by a positive or negative pulse. The
binary 1 pulse must alternate in polarity. The advantages of this scheme
are no loss of synchronization. If a long string of 1s occurs, no net dc
component and the less bandwidth for resulting signal. Also, the pulse
alternation property provides a simple means of error
detection.
2M
b. Pseudo ternary: In this case, binary 1 is represented by the
absence of line signal, and the binary 0 is represented by alternating
positive and negative pulses. There is no particular advantage of one
technique versus the other, and each is the basic of some applications.
This code provides synchronization. But, a long string of 0s is the case of
AMI or 1s in the case of pseudoternary still presents a
problem
2M
c. Manchester Code: In Manchester coding states there will always be
a transition at the middle of each bit period. This provides means of
adding the data rate clock to the message to be used on the receiving
end.
1M
d. Differential Manchester: In this case the mid bit transition is used
only to provide clocking. The encoding of a 0 is represented by the
presence of a transition at the beginning of the bit period. All of the
biphase techniques require at least one transition per bit time and may
have as many as two transitions. Thus, the maximum modulation rate is
twice that for NRZ. 2M
Advantages of biphase schemes are:
(1x3)M

Synchronization: Because there is a predictable transition during
each bit time, the receiver can synchronize on that transition.

No dc component: Biphase codes have no dc component.

Error detection: The absence of an expected transition can be
used to detect errors. Noise on the line would have to invert
both the signal before and after the expected transition to cause
an undetected error.
QUESTION_TYPE DESCRIPTIVE_QUESTION
QUESTION_ID
73515
QUESTION_TEXT Explain the various types of communications.
The various types of communication are:
SCHEME OF
EVALUATION

Point to point communication: In this type, communication
takes place between two end points. For instance, in the case of
voice communication using telephones, there is one calling party
and one called party. Hence the communication is called point to
point.
2M

Point to multipoint communication: In this type of
communication, there is one sender and multiple recipients. For
example, in voice conferencing, one person will be talking but
many others can listen. The message from the sender has to be
multicast to many others.
2M

Broadcasting: In this, there is a central location from which
information is sent to many recipients, as in case of audio or video
broadcasting. In a broadcasting system, the listeners are passive,
and there is no reverse communication path.
1M

Simplex communication: In simplex communication,
communication is possible only in one direction. There is one
sender and one receiver, the sender and the receiver cannot
change roles.
1M

Half Duplex communication: In this, communication is
possible in both directions between two entities, but one at a
time. A walkie talkie uses this approach. The person who wants to
talk presses a talk button on his handset to start talking, and the
other persons handset will be in the receive mode. When the
sender finishes, he terminates it with an over message. These types
of systems require limited channel bandwidth, so they are low cost
systems.
2M
Full Duplex communication: IN a full duplex communication
system, the two parties, the caller and the called can
communicate simultaneously, as in a telephone system. The
ability of the communication system to transport data in both
directions defines the system as full
duplex.
2M
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
73517
Write a note on bridges.
QUESTION_TEXT
SCHEME OF
EVALUATION
The bridge provides a means of interconnecting similar LANs. The
bridge is designed for use between local area networks that use identical
protocols for the physical and link layers and the amount of processing
required at the bridge is minimal.
(4 marks)
The functions of bridge are few:
Read all frames transmitted on A and accept those addressed to any
station on B
Using the medium access control protocol for B
Do the same for B-to-A traffic.
Several design aspects are:
The bridge makes no modifications to the content of the frames it
receives.
The bridge should contain enough buffer space to meet peak demands
The bridge must contain addressing and routing intelligence.
A bridge may connect more than two LANs
(6 marks)
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.