Ans.An
error is the change or the mismatching take place between the data unit
sent by transmitter and the data unit received by the receiver e.g.
10101010 sent by sender 10101011 received by receiver. Here is an error
of 1 bit.
Q. 2. Define Error Control.
Ans.Error
control refers to mechanisms to detect and correct errors that occur in
the transmission of frames. The most common techniques for error
control are based on some or all of the following:
1, Error detection
2. Positive acknowledgement
3. Retransmission after time-out
4. Negative acknowledgement and retransmission.
These mechanisms are also referred as automatic repeat request (ARC)).
Q. 3. What are three types of redundancy checks used in data communication?
Ans. Error
detection uses the concept of redundancy, which means adding extra bits
for detecting errors at the destination there ate three types of
redundancy checks are common in data communication:
(a) Parity check
(h) Cyclic Redundancy check (CRC)
(c) Checksum.
Q. 4. How can the simple parity bit detect a damaged data unit?
Ans.In
this technique, a redundant bit called a parity bit, is added to every
data unit so that the total number of Is in the unit becomes even (or
odd). Suppose we want to transmit 1100001. Adding the number of 1’s
gives us 3, an odd number. Before transmitting, we pass the data unit
through a parity generator. The parity generator counts the 1’s and
appends the parity bit to the end (al in this case).
Q. 5. What is the difference between even parity and odd parity?
Ans.In case of redundancy check method we have to append the data unit with some extra bits. These extra bits are called parity.
This parity or parity hit can be even or odd.
in
case of even parity we have to make number of 1’s even, including the
parity hit e.g. 1110001 is the data unit where the no. of l’s is already
even then we will insert 0 at the next to data unit it’, 1110001. In
case of odd parity we have to make no. of l’s odd, including the parity
bit. e.g. 1111000 is the data unit, where the no. of 1’s is even then we
will insert I at the next to data unit i.e. 11110001.
Q. 6. Define code world?
Ans.The
code word is the n bit encoded block of bits. As already seen it
contains message bits and parity or redundant bits as shown in the
following figure.
Q. 7. Define Code rate?
Ans,The code rate is defined as the ratio of number of message bits (K) to the total number of hits (n) in the code word.
Q. 8. Define code efficiency?
Ans.The code efficiency is defined as the ratio of message bits to the number of transmitted bits per block.
Q. 9. What are the disadvantages of coding?
Ans.(1) Coding makes the system complex.
(2)
As increased transmission bandwidth is required in order to transmit
the encoded signal. This is due to the additional hits added by the
encoder.
Q. 10. Suppose the sender wants the word “HELLO”. In ASCII the five characters are coded as:
What will be the combination of actual bits to send?
Ans. 11101110 11011110 11100100 11011000 11001001
Q. 11. How the receiver will detect that there is an error in:
Ans. The receiver counts the 1’s in each character and comes up with even numbers (b, 6, 4, 4, 4). The data are accepted.
Q. 12. Suppose the word HELLO is corrupted during transmission?
How receiver will check it out?
Ans. The
receiver counts the 1’s in each character and comes up with even and
odd numbers (7, 6, 5, 4, 4). The receiver knows that the data are
corrupted, discards them and asks for Retransmission.
Q. 13. Explain about error correction.
Ans. Error correction is the mechanism by which we can make changes in the received erroneous data to make it free from error.
The two most common error correction mechanisms are:
(i) Error correction by Retransmission.
(ii) Forward Error Correction.
Q. 14. What is check sum?
Ans.Checksum
is the one of the method used for error detection, based on the concept
of redundancy. In this mechanism, the unit is divided into K sections,
each of n bits. All sections are added using ones complement to get the
sum. This is complemented and becomes the check sum. There after this
check sum is sent with the data. At the receiver side the unit is
divided into K sections each of n bits. All sections are added using
ones complement to get the sum. The sum is complemented. If the result
is zero data are accepted otherwise rejected.
Q. 15. Discuss the two dimensional parity check and the types of errors it can and cannot detect.
Ans. Apart from simple parity check two-dimensional parity is the better approach.
In this method, a block of bits is organized in a table (rows and columns).
First we calculate the parity bit for each data unit then we organize them into table.
Data and Parity bits
A
redundancy of n bits can easily detect a burst error of n bits. A burst
error of more than n bits is also detected by this method with a very
high probability.
But
if 2 bits in our data unit are damaged and two bits in exactly the same
positions in another data unit are also damaged, the checker will not
detect an error.
Q. 16. What are the different types of error?
Or
How a single bit error does differ from a burst error?
Ans. A
single bit error is an isolated error condition that alters one bit but
does not affect nearby bits. On the other hand A burst error is a
contiguous sequence of bits in which the first and last bits and any
number of intermediate bits are received in error.
A
single bit can occur in the preserve of while noise, when a slight
random deterioration of single-to-noise ratio is sufficient to confuse
the receiver’s decision of a single bit. On the other hand burst errors
are more common and more difficult to deal with. Burst error can be
caused by impulse noise.
Q. 17. What is Error detection?
Ans. Regardless
of the design of the transmission system, there will be errors,
resulting in the change of one or more bits in a transmitted frame. When
a code word is transmitted one or more number of transmitted bits will
be reversed due to transmission impairments. Thus error will be
introduced. It is possible to detect these errors if the received code
word is not one of the valid code words. To detect the errors at the
receiver, the valid code words should be separated by a distance of more
than 1.
The
concept of including extra information in the transmission of error
detection is a good one. But instead of repeating the entire data
stream, a shorter group of bits may be appended to the end of each unit.
This technique is called redundancy because the extra bits are
redundant to the information; they are discarded as soon as the accuracy
of the transmission has been determined.
Q. 18. Discuss the concept of redundancy in error detection.
Ans. It
is a most common and powerful technique for the detection of errors. In
this technique extra bits are added. But instead of repeating the
entire data stream, a shorter group of bits may be appended to the end
of each unit. The technique is called redundancy because the extra bits
are redundant to the information. They are discarded as soon as the
accuracy of transmission has been determined.
The following fig. shows the process of using redundant bits to check the accuracy of data unit.
Once
the data stream has been generated. It passes through a device that
analyzes it and adds on appropriately’ coded redundancy check. The
receiver puts the entire stream through a checking function. If the
received bit stream passes the checking criteria, the data portion of
the data unit is accepted and redundant bits are discarded.
Q.19. Explain any one Mechanism used for error detection?
Or
What is the Parity check Method of Error detection?
Ans. The most common and least expensive mechanism for error detection is the parity check.
Parity checking can be simple or two-dimensional.
Simple Parity Check
In
this technique, a redundant bit, called a parity bit, is added to every
data unit so that the total number of Is in the unit (including the
parity bit) becomes even (or odd). Suppose we want to transmit the
binary data unit 1100001
Transmission Mode
Adding
the no. of is giving us 3 an odd number. Before transmitting we pass
the data unit through a parity generator. The parity generator counts
the is and appends the parity bit to the end. The total no. of is now 4,
an even number. The system now transmits the entire expanded unit
across the network link. When it reaches its destination, the receiver
puts all 8 bits through an even parity checking function. If the
receiver sees 11000011, it counts four is, an even number and the data
unit passes. But, if instead of 11000011, the receiver sees 11001011
then when the parity checker counts the Is it gets 5 an odd number. The
receiver knows that an error has been introduced into the data somewhere
and therefore rejects the whole unit.
Two Dimensional Parity Check
A
better approach is the two dimensional parity check in this method, a
block of bits is organised in a table (rows and columns). First we
calculate the parity bit for each data unit. Then we organise them into
table. Shows in fig we have four data units shown in four rows and eight
columns. We then calculate the parity hit for each column and create a
new row of 8 bits. They are the parity bits for the whole block. The
first parity bit in the fifth row is calculated based on all first bits,
the second parity bit is calculated based on all second bits, and so
on. We then attach the 8 parity bits to the original data and sent them
to the receiver.
Q.20. Explain CRC method of Error Detection?
Ans. Cyclic Redundancy Check (CRC):
Cyclic Redundancy check method is most powerful mechanism of error
detecting. Unlike the parity check which is based on addition, CRC is
based on binary division.
In
CRC, instead of adding bits to achieve a desired parity, a sequence of
redundant bits, called the CRC or the CRC remainder, is appended to the
end of a data unit so that the resulting data unit becomes exactly
divisible by a second predetermined binary number. At its destination
the incoming data unit is divided by the same number. If at this step
there is no remainder, the data unit is assumed to be intact and is
therefore accepted. A remainder indicates that the data unit has been
damaged in transit and therefore must be rejected.
The
redundancy bits used by CRC are derived by dividing the data unit by a
predetermined divisor, the remainder is the CRC. A CRC must have two
qualities. It must have exactly one less bit than the divisor, and
appending it to the end of the data string must make the resulting bit
sequence exactly divisible by the divisor.
CRC generator and checker
First,
a string of n 0’s is appended to the data unit. The number n is less
than the number of bits in the predetermined divisor, which are n + 1
bits.
Second,
the newly formed data unit is divided by the divisor, using a process
called binary division the remainder resulting from this division is the
CRC.
Third,
the CRC of n bits derived in step 2 replaces the appended Os at the end
of the data unit. The data unit arrives at the receiver data first
followed by the CRC. The receiver treats the whole string as a unit and
divides it by the same divisor that was used to find the CRC remainder.
If
the string arrives without error, the CRC checker yields a remainder of
zero and the data unit passes. If the string has been changed in
transit the division yields a non zero remainder and the data unit does
not pass.
Q.21. How is the check sum method of error detection take place?
Ans. Checksum is the third mechanism for error detection which is also based on the concept of redundancy.
Check sum Generator
In
the sender, the check sum generator subdivides the data unit into equal
segments of n bits. These segments are added using ones complement
arithmetic in such a way
that
the total is also n bits long. That total is then complemented and
appended to the and o the original data unit as redundancy bits called
the check sum field. The extended data unit is transmitted across the
network. So if the some of data segment is T, the checksum will be T.
Check sum Checker
The
receiver subdivides the data unit as above and adds all segments and
complements the result. If the extended data unit is intact, the total
value found by adding the data segments and the check sum field should
be zero If the result is not zero, the packet contains an error and the
receiver rejects it.
Q.22.
How the data communication between sender and the receiver will take
place where the error detection method is check sum and the data is :
Ans. Sender
The numbers are added using one’s complement arithmetic
Q. 23. What is hamming code of Error Correction? How it calculate, the redundancy?
OR
Explain any one method used for error correction.
Ans. The hamming code can be applied to data units of any length and uses the relationship between data and redundancy bits.
Suppose
there are 7 bits ASCII codes which requires 4 redundancy bits that can
be added to the end of the data unit or interspersed with the original
data bits. These units are position in 1, 2, 4, arid 8 (the position is
in an 11 bit sequence that are powers of 2). We prefer these bits are
r1, r2, r4 and r8.
Q. 24. What are various error correction codes?
Ans. A mechanism that can handle correction of an error heading of error correction code categories under the
There are two methods for error correction.
(1) Error correction by retransmission.
(2) Forward error correction.
Error Correction by Retransmission
In
error correction by retransmission, when an error is discovered, the
receiver can have the sender retransmit the entire data unit.
Forward Error Correction
In
forward error correction (FEC), a receiver can use an error-correcting
mode, which automatically corrects certain errors. In theory it is
possible to correct any error automatically. Error correcting codes
however are more sophisticated than error detection codes and require
more redundancy bits.
e.g.
To correct a single bit error in an ASCII character, the error
correction code must determine which of the 7 bits has changed In this
case we have to distinguish between eight different states no error,
error in position 2, and so on, up to the error in position 7. To do so
requires enough bits to show all eight states.
At
first glance, it seems that or 3-bit redundancy code should be adequate
because 3 bits can show eight different states (000 to 111) and can
therefore indicate the locations of eight different possibilities. To
calculate the no. of redundancy bits. We should consider
Where m is the no. of bits to be transfer r stands for the no. of redundancy. By this manner.
There is the practical solution for this method that is “Hamming Code”.
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