GAT General 2013 Schedule (Note it Down)
For more information visit NTS official web site www.nts.org.pk
Q. 1. What is an Error?
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)
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?
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?
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
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 organized in a table (rows and columns). First we calculate the parity bit for each data unit. Then we organize 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 :
The numbers are added using one’s complement arithmetic
Q. 23. What is hamming code of Error Correction? How it calculate, the redundancy?
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”.
Very Impotent Note
The above questions & answers had been taken from a website. Some of the answers contain figures as well you can find all those pictures from the website link is given blow