L13 - Data Communication, Error Detection, CRC

Question 1

What is meant by the ‘curse of the inverse-square law’?

Answer 1

When transmitting to a system, if the power received by the system is low, noise becomes a severe problem for the communication.

Question 2

What is mean by Error Detection?

Answer 2

Detecting the error bits within the data being transmitted.

Question 3

What is Error Correction Coding?

Answer 3

Adding redundancy to data being transmitted in order to safeguard against noise and improve communication reliability.

Question 4

Define noise

Answer 4

Data communication interference caused by bit flipping in the raw information.

Question 5

What are the 2 types of algorithms that deal with noise interference?

Answer 5

Error Detection : Detects the data packet cause the error during communication.

Error Correction : Corrects detected error.

Question 6

What are the 2 main methods of error correction?

Answer 6

1. Reverse error correction
2. Forward error correction

Question 7

Explain Reverse Error Correction…

Answer 7

1. The message receiver identifies the error in the data.
2. Receiver informs sender, who re-transmits the information until receiver confirms its error free.

Question 8

When is Reverse Error Correction not suitable?

Answer 8

For real-time communication.

Question 9

When communicating to a system, what is the the power proportional to?

Answer 9

1 / (distance^2)

Question 10

What does compression do?

Answer 10

Removes redundancy from raw data, reducing chance of interference.

Question 11

What are the 2 methods of detection bit errors?

Answer 11

Parity check
Cyclic redundancy check

Question 12

What is the Parity Check? Explain how it works…

Answer 12

1. An error detection method
   2. Adds an extra 1 bit to the data being sent and sends the data.
   3. Upon receiving, the receiver operates XOR on the data to establish the data’s parity bit.
   4. If parity bit is same as it was when sent, the data is error free.

Question 12

What is the difference between Odd and Even Parity Checks?

Answer 12

1. Odd -> Adds 1 to ensure an off number of 1’s
   2. Even -> Adds 1 to ensure an even number of 1’s

Question 12

What is an issue with Parity Check error detection? What is the solution to this?

Answer 12

1. Only detects errors if 1 bit has been flipped. If 2, then the number of 1’s will be even, thus the parity will be the same. Thus, the error won’t be detected.
   2. Cyclic Redundancy Check is a solution to this.

Question 13

How many bits does the encoder/decoder have for CRC-4, CRC-8, CRC-16 functions?

Answer 13

4 bits, 8 bits, 16 bits.

Question 14

What operation is performed on bits in a CRC function?

Answer 14

XOR

Question 15

What are the 4 steps of CRC Generation at the sender side?

Answer 15

1. Find length L of the divisor.
   2. Append L-1 bits to the original message.
   3. Perform binary division operation.
   4. Remainder of the division is the CRC ( must be L-1 bits )

Question 16

What are the steps for the receiver to take during CRC?

Answer 16

Upon receiving data with the sender CRC check appended to it, use the divisor to perform XOR on the data.

Question 17

How does the receiver know that there are no errors in the CRC transmitted data?

Answer 17

When the result of XOR on the transmitted data is all 0’s bits.

Question 18

How does CRC work?

Answer 18

1. Establish the sender CRC bit code
   
   2. Append the sender CRC code to the data to be transmitted.
   3. Send message
   4. Receiver performs XOR on transmitted data

Question 19

How can we represent the encoder / decoder of the CRC in polynomial form? Write 1101 in polynomial form…

Answer 19

1. Can be written such that the bit is the coefficient of each variable.
2. 1101 = 1X^3 + 1X^2 + 0X^1 + 1X^0 = X^3 + X^2 + X^0

Question 20

What is the advantage of writing the encoder/decoder in polynomial form?

Answer 20

Polynomial division and multiplication enables efficient encoding and decoding.