L13 - Data Communication, Error Detection, CRC Flashcards
What is meant by the ‘curse of the inverse-square law’?
When transmitting to a system, if the power received by the system is low, noise becomes a severe problem for the communication.
What is mean by Error Detection?
Detecting the error bits within the data being transmitted.
What is Error Correction Coding?
Adding redundancy to data being transmitted in order to safeguard against noise and improve communication reliability.
Define noise
Data communication interference caused by bit flipping in the raw information.
What are the 2 types of algorithms that deal with noise interference?
Error Detection : Detects the data packet cause the error during communication.
Error Correction : Corrects detected error.
What are the 2 main methods of error correction?
- Reverse error correction
- Forward error correction
Explain Reverse Error Correction…
- The message receiver identifies the error in the data.
- Receiver informs sender, who re-transmits the information until receiver confirms its error free.
When is Reverse Error Correction not suitable?
For real-time communication.
When communicating to a system, what is the the power proportional to?
1 / (distance^2)
What does compression do?
Removes redundancy from raw data, reducing chance of interference.
What are the 2 methods of detection bit errors?
Parity check
Cyclic redundancy check
What is the Parity Check? Explain how it works…
- An error detection method
- Adds an extra 1 bit to the data being sent and sends the data.
- Upon receiving, the receiver operates XOR on the data to establish the data’s parity bit.
- If parity bit is same as it was when sent, the data is error free.
What is the difference between Odd and Even Parity Checks?
- Odd -> Adds 1 to ensure an off number of 1’s
- Even -> Adds 1 to ensure an even number of 1’s
What is an issue with Parity Check error detection? What is the solution to this?
- 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.
- Cyclic Redundancy Check is a solution to this.
How many bits does the encoder/decoder have for CRC-4, CRC-8, CRC-16 functions?
4 bits, 8 bits, 16 bits.
What operation is performed on bits in a CRC function?
XOR
What are the 4 steps of CRC Generation at the sender side?
- Find length L of the divisor.
- Append L-1 bits to the original message.
- Perform binary division operation.
- Remainder of the division is the CRC ( must be L-1 bits )
What are the steps for the receiver to take during CRC?
Upon receiving data with the sender CRC check appended to it, use the divisor to perform XOR on the data.
How does the receiver know that there are no errors in the CRC transmitted data?
When the result of XOR on the transmitted data is all 0’s bits.
How does CRC work?
- Establish the sender CRC bit code
- Append the sender CRC code to the data to be transmitted.
- Send message
- Receiver performs XOR on transmitted data
How can we represent the encoder / decoder of the CRC in polynomial form? Write 1101 in polynomial form…
- Can be written such that the bit is the coefficient of each variable.
- 1101 = 1X^3 + 1X^2 + 0X^1 + 1X^0 = X^3 + X^2 + X^0
What is the advantage of writing the encoder/decoder in polynomial form?
Polynomial division and multiplication enables efficient encoding and decoding.