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# CRC In Computer Networks

Last Updated on June 16, 2023 by Mayank Dham In computer networks, the reliable and accurate transmission of data is of paramount importance. Errors during data transmission can lead to corrupt or invalid data, compromising the integrity and reliability of the entire network. Cyclic Redundancy Check (CRC) is a widely used error detection technique that ensures data integrity by detecting and correcting errors that may occur during data transmission. In this article, we will delve into the intricacies of CRC, exploring its principles, advantages, implementation, and significance in computer networks.

## Understanding CRC

Cyclic Redundancy Check (CRC) is an error detection technique that uses a mathematical algorithm to verify the integrity of the data being transmitted. It appends a small, fixed-length checksum to the data, which is calculated based on the content of the message. The receiving end can then verify the integrity of the data by performing the same calculation and comparing the checksums. ## Principles of CRC

The CRC algorithm operates on binary data, treating it as a polynomial. The data is divided into blocks of a fixed size, and a generator polynomial is used to perform a mathematical calculation. This calculation results in a remainder, which becomes the checksum. The checksum is appended to the original data, creating the transmitted message.
During transmission, both the sender and the receiver agree on the same generator polynomial. The receiver performs the same CRC calculation on the received message and checks if the calculated checksum matches the appended checksum. If the checksums match, it indicates that the data has been transmitted without errors. However, if the checksums do not match, it signifies the presence of errors in the data.

1. Efficiency: CRC is computationally efficient, as it involves simple bitwise operations and does not require complex mathematical computations. This makes it suitable for real-time applications and high-speed data transmission.

2. Error Detection: CRC is capable of detecting various types of errors, including single-bit errors, burst errors, and most common transmission errors. Its ability to detect errors allows for early identification and retransmission of erroneous data.

3. Simplicity: The implementation of CRC is relatively straightforward, with readily available algorithms and libraries. It can be easily incorporated into existing network protocols and hardware components.

## CRC Implementation

The implementation of the CRC involves several key steps:

1. Message and Polynomial Selection: The sender selects the message to be transmitted and chooses a suitable generator polynomial. The polynomial is typically represented as a binary number, often in the form of a simple bit pattern.

3. Checksum Calculation: The sender performs the CRC calculation on the message, dividing it by the chosen generator polynomial. The remainder obtained from this division is the checksum.

5. Checksum Appending: The sender appends the calculated checksum to the original message, creating the transmitted message that includes both the data and the checksum.

7. Checksum Verification: The receiver receives the transmitted message and performs the same CRC calculation using the agreed-upon generator polynomial. It compares the calculated checksum with the received checksum to check for errors.

9. Error Handling: If the received checksum matches the calculated checksum, the data is considered error-free. However, if the checksums do not match, the receiver notifies the sender of the error, and appropriate error handling mechanisms, such as retransmission, can be implemented.

### Code Implementation of CRC in C++ Using Bit Manipulation

```#include <iostream>
#include <math.h>
#include <stdio.h>
using namespace std;

string toBin(long long int num)
{
string bin = "";
while (num) {
if (num & 1)
bin = "1" + bin;
else
bin = "0" + bin;
num = num >> 1;
}
return bin;
}

long long int toDec(string bin)
{
long long int num = 0;
for (int i = 0; i < bin.length(); i++) {
if (bin.at(i) == '1')
num += 1 << (bin.length() - i - 1);
}
return num;
}

void CRC(string dataword, string generator)
{
int l_gen = generator.length();
long long int gen = toDec(generator);

long long int dword = toDec(dataword);

long long int dividend = dword << (l_gen - 1);
int shft = (int)ceill(log2l(dividend + 1)) - l_gen;
long long int rem;

while ((dividend >= gen) || (shft >= 0)) {
rem = (dividend >> shft) ^ gen;
dividend = (dividend & ((1 << shft) - 1))
| (rem << shft);

// change shft variable
shft = (int)ceill(log2l(dividend + 1)) - l_gen;
}
long long int codeword
= (dword << (l_gen - 1)) | dividend;
cout << "Remainder: " << toBin(dividend) << endl;
cout << "Codeword : " << toBin(codeword) << endl;
}

int main()
{
string dataword, generator;
dataword = "10011101";
generator = "1001";
CRC(dataword, generator);
return 0;
}
```

## Significance of CRC in Computer Networks

CRC plays a vital role in ensuring data integrity and reliability in computer networks. Its significance can be highlighted in the following ways:

1. Error Detection: CRC enables the detection of errors during data transmission, allowing for prompt identification and correction. By detecting errors, CRC helps maintain data integrity and prevents the propagation of corrupted data within the network.

2. Data Transfer: The reliable error detection provided by CRC ensures that erroneous data is not processed or acted upon. This enhances the overall efficiency of data transfer and reduces the need for additional error correction mechanisms.

3. Network Performance: With CRC, network performance is improved by reducing the overhead associated with error correction. By detecting errors at the receiver end, CRC minimizes the need for retransmission or complex error recovery processes, leading to smoother and faster data transmission.

4. Standardization: CRC has been widely adopted as a standard error detection technique in various network protocols, such as Ethernet and Wi-Fi. Its standardized implementation ensures interoperability among different network devices and facilitates seamless communication.

Conclusion
CRC is a fundamental error detection technique in computer networks that ensures data integrity during transmission. By appending a checksum to the data and performing checksum verification at the receiver end, CRC detects errors and enables prompt error correction. Its efficiency, simplicity, and effectiveness make it an integral part of network protocols and hardware implementations. As computer networks continue to evolve and expand, the role of CRC remains crucial in maintaining the integrity and reliability of data transmission.

Q1. What is the purpose of CRC in computer networks?
CRC (Cyclic Redundancy Check) is used in computer networks to ensure data integrity during transmission. It detects errors that may occur in the data and allows the receiver to verify the accuracy of the received information.

Q2. How does CRC detect errors in data transmission?
CRC employs a mathematical algorithm that generates a checksum based on the content of the data. The sender calculates the checksum and appends it to the data being transmitted. At the receiver’s end, the same algorithm is applied to the received data, and the resulting checksum is compared with the transmitted checksum. If they match, it indicates that the data is likely error-free. If they don’t match, it signifies the presence of errors.

Q3. Can CRC correct errors in data transmission?
No, CRC is an error detection technique that does not correct errors. It can only detect the presence of errors. When errors are detected, appropriate measures, such as the retransmission of data, can be taken to ensure error-free transmission.

Q4. Is CRC used in all types of data transmission?
CRC is commonly used in various data transmission scenarios, including wired and wireless networks. It is widely implemented in network protocols, such as Ethernet, Wi-Fi, and Bluetooth, to ensure reliable data transmission and minimize the impact of errors.

Q5. How efficient is CRC in detecting errors in data transmission?
The CRC is known for its efficiency in error detection. It can detect a wide range of errors, including single-bit errors, burst errors, and the most common transmission errors. Its mathematical calculations are computationally efficient, allowing for real-time error detection without significant processing overhead.