Bit Stuffing in C

Bit stuffing is a widely used technique in digital communication to ensure reliable data transmission. In simple terms, bit stuffing involves adding extra bits to the transmitted data to ensure that the receiver can accurately detect the start and end of the data frame. This technique is particularly useful in situations where data frames are transmitted over unreliable channels, such as wireless networks or noisy copper wires. In this article, we will discuss bit stuffing in C programming language and how it can be implemented using bitwise operators.

The basic concept of bit stuffing is straightforward. The sender of the data frame adds an extra bit after every sequence of five consecutive 1s in the data being transmitted. This added bit is always a 0 and does not interfere with the actual data being transmitted. The receiver of the data frame then looks for this sequence of six consecutive 1s, which indicates the start of a data frame. The receiver then extracts the data from the frame, removes the extra 0s, and processes the data as normal. The process of adding and removing extra bits is known as bit stuffing and de-stuffing, respectively.

In C programming language, bit stuffing can be implemented using bitwise operators. Bitwise operators are used to manipulate individual bits within a byte, which is the smallest unit of data that can be processed by a computer. There are six bitwise operators in C:

• & (bitwise AND)
• | (bitwise OR)
• ^ (bitwise XOR)
• ~ (bitwise NOT)
• << (bitwise left shift)
• (bitwise right shift)

These operators are used to perform operations on individual bits within a byte, such as setting a bit to 1, clearing a bit to 0, or flipping the value of a bit.

Now we will take a closer look at how bit stuffing can be implemented in C programming language. We will start by defining a simple data frame consisting of six bytes:

In this example, the first and last bytes of the data frame are the start and end markers, respectively. The four bytes in between represent the actual data being transmitted.

To implement bit stuffing, we can use a for loop to iterate over each byte in the data frame. Within the loop, we can use an if statement to check if the current byte contains a sequence of five consecutive 1s. If it does, we can insert an extra 0 bit after the sequence of 1s. Below is the code to implement bit stuffing:

Explanation:

In this code, we use a bitwise AND operation to check if the current byte and the previous and next bytes contain a sequence of five consecutive 1s. The & operator returns a value that contains a 1 in each bit position where both operands have a 1. After that, we compare the result of the & operation with the binary value 0xF8, which represents a sequence of five consecutive 1s followed by a 0. If the result of the & operation matches 0xF8, we know that the current byte contains a sequence of five consecutive 1s.

To insert an extra 0 bit after the sequence of 1s, we use the left shift operator (<<) to shift the bits the bit-stuffed data frame:

Output:

The output of this code will be:

7E FE 01 23 45 67 7E

Explanation:

In this bit-stuffed data frame, we can see that an extra 0 bit has been inserted after the sequence of five consecutive 1s in the second byte (0xFE). The extra 0 bit does not interfere with the actual data being transmitted, but it ensures that the receiver can accurately detect the start and end of the data frame.

To de-stuff the bit-stuffed data frame, the receiver needs to reverse the process of bit stuffing. The receiver needs to look for a sequence of six consecutive 1s, which indicates the start of a data frame. After that, the receiver can extract the data from the frame, remove the extra 0 bits, and process the data as normal.

Below is the code to implement bit de-stuffing:

In this code, we use a bitwise AND operation to check if the current byte and the previous byte contain a sequence of six consecutive 1s. The & operator returns a value that contains a 1 in each bit position where both operands have a 1. After that, we compare the result of the & operation with the binary value 0xFC, which represents a sequence of six consecutive 1s followed by two 0s. If the result of the & operation matches 0xFC, we know that the current byte contains an extra 0 bit that needs to be removed.

To remove the extra 0 bit, we use the right shift operator (>>) and the left shift operator (<<) to shift the bits within the byte. After that, we combine the shifted bits with the next byte in the data frame to form a single byte. This process is repeated for each byte in the data frame, and the resulting de-stuffed data frame is stored in the data array.

Below is the code to print the de-stuffed data frame:

The output of this code will be:

7E 01 23 45 67 7E

In this de-stuffed data frame, we can see that the extra 0 bit in the second byte has been removed, and the original data has been restored.

Below is an example of how to implement bit stuffing in C:

Output:

Original data: 7E 01 23 45 67 7E 
Stuffed data: 7D 5E 01 23 45 67 7D 5E 7E

Explanation:

In this example, the original data is stored in the data array, and the stuffed data is stored in the stuffedData array. The length variable is used to store the length of the original data.

The for loop iterates through the original data and adds an extra bit after every sequence of five consecutive 1s using the if statement. If the current byte is either the FLAG or ESCAPE byte, it is escaped and XORed with XOR.

If the current byte is equal to 0xFF and the next byte is also equal to 0xFF, an extra byte with the value of 0x00 is added to the stuffed data. Finally, the FLAG byte is added to the end of the stuffed data.

Conclusion:

In conclusion, bit stuffing is a useful technique in digital communication to ensure reliable data transmission. Bit stuffing can be easily implemented in C programming language using bitwise operators. The sender can add an extra 0 bit after every sequence of five consecutive 1s in the data frame, and the receiver can remove the extra 0 bit to restore the original data. By using bit stuffing, the receiver can accurately detect the start and end of the data frame, even if the actual data contains a sequence of five consecutive 1s.

Bit stuffing is commonly used in communication protocols such as HDLC (High-Level Data Link Control) and PPP (Point-to-Point Protocol). These protocols use bit stuffing to ensure reliable data transmission over noisy or error-prone communication channels. Bit stuffing is also used in digital audio and video encoding to ensure that the encoded data stream does not contain a sequence of consecutive 1s, which can cause synchronization issues.