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Error Detection in Data link layer or Transport layer
- 2. WHAT IS ERROR IN NETWORKING The data link layer is responsible for the reliable transmission of data frames between two directly connected nodes over a physical medium, such as ethernet or wi-fi. Errors in the data link layer refer to the corruption or loss of data during this transmission process. There are several types of errors that can occur at the data link layer. Bit Errors Frame Loss Frame Duplication Frame Mis ordering Frame Corruption Collision
- 3. TYPES OF ERROR Single bit error: • In a frame, there is only one bit, anywhere though, which is corrupt. Multiple bits error: • Frame is received with more than one bits in corrupted state. Burst error: • Frame contains more than 1 consecutive bits corrupted
- 4. SINGLE BIT ERROR Single-bit error does not appear more likely in serial data transmission. For example, sender sends the data at 10 mbps, this means that the bit lasts only for 1 ‘s and for a single-bit error to occurred, a noise must be more than 1 ‘s. Single-bit error mainly occurs in parallel data transmission. For example, if eight wires are used to send the eight bits of a byte, if one of the wire is noisy, then single-bit is corrupted per byte.
- 5. WAYS TO CHECK Parity bit: in a parity bit scheme, an additional bit is added to each byte or character to make the total number of ones either even (even parity) or odd (odd parity). The receiver can then check if the number of ones in the received data matches the expected parity, thus detecting single-bit errors. Checksum: checksums involve performing a mathematical calculation on the data and sending the result along with the data. The receiver recalculates the checksum and compares it to the received value. If they don't match, an error is detected.
- 6. MULTIPLE-BIT ERRORS A multiple-bit error occurs when more than one bit in a data frame is changed during transmission. These errors are more likely to go undetected by simple error detection techniques like parity bit and checksum. However, more advanced techniques like cyclic redundancy check (CRC) can still be effective in detecting multiple-bit errors.
- 7. WAYS TO CHECK Cyclic Redundancy Check (CRC): CRC is a powerful error detection technique that uses polynomial codes to generate a checksum. The receiver performs the same calculation and checks whether the received CRC matches the calculated CRC. CRC is capable of detecting a wide range of errors, including multiple-bit errors.
- 8. BURST ERRORS Burst errors are consecutive errors that occur within a short span of time. These errors are typically caused by factors like signal fading, interference, or noise that affect a group of adjacent bits. Burst errors can be particularly problematic, as they can cause significant data corruption.
- 9. WAYS TO CHECK Hamming code: hamming codes are error-correcting codes that add redundant bits to the data. These codes are designed to correct single-bit errors and detect multiple-bit errors. They are particularly effective at correcting burst errors of a certain length. Interleaving: interleaving involves rearranging the data bits before transmission so that consecutive bits are separated by a fixed distance. This helps to spread out burst errors and reduces the likelihood of multiple errors affecting adjacent bits.
- 10. ERROR DETECTING TECHNIQUES Single parity check Two- dimensional parity check Checksum Cyclic redundancy check
- 11. SINGLE BIT PARITY ERROR DETECTION Parity bit is a simple error detection technique commonly used to detect single-bit errors in data transmission. It involves adding an extra bit, known as the parity bit, to the original data before transmission. The parity bit is set to either 0 or 1, depending on whether the total number of 1s in the data (including the parity bit) is meant to be even (even parity) or odd (odd parity). Even Parity Odd Parity
- 12. EVEN PARITY CHECKING If the total number of 1s in the data (including the parity bit) is an even number, the parity bit is set to 0. This ensures that the total number of 1s remains even. If a single bit is flipped during transmission, it will result in an odd number of 1s in the received data. The parity check at the receiving end will then detect this error, as the total number of 1s will no longer be even.
- 13. ODD PARITY CHECKING If the total number of 1s in the data (including the parity bit) is an odd number, the parity bit is set to 1. This ensures that the total number of 1s remains odd. If a single bit is flipped during transmission, it will result in an even number of 1s in the received data. The parity check at the receiving end will then detect this error, as the total number of 1s will no longer be odd.
- 15. TWO-DIMENSIONAL PARITY CHECK Performance can be improved by using Two-Dimensional Parity Check which organizes the data in the form of a table. Parity check bits are computed for each row, which is equivalent to the single-parity check. In Two-Dimensional Parity check, a block of bits is divided into rows, and the redundant row of bits is added to the whole block. At the receiving end, the parity bits are compared with the parity bits computed from the received data.
- 17. CHECKSUM Checksum Generator • A Checksum is generated at the sending side. Checksum generator subdivides the data into equal segments of n bits each, and all these segments are added together by using one's complement arithmetic. The sum is complemented and appended to the original data, known as checksum field. The extended data is transmitted across the network. Checksum Checker • A Checksum is verified at the receiving side. The receiver subdivides the incoming data into equal segments of n bits each, and all these segments are added together, and then this sum is complemented. If the complement of the sum is zero, then the data is accepted otherwise data is rejected.
- 19. EXAMPLE Suppose that the sender wants to send 4 frames each of 8 bits, where the frames are 11001100, 10101010, 11110000 and 11000011.The sender adds the bits using 1s complement arithmetic. While adding two numbers using 1s complement arithmetic, if there is a carry over, it is added to the sum. The sender adds the bits using 1s complement arithmetic. While adding two numbers using 1s complement arithmetic, if there is a carry over, it is added to the sum. After adding all the 4 frames, the sender complements the sum to get the checksum, 11010011, and sends it along with the data frames. The receiver performs 1s complement arithmetic sum of all the frames including the checksum. The result is complemented and found to be 0. Hence, the receiver assumes that no error has occurred.
- 21. CYCLIC REDUNDANCY CHECK (CRC) In CRC technique, a string of n 0s is appended to the data unit, and this n number is less than the number of bits in a predetermined number, known as division which is n+1 bits. Secondly, the newly extended data is divided by a divisor using a process is known as binary division. The remainder generated from this division is known as CRC remainder. Thirdly, the CRC remainder replaces the appended 0s at the end of the original data. This newly generated unit is sent to the receiver. The receiver receives the data followed by the CRC remainder. The receiver will treat this whole unit as a single unit, and it is divided by the same divisor that was used to find the CRC remainder.
- 23. CRC GENERATOR A CRC generator uses a modulo-2 division. Firstly, three zeroes are appended at the end of the data as the length of the divisor is 4 and we know that the length of the string 0s to be appended is always one less than the length of the divisor. Now, the string becomes 11100000, and the resultant string is divided by the divisor 1001. The remainder generated from the binary division is known as CRC remainder. The generated value of the CRC remainder is 111. CRC remainder replaces the appended string of 0s at the end of the data unit, and the final string would be 11100111 which is sent across the network.
- 24. CRC CHECKER The functionality of the CRC checker is similar to the CRC generator. When the string 11100111 is received at the receiving end, then CRC checker performs the modulo-2 division. A string is divided by the same divisor, i.e., 1001. In this case, CRC checker generates the remainder of zero. Therefore, the data is accepted.