OSI Layer 2 error correction
Please expain in detail how OSI Layer 2 handles error correction.
The OSI (Open Systems Interconnection) model is a conceptual framework that characterizes and standardizes the functions of a telecommunication or computing system without regard to its underlying internal structure and technology. Its goal is the interoperability of diverse communication systems with standard protocols. The model partitions a communication system into abstraction layers. The OSI model has seven layers, with Layer 2 being the Data Link Layer.
The Data Link Layer (Layer 2) provides node-to-node data transfer—a link between two directly connected nodes. It detects and possibly corrects errors that may occur in the Physical Layer (Layer 1). Here's how Layer 2 handles error detection and correction:
1. Error Detection
Layer 2 incorporates mechanisms to detect errors that occur during transmission. The most common methods are:
Parity Checks: Adding a parity bit can help detect if an error has occurred. Even parity means the number of 1-bits is even, while odd parity means it's odd. If the received parity doesn't match the expected, an error is detected.
Checksums: The sender computes a value (the checksum) based on the contents of the data packet, which is then sent along with the packet. The receiver computes the checksum again on the received data and compares it with the transmitted checksum. If they don't match, an error is detected.
Cyclic Redundancy Checks (CRC): CRC is more powerful than a checksum and is used to detect errors in digital networks. CRC calculates a cyclic redundancy check value based on the data packet and sends this value with the packet. The receiver performs the same calculation and compares the result with the received CRC value. A mismatch indicates an error.
2. Error Correction
Error correction at Layer 2 can be handled in a couple of ways:
Automatic Repeat Request (ARQ): While not an error correction mechanism per se, ARQ is a way of dealing with errors. When an error is detected using one of the detection methods, the receiver can request that the data be retransmitted. This is the most common method of handling errors at Layer 2. There are several types of ARQ protocols, including Stop-and-Wait ARQ, Go-Back-N ARQ, and Selective Repeat ARQ.
Forward Error Correction (FEC): In some systems, the Data Link Layer can employ FEC techniques to not only detect but also correct errors without needing retransmission. This is done by adding redundant data to the transmitted information, which can be used by the receiver to detect and correct errors up to a certain complexity. FEC is useful in situations where retransmissions are costly or impossible, such as in live satellite communications.
In practice, most Layer 2 protocols focus on error detection and rely on retransmissions (ARQ) for error correction rather than employing FEC, which adds overhead and complexity. The choice between using ARQ and FEC depends on the specific requirements of the communication system, including factors like bandwidth availability, latency constraints, and the error characteristics of the physical medium.