Data-link layer uses the techniques of error control simply to ensure and confirm that all the data frames or packets, i.e. bit streams of data, are transmitted or transferred from sender to receiver with certain accuracy. Using or providing error control at this data link layer is an optimization, it was never a requirement. Error control is basically process in data link layer of detecting or identifying and re-transmitting data frames that might be lost or corrupted during transmission. In both of these cases, receiver or destination does not receive correct data frame and sender or source does not even know anything about any such loss regarding data frames. Therefore, in such type of cases, both sender and receiver are provided with some essential protocols that are required to detect or identify such types of errors as loss of data frames. The Data-link layer follows a technique known as re-transmission of frames to detect or identify transit errors and also to take necessary actions that are required to reduce or remove such errors. Each and every time an error is detected during transmission, particular data frames are retransmitted and this process is known as ARQ (Automatic Repeat Request).
Ways of doing Error Control : There are basically two ways of doing Error control as given below :
Ways of Error Control
- Error Detection : Error detection, as the name suggests, simply means detection or identification of errors. These errors may occur due to noise or any other impairments during transmission from transmitter to the receiver, in communication system. It is a class of techniques for detecting garbled i.e. unclear and distorted data or messages.
- Error Correction : Error correction, as the name suggests, simply means correction or solving or fixing of errors. It simply means reconstruction and rehabilitation of original data that is error-free. But error correction method is very costly and very hard.
Various Techniques for Flow Control : There are various techniques of error control as given below :
1. Stop-and-Wait ARQ : Stop-and-Wait ARQ is also known as alternating bit protocol. It is one of the simplest flow and error control techniques or mechanisms. This mechanism is generally required in telecommunications to transmit data or information between two connected devices. Receiver simply indicates its readiness to receive data for each frame. In these, sender sends information or data packets to receiver. Sender then stops and waits for ACK (Acknowledgment) from receiver. Further, if ACK does not arrive within given time period i.e., time-out, sender then again resends frame and waits for ACK. But, if sender receives ACK, then it will transmit the next data packet to receiver and then again wait for ACK from receiver. This process to stop and wait continues until sender has no data frame or packet to send.
2. Sliding Window ARQ : This technique is generally used for continuous transmission error control. It is further categorized into two categories as given below :
- Go-Back-N ARQ : Go-Back-N ARQ is form of ARQ protocol in which transmission process continues to send or transmit total number of frames that are specified by window size even without receiving an ACK (Acknowledgement) packet from the receiver. It uses sliding window flow control protocol. If no errors occur, then operation is identical to sliding window.
- Selective Repeat ARQ : Selective Repeat ARQ is also form of ARQ protocol in which only suspected or damaged or lost data frames are only retransmitted. This technique is similar to Go-Back-N ARQ though much more efficient than the Go-Back-N ARQ technique due to reason that it reduces number of retransmission. In this, the sender only retransmits frames for which NAK is received. But this technique is used less because of more complexity between sender and receiver and each frame must be needed to be acknowledged individually.
The main difference between Go Back ARQ and Selective Repeat ARQ is that in Go Back ARQ, the sender has to retransmit the whole window of frame again if any of the frame is lost but in Selective Repeat ARQ only the data frame that is lost is retransmitted.