Error control in the Data Link Layer ensures that frames transmitted by the sender are received correctly and reliably by the receiver. It focuses on:
- Detecting errors that occur during data transmission
- Correcting errors through retransmission when required
- Ensuring reliable frame delivery over a noisy communication channel
The primary mechanism used for error control is Automatic Repeat Request (ARQ).
In ARQ, if an error is detected or an ACK is not received, the sender retransmits the affected frame(s) until they are successfully received.
There are basically two ways of doing Error control, as given below :
- Error Detection: Error detection refers to the identification of errors in transmitted data. These errors may occur due to noise or other transmission impairments while data is being sent from the transmitter to the receiver in a communication system. It involves techniques used to detect corrupted, distorted, or unclear data before it is processed further.
- Error Correction: Error correction refers to the process of fixing detected errors to recover the original, error-free data. It involves reconstructing the correct data either by using additional information or through retransmission. However, error correction techniques are generally complex, costly, and resource-intensive, which is why they are used selectively.
1. Stop-and-Wait ARQ: In Stop-and-Wait ARQ, the sender transmits one data frame at a time and waits for an ACK (Acknowledgment) from the receiver. If the ACK is not received within a time limit, the sender retransmits the frame. Once the ACK is received, the sender sends the next frame. This process continues until all frames are successfully delivered.
2. Sliding Window ARQ: Sliding Window ARQ is used for continuous data transmission, allowing the sender to transmit multiple frames without waiting for an acknowledgment for each one. This improves channel utilization and overall efficiency. Sliding Window ARQ is mainly divided into two types:
Techniques of error control
Stop-and-Wait ARQ:
In Stop-and-Wait ARQ, the sender transmits one data frame at a time and waits for an ACK (Acknowledgment) from the receiver. If the ACK is not received within a time limit, the sender retransmits the frame. Once the ACK is received, the sender sends the next frame. This process continues until all frames are successfully delivered.
Sliding Window ARQ:
Sliding Window ARQ is used for continuous data transmission, allowing the sender to transmit multiple frames without waiting for an acknowledgment for each one. This improves channel utilization and overall efficiency. Sliding Window ARQ is mainly divided into two types:
Go-Back-N ARQ:
It is an error control protocol in which the sender is allowed to send multiple frames (up to a fixed window size) without waiting for ACKs.
- The receiver sends ACKs for correctly received frames.
- If a frame is lost or arrives with an error, the receiver discards that frame and all subsequent frames.
- The sender then retransmits the erroneous frame along with all the frames sent after it.
- When no errors occur, the protocol behaves like a normal sliding window mechanism.
This method is simple to implement but may cause unnecessary retransmissions.
Selective Repeat ARQ:
It is an error control protocol in which only the frames that are lost or corrupted are retransmitted, unlike Go-Back-N where multiple frames are resent.
- It improves efficiency by avoiding unnecessary retransmissions.
- The sender retransmits only those frames for which a NAK is received or a timeout occurs.
- Each frame is acknowledged individually, allowing correctly received frames to be accepted and stored.
- Due to the need for additional buffering and tracking, the protocol is more complex, which limits its use compared to simpler ARQ schemes.
The main difference between Go-Back-N ARQ and Selective Repeat ARQ is in the retransmission strategy. In Go-Back-N ARQ, if a frame is lost or corrupted, the sender retransmits that frame along with all subsequent frames in the window. In contrast, Selective Repeat ARQ retransmits only the specific frame that is lost or damaged, making it more efficient.