• Table of Contents

Error Control

The error control mechanism may involve two possible ways.

• The first one is error detection, which we’ve covered in 7.2 Data Link Layer - Error Detection.
• The second is error correction is error correction, which we’ll cover now.

Error correction goes a step further by not only identifying errors but also attempting to correct them. One technique we’ll look at is retransmission.

<aside> <img src="/icons/map-pin_gray.svg" alt="/icons/map-pin_gray.svg" width="40px" /> Error control measures in the lower layers don't guarantee error-free data at higher layers.

</aside>

Retransmission

Retransmission is crucial for error correction through the data link and transport layers:

1. Data Link Layer:
   • Monitors incoming frames for errors, especially in situations like WiFi with interference.
   • Retransmits lost frames to ensure reliable local network communication.
2. Transport Layer:
   • Addresses packet loss due to network congestion or other factors.
   • Employs mechanisms for retransmitting lost IP packets, ensuring end-to-end reliability.

Retransmission utilizes acknowledgment ($\text{ACK}$) and timeouts for identifying and recovering lost data.

Rules for Retransmission

From the sender:

• The sender automatically resends the frame or packet if it does not receive an acknowledgment ($\text{ACK}$) from the receiver after a predefined time period called a timeout.
• If the sender doesn't receive an $\text{ACK}$ within the specified time, it assumes that the original frame or packet was lost or corrupted during transmission and initiates retransmission.

From the receiver:

• The receiver automatically acknowledges correct frames or packets with an $\text{ACK}$ message. This $\text{ACK}$ serves to inform the sender that the frame or packet was correctly received.
• By sending an $\text{ACK}$, the receiver lets the sender know that the data was successfully received and doesn't need to be resent.