A Comprehensive Guide to Flow Control in Elementary Data Link Protocols
What is Flow Control?
Flow control regulates the rate of data transmission between devices to prevent a fast sender from overwhelming a slow receiver. It ensures that the sender does not transmit more frames than the receiver can handle.
Types of Flow Control
- Stop-and-Wait: The sender transmits one frame and waits for an acknowledgement before sending the next frame. This prevents the sender from overwhelming the receiver.
- Sliding Window: The sender can transmit multiple frames before receiving an acknowledgement. The receiver specifies a window size indicating how many frames it can accept. The sender can only send frames within the window.
Stop-and-Wait Flow Control
In the Simplex Stop-and-Wait protocol:
- The sender transmits a frame and starts a timer.
- If an acknowledgement is received before the timer expires, the sender transmits the next frame.
- If the timer expires before an acknowledgement is received, the frame is retransmitted.
This ensures the receiver is not overwhelmed by a fast sender.
Sliding Window Flow Control
Sliding window protocols allow the sender to transmit multiple frames before receiving acknowledgements. The receiver specifies a window size indicating how many frames it can accept. The sender can only send frames within the window.
As frames are acknowledged, the window slides to allow more frames to be sent. This improves efficiency compared to stop-and-wait.
Variations include:
- Go-Back-N: If a frame is lost or corrupted, the receiver discards all subsequent frames and the sender retransmits from the lost frame.
- Selective Repeat: The receiver buffers correctly received frames even if some are lost or corrupted. Only lost/corrupted frames are retransmitted.
Flow control is a critical function of the data link layer to ensure reliable and efficient data transmission between devices with different processing capabilities. Stop-and-wait and sliding window protocols provide effective flow control mechanisms for elementary data link protocols.