Stop-and-Wait Automatic Repeat Request (ARQ) is a flow control protocol used in data link layer to ensure reliable data transmission between a sender and a receiver. In this protocol, the sender transmits a single data frame to the receiver and waits for an acknowledgment (ACK) signal from the receiver. The receiver sends the ACK signal to the sender after receiving the data frame successfully.
If the sender does not receive the ACK signal within a certain time period, it assumes that the data frame was not received correctly and sends the same data frame again. This process continues until the receiver sends an ACK signal to the sender.
It has two possible cases:
When there is no error:
In this case, the sender sends a frame and waits for the acknowledgment from the receiver. The receiver receives the frame and sends an acknowledgment (ACK) back to the sender. Upon receiving the ACK, the sender sends the next frame, and the process continues until all the frames are transmitted successfully.
When there is an error:
In this case, if the sender does not receive an ACK within a certain amount of time, it assumes that there is an error and retransmits the frame. The receiver receives the frame and checks for errors. If there are no errors, it sends an ACK back to the sender. If there are errors, the receiver discards the frame and sends a negative acknowledgment (NAK) to the sender. Upon receiving the NAK, the sender retransmits the frame.
When ACK is not received:
If the sender does not receive an acknowledgment (ACK) from the receiver within a specified time period (also called timeout interval), it assumes that the frame it transmitted was lost or damaged and retransmits the frame. This scenario can occur due to several reasons, such as channel noise, congestion, or faulty hardware.
Advantages of Stop-and-Wait ARQ:
- It is simple to implement and requires minimal buffering.
- It ensures that data is delivered in the correct sequence, as the receiver only accepts one frame at a time.
- It provides reliable communication by ensuring that lost or damaged frames are retransmitted.
Disadvantages of Stop-and-Wait ARQ:
- It can be inefficient, as the sender must wait for an acknowledgment before sending the next frame.
- It can result in lower throughput, as the transmission rate is limited by the round-trip time of each frame.
- It can lead to increased latency, as frames may be retransmitted multiple times if they are lost or damaged.