Stop-and-Wait

Senders keeps a copy of the last frame until it receives an acknowledgement.
For identification, both data frames and acknowledgements (ACK) frames are numbered alternatively 0 and 1.
Sender has a control variable (S) that holds the number of the recently sent frame. (0 or 1)
Receiver has a control variable (R) that holds the number of the next frame expected (0 or 1).
Sender starts a timer when it sends a frame. If an ACK is not received within a allocated time period, the sender assumes that the frame was lost or damaged and resends it.
Receiver send only positive ACK if the frame is intact.
ACK number always defines the number of the next expected frame.

When a receiver receives a damaged frame, it discards it and keeps its value of R.
After the timer at the sender expires, another copy of frame 1 is sent.

If the sender receives a damaged ACK, it discards it.
When the timer of the sender expires, the sender retransmits frame 1.
Receiver has already received frame 1 and expecting to receive frame 0 (R=0). Therefore it discards the second copy of frame 1.

The ACK can be delayed at the receiver or due to some problem
It is received after the timer for frame 0 has expired.
Sender retransmitted a copy of frame 0. However, R =1 means receiver expects to see frame 1. Receiver discards the duplicate frame 0.
Sender receives 2 ACKs, it discards the second ACK.

A method to combine a data frame with ACK.
Station A and B both have data to send.
Instead of sending separately, station A sends a data frame that includes an ACK.
Station B does the same thing.
Piggybacking saves bandwidth.

In stop-and-wait, at any point in time, there is only one frame that is sent and waiting to be acknowledged.
This is not a good use of transmission medium.
To improve efficiency, multiple frames should be in transition while waiting for ACK.