What is Piggybacking?
In piggybacking technique, when the receiver receives the data frame (message), it waits and doesn't send the acknowledgment (ACK) immediately. The receiver waits until it has a message to send and attaches the ACK to this message. This helps it to make better use of bandwidth.
But a major disadvantage is that if the receiver waits too long for transmitting the acknowledgement, retransmission of frame would take place. To avoid the delay, the waiting period is fixed. If the delay is long, the receiver sends a separate ACK frame.