<P> Stop - and - wait ARQ, also referred to as alternating bit protocol, is a method in telecommunications to send information between two connected devices . It ensures that information is not lost due to dropped packets and that packets are received in the correct order . It is the simplest automatic repeat - request (ARQ) mechanism . A stop - and - wait ARQ sender sends one frame at a time; it is a special case of the general sliding window protocol with transmit and receive window sizes equal to one and greater than one respectively . After sending each frame, the sender doesn't send any further frames until it receives an acknowledgement (ACK) signal . After receiving a valid frame, the receiver sends an ACK . If the ACK does not reach the sender before a certain time, known as the timeout, the sender sends the same frame again . The timeout countdown is reset after each frame transmission . The above behavior is a basic example of Stop - and - Wait . However, real - life implementations vary to address certain issues of design . </P> <P> Typically the transmitter adds a redundancy check number to the end of each frame . The receiver uses the redundancy check number to check for possible damage . If the receiver sees that the frame is good, it sends an ACK . If the receiver sees that the frame is damaged, the receiver discards it and does not send an ACK--pretending that the frame was completely lost, not merely damaged . </P> <P> One problem is when the ACK sent by the receiver is damaged or lost . In this case, the sender doesn't receive the ACK, times out, and sends the frame again . Now the receiver has two copies of the same frame, and doesn't know if the second one is a duplicate frame or the next frame of the sequence carrying identical data . </P> <P> Another problem is when the transmission medium has such a long latency that the sender's timeout runs out before the frame reaches the receiver . In this case the sender resends the same packet . Eventually the receiver gets two copies of the same frame, and sends an ACK for each one . The sender, waiting for a single ACK, receives two ACKs, which may cause problems if it assumes that the second ACK is for the next frame in the sequence . </P>

Differentiate between stop-and wait arq and go-back-n protocol