## Types of ARQ operation

Stop and Wait ARQ – This is the simplest ARQ method where the transmitter waits after each message for an acknowledgement of correct reception (known as an ACK) from the receiver. If the message is received in error, a negative acknowledgment (NAK) is returned. While this process is taking place, any new messages must be stored in a buffer at the transmitter site. |

Go Back N ARQ – As the name suggests, the transmitter in this case continues to transmit messages in sequence until a NAK is received. The NAK identifies which message was in error and the transmitter then 'back-tracks' to this message, starting to retransmit all messages in the sequence from when the error occurred. Clearly, this has less signalling overhead (no ACKs used) than the Stop and Wait protocol. Selective ARQ – By making the protocol slightly more complex, and by providing a buffer in the receiver as well as the transmitter, it is of course possible for the receiver to inform the transmitter of the specific message or packet that is in error. The transmitter then needs to only send this specific message which the receiver can reinsert in the correct place in the receiver buffer. Although the most complex, this is also the most efficient type of ARQ protocol and the most widely used. There are several variants of this protocol optimized for a given set of channel characteristics.