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.
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