2013/6/2
3
c. What is the main effect of introducing the
immediate-retransmission feature?
y C) The main effect is a speed up in the error recovery process.
The main effect is that the expected time for transmission
is reduced because when the error is detected a NAK is send
and the sender can stop the transmission and initiate the
retransmission of the frame. If the error is in the ACK then
the sender will not have to wait for the time out. Always
when there is an error in the ACK or NAK the last frame sent
has to be retransmitted because the sender does not know if
the frame was received with or without errors.
5.12
In Stop-and-Wait ARQ why should the receiver always
send an acknowledgment message each time it receives a
frame with the wrong sequence number?
The sender cannot send the next frame until it has received
the ACK for the last frame so, if the receiver gets a frame with
the wrong sequence it has to be a retransmission of the previous
frame received. This means that the ACK was lost so the
Solution:
receiver has to ACK again to indicate the sender that it has
received the frame.
