When the sender is responding to this event it cannot

3 is the response to one single event. When the sender is
responding to this event, it cannot accept the triggering of
other events. This means that when ACK 2 arrives, the
sender is still busy with sending frame 3.
Example 11.7

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The physical layer must wait until this event is completed
and the data link layer goes back to its sleeping state. We
have shown a vertical line to indicate the delay. It is the
same story with ACK 3; but when ACK 3 arrives, the
sender is busy responding to ACK 2. It happens again when
ACK 4 arrives. Note that before the second timer expires,
all outstanding frames have been sent and the timer is
stopped.
Example 11.7 (continued)

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Figure 11.17
Flow diagram for Example 11.7

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Stop-and-Wait ARQ is a special case of Go-Back-N ARQ in which the size of
the send window is 1.
Note

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Figure 11.18
Send window for Selective Repeat ARQ

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Figure 11.19
Receive window for Selective Repeat ARQ

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Figure 11.20
Design of Selective Repeat ARQ

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Figure 11.21
Selective Repeat ARQ, window size

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In Selective Repeat ARQ, the size of the sender and receiver window
must be at most one-half of 2m.
Note