Distinguish between Go Back N ARQ and selective reject ARQ Sketch the frame flow diagram for Go Back ARQ and 3 bit sequence number field and window size of 5, showing the following events:

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Protocol	Go Back N ARQ	Selective repeat ARQ
Bandwidth utilization	Medium	High
Maximum sender window size	2^ m-1	2^(m-1)
Maximum receiver window size	1	2^(m-1)
Pipelining	Implemented	Implemented
Out of order frames	Discarded	Accepted

Movement of sender's window:

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Fig a: Sender sliding window

Figure a. shows the sender's window. If the sender's window size is 4 and frames 1 and 2 are sent but acknowledgement has not been received so far, then as shown in the figure b, the sender's windows will only contain two frames that is, 3 and 4.

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Fig b: Sender's window after sending first two frames but no acknowledgement

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Fig c: Sender's window after receiving acknowledgement bearing numbers

Now if the sender receives acknowledgement bearing number 3 then it understands that the receiver has correctly received frames 1 and 2.

The senders window now expands and includes the next two frames as shown in the figure c. In this way the left edge of the sender's windows will shift right when the data frames are sent and the right edge of the sender's windows will shift right when the acknowledgement is received.

Movement of receiver's windows:

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Fig d : Receiver's sliding window

Figure d shows the receiver's window. Its left edge shifts right on receiving each data frame, where as its right age shifts right when an acknowledgement is sent.

If we take the same example that we discussed for the sender's window then the position of receivers windows are as shown in the figures e and f.

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Fig e : Two frames (1 and 2) received but no acknowledgement sent

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Fig f: After sending the acknowledgement

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