This document summarizes Go-Back-N ARQ and Selective Repeat ARQ protocols. It discusses the key aspects of each protocol including how they handle sequence numbers, sliding windows, control variables, timers, acknowledgements, and dealing with damaged or lost frames. Selective Repeat ARQ is more efficient for noisy links as it only resends the damaged frame rather than multiple frames like Go-Back-N ARQ. The document provides details on the operation of each protocol and differences in how they handle the receiver window size and control variables.

1. DATA COMMUNICATIONS &
NETWORKING
LECTURE-
24
Course Instructor : Sehrish Rafiq
Department Of Computer Science
University Of Peshawar

2. LECTURE OVERVIEW
 Go-Back N-ARQ
 Selective Repeat ARQ

3. GO-BACK-N ARQ
&
SELECTIVE- REPEAT-ARQ
 To improve the efficiency, multiple frames should
be in-transition while waiting for
acknowledgement.
 Go-Back N-ARQ
 Selective Repeat ARQ

4. GO-BACK N-ARQ
 In Go-Back N-ARQ ,we can send W frames before
worrying about acknowledgements.
 We keep a copy of these frames until an
acknowledgement arrive.

5. SEQUENCE NUMBERS
 Frames from a sending station are numbered
sequentially.
 sequence number is included in the header ,we
need to set a limit.
 If the header of the frame allows m bits for the
sequence number the sequence number range
from 0 to 2m
-1.
 E.g.
 For m=3 the sequence numbers would be 0 to 7.
 However we can repeat the sequence.

6. SENDER SLIDING WINDOW
 At the sender site to hold the outstanding frames
until they are acknowledged we use the concept
of a window.
 We imagine that all frames are stored in a
buffer.
 The outstanding frames are enclosed in a
window.
 The frames to the left of the window are those
that have already being acknowledged.

7. SENDER SLIDING WINDOW
 Those to the right of the window cannot be sent
before the window slides over them.
 The size of the window is at most 2m
-1.
 The size of this window is fixed.
 The window slides to include new unsent frames
when correct acknowledgements are received.

8. SENDER SLIDING WINDOW

9. RECEIVER SLIDING WINDOW
 The size of the window at the receiver site in this
protocol is 1.
 The receiver is always looking for a specific frame
to arrive in a specific order.
 Any frame arriving out of order is discarded and
needs to be resent.
 The receiver window also slides .

10. RECEIVER WINDOW

11. CONTROL VARIABLES
 The sender has three variables , S, SF and SL.
 The S variable holds the sequence number of the
recently sent frame.
 SF holds the sequence number of the first frame of
the window.
 SL holds the sequence number of the last frame of
the window.
 The size of the window is W= SL - SF +1.
 The receiver only has one variable R holding the
sequence number of the frame it expects to
receive.

12. CONTROL VARIABLES CONT…
 If the sequence number of the received frame is
the same as the value of R, the frame is accepted,
if not it is rejected.

13. TIMERS
 The sender sets a timer for each frame sent.
 The receiver has no timers.

14. ACKNOWLEDGEMENT
 The receiver sends positive acknowledgements if
a frame has arrived safe and sound and in order.
 If a frame is damaged or is received out of
order ,the receiver is silent and will discard all
subsequent frames until it receives the one it is
expecting .
 The silence of the receiver causes the timer of the
unacknowledged frame to expire.
 This in turn causes the sender to go back and
resend all frames beginning one with the expired
timer.

15. GO-BACK-N-ARQ NORMAL
OPERATION

16. DAMAGED OR LOST FRAME

17. DAMAGED OR LOST
ACKNOWLEDGEMENT
 If an acknowledgement is damaged or lost we can
have two situations.
 If the next acknowledgement arrives before the
expiration of any timer there is no need for
retransmission of frames because
acknowledgements are cumulative in this
protocol.
 However if the next ACK arrives after the
timeout, the frame and all the frames after that
are resent.
 The receiver never resends an ACK.

18. SELECTIVE REPEAT ARQ
 Go-Back-N-ARQ protocol is very inefficient for a
noisy link.
 In a noisy link a frame has a high probability of
damage, which means the resending of multiple
frames.
 This resending uses up the bandwidth and slows
down the transmission.
 For noisy links there is another mechanism that
does not resend N frames when just one frame is
damaged, only the damaged frame is resent.
 This mechanism is called Selective-Repeat-ARQ.

19. SENDER AND RECEIVER
WINDOWS
 The configuration of the sender and its control
variables for Selective-Repeat ARQ are the same
as for Go-Back-N ARQ.
 However, the size of the window should be at
most one half of the value 2m
.
 The receiver window must have the same size.
 This window however specifies the range of the
accepted received frame.
 In selective –Repeat ARQ the receiver is looking
for a range of sequence numbers.

20. CONTROL VARIABLES
 The receiver has two control variables RF and RL
to define the boundaries of the window.

21. SELECTIVE REPEAT ARQ, LOST
FRAME

22. THANKS!!!