computer Network

1. 1
 It coordinates the amount of data that can be sent before
receiving acknowledgment.
 It provides the receiver’s acknowledgment of frames
received corrupted.
Flow control

2. 2
Stop and Wait

3. 3
Normal operation
In Stop-and-Wait ARQ, numbering frames prevents the
retaining of duplicate frames.

4. 4
Stop-and-Wait ARQ, lost frame

5. 5
Stop-and-Wait ARQ, lost ACK frame

6. 6

7. 7
Sliding Window

8. 8
Sliding Window
Sender Sliding Window

9. 9
Sliding Window
Receiver Sliding Window

10. 10
Sliding Window Example

11. 11

12. 12

13. 13
Error Control

14. 14

15. Go Back N ARQ
What is Go Back N ARQ?
• This protocol is based on the method of using the
Sliding Window Protocol as the basis of data
exchange, where the ’N’ in the protocol represents
the window size.
• The term ARQ represents the Automatic Repeat
Request, which refers to sending multiple frames
simultaneously from the sender to the receiver end.
15

16. Go Back N ARQ
Important Feature of the Protocol
• The frames shared in the protocol are sequenced for
better efficiency, to avoid any retransmission of
shared data, and differentiate between the frames.
16
0 1 2 3 4 5 6 7
Sequence Number

17. Go Back N ARQ
Important Feature of the Protocol
17
The protocol is designed to share
multiple frames at a time, with the
receiver end, before expecting any
acknowledgment from it. This
simultaneous exchange of data is
termed protocol pipelining.

18. Go Back N ARQ
Important Feature of the Protocol
18
If the acknowledgment is not
shared to the sender side within a
certain time frame, all the frames
after the non-acknowledged frame
are to be retransmitted to the
receiver side.

19. Go Back N ARQ
Working of the Protocol
The working of the Go-Back-N ARQ protocol
involves applying the sliding window method for the
basis of sharing data, and the number of frames to be
shared is decided by the window size.
19

20. Go Back N ARQ
Working of the Protocol
1. To begin with, the sender side will share the data
frames simultaneously according to the window size
assigned, over to the receiver side, and wait for the
acknowledgment.
20

21. Go Back N ARQ
Working of the Protocol
2. After the receiver side receives the frames, it will
use the first frame and send the acknowledgment to
the sender side.
21

22. Go Back N ARQ
Working of the Protocol
3. After the sender receives the acknowledgment for
the first frame, the sender will share the next frame
with the receiver.
22

23. Go Back N ARQ
Working of the Protocol
4.This exchange continues until, due to some external
or internal interruption in the network, the
acknowledgment is not received by the sender side
23

24. Go Back N ARQ
Working of the Protocol
5.Then, the sender side will go back to the
unacknowledged frame and retransmit that frame,
along with all the frames shared after that frame with
the receiver. This represents the Go-Back-N ARQ
protocol method.
24

25. Go Back N ARQ
Advantages
• Multiple frames can be simultaneous to the receiver
side.
• Increase the efficiency of the data transfer and has
more control over the flow of frames.
• Time delay is less for sharing data frames.
25

26. Go Back N ARQ
Disadvantages
• The storage of data frames at the receiver side.
• Retransmission of frames, when the
acknowledgement is not received by the sender end.
26

27. Go Back N ARQ
Problem Statement
In a network channel, the number of frames given is
10 with the window’s size assigned as 3 and every 4th
packet is lost. Find the total number of messages sent
from the sender to the receiver side.
a) 27
b) 25
c) 22
d) 29
27

28. Go Back N ARQ
Problem Statement
In a network channel, the number of frames given is
10 with the window’s size assigned as 3 and every 4th
packet is lost. Find the total number of messages sent
from the sender to the receiver side.
1 2 3 4 5 6 4 5 6 7 5 6 7 8 6 7 8 9 7 8 9 10 8 9 10 9 10
28

29. Go Back N ARQ
Problem Statement
In a network channel, the number of frames given is
10 with the window’s size assigned as 3 and every 4th
packet is lost. Find the total number of messages sent
from the sender to the receiver side.
a) 27
b) 25
c) 22
d) 29
29

30. 30

31. 31

32. 32

33. 33

34. 34

35. Comparison
Stop and Wait
ARQ
Go back N
Selective
Repeat
Remarks
Efficiency 1 / (1+2a) N / (1+2a) N / (1+2a)
Go back N and
Selective
Repeat gives
better
efficiency than
Stop and Wait
ARQ.
35

36. Comparison
Stop and Wait
ARQ
Go back N
Selective
Repeat
Remarks
Window Size
Sender Window
Size = 1
Receiver
Window Size =
1
Sender Window
Size = N
Receiver
Window Size =
1
Sender Window
Size = N
Receiver
Window Size =
N
Buffer
requirement in
Selective
Repeat is very
large.
If the system
does not have
lots of memory,
then it is better
to choose Go
back N.
36

37. Comparison
Stop and Wait
ARQ
Go back N
Selective
Repeat
Remarks
Minimum
number of
sequence
numbers
required
2 N+1 2 x N
Selective
Repeat requires
large number of
bits in sequence
number field.
37

38. Comparison
Stop and Wait
ARQ
Go back N
Selective
Repeat
Remarks
Retransmissions
required if a
packet is lost
Only the lost
packet is
retransmitted
The entire
window is
retransmitted
Only the lost
packet is
retransmitted
Selective Repeat
is far better than
Go back N in
terms of
retransmissions
required.
38

39. Comparison
Stop and Wait
ARQ
Go back N
Selective
Repeat
Remarks
Bandwidth
Requirement
Bandwidth
requirement is
Low
Bandwidth
requirement is
high because
even if a single
packet is lost,
entire window
has to be
retransmitted.
Thus, if error rate
is high, it wastes
a lot of
bandwidth.
Bandwidth
requirement is
moderate
Selective Repeat
is better than Go
back N in terms
of bandwidth
requirement.
39

40. Comparison
Stop and Wait
ARQ
Go back N
Selective
Repeat
Remarks
CPU usage Low Moderate
High due to
searching and
sorting required
at sender and
receiver side
Go back N is
better than
Selective Repeat
in terms of CPU
usage.
40

41. Comparison
Stop and Wait
ARQ
Go back N
Selective
Repeat
Remarks
Level of
difficulty in
Implementation
Low Moderate
Complex as it
requires extra
logic and sorting
and searching
Go back N is
better than
Selective Repeat
in terms of
implementation
difficulty.
41

42. Comparison
Stop and Wait
ARQ
Go back N
Selective
Repeat
Remarks
Acknowledgeme
nts
Uses independent
acknowledgemen
t for each packet
Uses cumulative
acknowledgemen
ts (but may use
independent
acknowledgemen
ts as well)
Uses independent
acknowledgemen
t for each packet
Sending
cumulative
acknowledgemen
ts reduces the
traffic in the
network but if it
is lost, then the
ACKs for all the
corresponding
packets are lost.
42

43. Comparison
Stop and Wait
ARQ
Go back N
Selective
Repeat
Remarks
Type of
Transmission
Half duplex Full duplex Full duplex
Go back N and
Selective Repeat
are better in
terms of channel
usage.
43

44. Conclusions
44
• Go back N is more often used than other protocols.
• SR protocol is less used because of its complexity.
• Stop and Wait ARQ is less used because of its low efficiency.
• Depending on the context and resources availability, Go
back N or Selective Repeat is employed.
• Selective Repeat and Stop and Wait ARQ are similar in
terms of retransmissions.
• Go back N and Selective Repeat are similar in terms of
efficiency if sender window sizes are same.
• SR protocol may be considered as a combination of
advantages of Stop and Wait ARQ and Go back N.
• SR protocol is superior to other protocols but because of its
complexity, it is less used.

45. Thanks!!
45