Flow control and error control are responsible for managing data transmission rates and ensuring reliable data transmission on the data link layer of the OSI model. Flow control involves the receiver notifying the sender of its buffer limit and acknowledging received data so the sender does not overflow the receiver's buffer. Error control involves the sender retransmitting frames if acknowledgment of receipt is not received. Three common error control protocols are stop-and-wait ARQ, go-back-N ARQ, and selective repeat ARQ, which differ in their window sizes and retransmission strategies.

1. Flow Control and
Error Control
Automatic Repeat Requests on
Data Link Layer
Presentation on

2. OSI Model Layers
Responsible for Flow and Error Control
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3. Flow Control
Sender Receiver
Data
When the receiver is
receiving some data from
the sender, it has to
mention it’s receiving limit
and ACK. If it doesn’t, the
sender may send some
enormous amount of data
which will cause overflow.
will send
10 bytes
limit: 5
bytes
sending
5B
no ACK
overflow
Data
sending
5B
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4. Flow Control
Sender Receiver
ACK
Data
Data
ACK
So, a set of procedure to
restrict the amount of
data the sender can
send before waiting for
acknowledgement is
introduced.
This is flow control.
will send
10 bytes
limit: 5
bytes
sending
5B
received
5B
sending
5B
received
5B
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5. Error Control
Sender Receiver
Frame 0
Similar to Flow Control, if a
sender is sending a frame
and does not receive any
acknowledgement, it will
re-send the frame
automatically.
will send
10 bytes
limit: 5
bytes
sending
5B
no ACK
error
Frame 0
sending
5B
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6. Error Control
Sender Receiver
ACK 0
Frame 0
Frame 1
ACK 1
But when a frame is sent
successfully and
receiver sends an
acknowledgement of it,
the sender can send the
next frame.
This is error control.
will send
10 bytes
limit: 5
bytes
sending
5B
received
5B
sending
5B
received
5B
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7. Protocols
By combining the Flow Control and Error Control, we get 2
different protocols to resolve error. As-
Noiseless Channel
Simplest
Stop-and-Wait
Noisy Channel
Stop-and-Wait
Go-Back-N
Selective Repeat
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8. Noisy Channel Protocols
Our discussion will be based on these 3 noisy channel
protocols-
Noisy Channel
Stop-and-Wait
Go-Back-N
Selective Repeat
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9. 1. Stop and Wait Auto Repeat Req.
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Sender Receiver
Sends one frame at a time Receives one frame at a time
Send the next packet only after
receiving acknowledgement for
the previous
Send acknowledgement after
receiving and consuming a data
packet
1 (S) + 1 (R) = 2 Sequence Number

10. 1. Stop and Wait Auto Repeat Req.
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Sender Receiver
i. Sent and Acknowledged
Frame 0
ACK 1
0 1 0 1 0 1
0 1 0 1 0 1
Start
Stop
Successful
0 1 0 1 0 1

11. 1. Stop and Wait Auto Repeat Req.
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Sender Receiver
ii. Frame lost on sender side
Frame 1
ACK 0
0 1 0 1 0 1
Start
Stop
0 1 0 1 0 1 Frame 1 (R)
0 1 0 1 0 1
0 1 0 1 0 1
Timed
Out,
Resent

12. 1. Stop and Wait Auto Repeat Req.
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Sender Receiver
iii. Acknowledge lost on receiver side
Frame 0
ACK 1
0 1 0 1 0 1
Start
Stop
0 1 0 1 0 1 Frame 0 (R)
0 1 0 1 0 1
0 1 0 1 0 1
Timed
Out,
Resent
0 1 0 1 0 1
ACK 1

13. 2. Go-Back-N Auto Repeat Req.
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Sender Receiver
Sender Window 2m - 1 Receiver Window 1
Send one or more packet only after
receiving acknowledgement
in order
Send acknowledgement after
receiving and consuming a data
packet
( 2m – 1 ) + 1 Sequence Number [( 2m – 1 ) = N]

14. 2. Go-Back-N Auto Repeat Req.
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Sender Receiver
i. Sent
Data: 2m = 23 = 8 (01234567)
Window: 23 – 1 = 7
0
1
2
3
4
5
6
1 0 7 6 5 4 3 2 1 0
Window

15. 2. Go-Back-N Auto Repeat Req.
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Sender Receiver
ii. Receiving ACK
0
1
2
3
4
5
6
7
0
1 0 7 6 5 4 3 2 1 0
Window

16. 2. Go-Back-N Auto Repeat Req.
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Sender Receiver
iii. ACK lost
0
1
2
3
4
5
6
1 0 7 6 5 4 3 2 1 0
Window
7
0
1
Timed
Out,
Resent

17. 2. Go-Back-N Auto Repeat Req.
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Sender Receiver
iv. Resend
0
1
1 0 7 6 5 4 3 2 1 0
Window
7
0
1
…
1
2
3
4
5
6
…

18. 2. Go-Back-N Auto Repeat Req.
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Sender Receiver
iv. Resend
0
1
7
0
1
…
1
2
3
4
5
6
…
Discard (Out of Sync)
1 0 7 6 5 4 3 2 1 0
Window

19. 2. Go-Back-N Auto Repeat Req.
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Sender Receiver
iv. Resend
0
1
1 0 7 6 5 4 3 2 1 0
Window
7
0
1
…
1
2
3
4
5
6
…
Discard (Out of Sync)
1

20. 3. Selective Repeat Auto Repeat Req.
Sender Receiver
Sender Window 2m - 1 Receiver Window 2m - 1
Sends the packets with error or
packets with no ACK when timed
out
Send acknowledgement after
receiving and consuming a data
packet
2 x (2m - 1) Sequence Number [( 2m – 1 ) = N]

21. 3. Selective Repeat Auto Repeat Req.
1 0 7 6 5 4 3 2 1 0
Window
Data: 2m = 23 = 8 (01234567)
Window: 23-1 = 4
Sender Receiver
0
1
2
3

22. 3. Selective Repeat Auto Repeat Req.
1 0 7 6 5 4 3 2 1 0
Window
Sender Receiver
0
1
2
3
0
1
4
5
2
2
3
6

23. Comparison
Stop-and-Wait ARQ Go-Back-N ARQ Selective Repeat ARQ
Single Frame Flow Control Window Flow Control Window Flow Control
For Lower Error Rate For Higher Error Rate For Higher Error Rate
Less Efficient Moderate Efficient Most Efficient

24. Comparison
Stop-and-Wait ARQ Go-Back-N ARQ Selective Repeat ARQ
Resent Only Single Lost
Packet
Resent The Lost Packet
Window
Resent The Lost Packets
Small Size Buffer Large Size Buffer Moderate Size Buffer

25. Conclusion
Stop-and-Wait ARQ is simple to implement and useful for low-
bandwidth connections, but its efficiency is very low for high-
bandwidth connections. Go-Back-N ARQ is more efficient than Stop-
and-Wait ARQ for high-bandwidth connections, but it can result in a
higher number of retransmissions and increased delay due to the use
of a sliding window. Selective-Repeat ARQ provides the best efficiency
and reduced retransmissions by allowing the receiver to accept out-of-
order packets.

26. Got Any Question?

27. Thank You! 