Flow control specifies how much data a sender can transmit before receiving permission to continue. There are two main types of flow control: stop-and-wait and sliding window. Stop-and-wait allows transmission of one frame at a time, while sliding window allows transmitting multiple frames before needing acknowledgement. Sliding window flow control uses variables like window size, last ACK received, and last frame sent to determine how transmission proceeds. It provides more efficiency than stop-and-wait. Automatic repeat request (ARQ) handles retransmission of lost or damaged frames through timeouts, negative acknowledgements, or cumulative acknowledgements depending on the specific ARQ protocol used.

1. Link Level Flow
Control

2. Flow Control
Flow Control – specifies how much data the sender can transmit
before receiving permission or acknowledgement to continue from the
receiver.
Flow Control
Sliding WindowStop and Wait

3. Stop and Wait
Sender sends one frame at a time stops until it receives the confirmation
(Acknowledgement) from the receiver and then sends the next frame.
Acknowledgement – small control frame that a protocol (receiver side) sends
back to the sender saying that it has received an earlier frame.
Control frame – frame with header only. (No data is in control frame)
Timeout period may vary for different frame transmission.
Advantage:
◦ Simplicity
Disadvantage:
◦ Inefficiency i.e., it allows sender to have only one outstanding frame on the link.

4. Stop and Wait

5. Sliding Window
Sender can transmit several frames continuously before needing an ACK.
If ACK received by the sender before continuous transmission is finished,
sender can transmit.
An ACK can acknowledge the correct receipt of multiple frames at a receiver
(cumulative ACK).
Sender assigns a frame number to each frame. Frames are numbered “modulo
n”.
i.e., 0 to n-1, 0, 1, 2 …n-1, 0, 1, 2 …n-1, 0, 1 … n-1…

6. Sliding Window

7. Sliding Window
 Sender maintains three variables,
o Sender Window Size (SWS)
o Last ACK Received (LAR) – frame number of the last
acknowledgement received.
o Last Frame Sent (LFS) – frame number of the last frame
sent.
LFS – LAR ≤ SWS

8. Sliding Window
Moves to the right frame by frame,
when a frame is sent
Moves to the right, the size of
several frames at a time, when an
ACK is received

9. Sliding Window
o When an acknowledgement received, the sender moves LAR to the
right, thereby allowing the sender to transmit another frame.
o Sender has to buffer the frame up to SWS frames for retransmission
purpose.

10. Sliding Window
 Receiver maintains three variables,
o Receiver Window Size (RWS) – upper bound on the
number of out of order frames that the receiver is willing to
accept.
o Largest Acceptable Frame (LAF) – frame number of the
largest acceptable frame.
o Last Frame Received (LFR) – frame number of the last
frame received.
LAF – LFR ≤ RWS

11. Sliding Window
Moves to the right frame by
frame, when a frame is
received
Moves to the right the size of
the several frames at a time,
when an ACK is sent.

12. Sliding Window
 When a frame with sequence number SeqNum arrives,
If SeqNum ≤ LFR or SeqNum > LAF
- Discard it (the frame is outside the receiver window)
If LFR < SeqNum ≤ LAF
- Accept it
- Now the receiver needs to decide whether or not to send
an ACK

13. Sliding Window

14. Automatic Repeat Request
Automatic Repeat Request (ARQ)
 Retransmission of lost or damaged frame is referred as Automatic Repeat
Request.
 Retransmission taken place,
o When error occurs in frame, receiver sends an NAK (Negative
Acknowledgement) explicitly to the sender.
o When the sender’s timer expires (due to error frame, frame lost or
ACK lost), it triggers the retransmission.
Timeout – the expected delay for sender to receive ACK for the frame

15. Automatic Repeat Request
Selective
Repeat ARQ
Sliding Window
ARQ
Stop and Wait
ARQ
ARQ
Go – Back - N
ARQ

16. Stop and Wait ARQ
 Extension of stop and wait protocol.
 Receiver sends an ACK if frame is correctly received. Sender waits for an ACK
before transmitting the next frame.
Retransmission,
- If sender does not receive anything before timeout period expires
(due to frame lost or ACK lost), it retransmits the frame.
- Suppose if sender receives the NAK (error in frame or damaged
frame), it retransmits that error frame.

17. Stop and Wait ARQ

18. Sliding Window ARQ
Sliding Window ARQ
Two approaches,
 Go – Back – N ARQ
 Selective Repeat ARQ
Go – Back – N ARQ:
– Go – Back – N uses sliding window flow control protocol.
– If no errors occur the operations are similar to sliding window.
– A station may send multiple frames as allowed by the window.
Retransmission,
 Receiver sends a NAK ‘i’ if frame ‘i’ contains error (or) frame ‘i’ was lost (i.e., out of order frame). After that
receiver discards all frames until the error frame (or) lost frame was correctly retransmitted.
 If sender receives the NAK for frame ‘i’ it will retransmit frame ‘i’ and all frames i+1, i+2… which have been
sent, but not been acknowledged.

19. Sliding Window ARQ

20. Sliding Window ARQ

21. Sliding Window ARQ

22. Sliding Window ARQ
Selective Repeat ARQ
– Here sender only retransmits frames for which NAK is received.
Advantage over Go – Back – N,
 Fewer retransmissions
Disadvantages,
 More complex at sender and receiver.
 Each frame must be acknowledged individually (no cumulative
acknowledgement).
Receiver may receive out of sequence frames.

23. Sliding Window ARQ
For ACK / NAK loss, Go – Back – N is used. i.e., selective repeat
work similar to Go – Back – N.