Go-Back-N (GBN) is an ARQ protocol that allows a sender to transmit multiple frames before receiving an acknowledgement. The sender maintains a window of size N, meaning it can transmit N frames before waiting for a response. The receiver window is always size 1, acknowledging frames individually. If a frame times out without an ACK, the sender retransmits that frame and all subsequent frames in the window. GBN improves efficiency over stop-and-wait by allowing transmission of multiple frames while reducing waiting time at the sender.

1. GO-BACK-N PROTOCOL
DONE BY:
STEFFY.D(140071601072)
SHEREEN FATHIMA.A(140071601068)
TASNIM TABASUM.R(140071601080)
COMPUTER NETWORKS LAB
B.S.ABDUR RAHMAN UNIVERSITY

2. CONTENTS
• ARQ
• Definition
• Why we need GBN?
• Sender window size.
• Receiver window size.
• Data packet is lost.
• Acknowledgement
• Advantages

3. Automatic Repeat Request(ARQ)
• Systems which automatically request the retransmission of missing
packets or packets with errors are called ARQ systems.
Three common schemes are used:
• Stop & Wait
• Go Back N
• Selective Repeat

4. DEFINITION
• Go-Back-N ARQ is a specific instance of the automatic repeat request
(ARQ) protocol, in which the sending process continues to send a number
of frames specified by a window size even without receiving an
acknowledgement(ACK) packet from the receiver.
• GBN is a practical approach of sliding window protocol.

5. Why we need GBN?
• To improve efficiency of transmission.
• To send more than one frames at a time.
• To reduce waiting time of the sender .
• (i.e.)Sender is waiting for an acknowledgement from the
receiver.

6. Sender sliding Window
• Sender can transmit N frames before receiving ACK.
• Its window size will be N.
• Sender will maintain a copy of sent packets in sent buffer until for the
particular sent packets are acknowledged.
• Sender will resend all packets if the timeout timer runs out.
• Once data get acknowledged by the receiver, that particular data will be
removed from the buffer.

7. Receiver sliding window
• Size of the window at the receiving
site is always 1 in this protocol.
• 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.
• Receiver window slides as shown in
fig. Receiver is waiting for frame 0
in part a.

8. Receiver sliding window
• Receiver sends positive ACK if a frame arrived safe and in order.
• The silence of the receiver causes the timer of the unacknowledged frame to expire.
• Then the sender resends all frames, beginning with the one with the expired timer.
• For example, suppose the sender has sent frame 6, but the timer for frame 3 expires (i.e.
frame 3 has not been acknowledged), then the sender goes back and sends frames 3, 4, 5,
6 again. Thus it is called Go-Back-N-ARQ
• The receiver does not have to acknowledge each frame received, it can send one
cumulative ACK for several frames.

9. Sender window size.
• Sender window size in GBN is N itself.
• N value must be greater than 1
• If N value is equal to 1,then it is stop & wait protocol.
Representation:
GBN=GB10=>N=10=>sender window size.

10. Receiver window size
• Receiver window size is always 1.
Let us consider an example,GB4=>N=4=>sender window size.
SENDER RECEIVER
3 2 1 0 0
1
0
2
3
1 2 3
Ack0
Ack1
Ack2
Ack3
Normal operation

11. Data packet is lost
0
1
2
3
3 2 1 0
SENDER RECEIVER
Ack 0
Ack 1
T0 timer
2
3
0 1
Retransmission
45
4
5
5
4
1
0
2
3

12. Acknowledgement
• There are two types of acknowledgement used in GBN
•Cumulative Acknowledgement
•Independent Acknowledgement

13. COMPARISON
Cumulative Acknowledgement
• Traffic is less
• Reliability is less
Independent Acknowledgement
• Traffic is more
• Reliability is more

14. Graphical Representations
Cumulative Acknowledgement
1 0
Independent Acknowledgement
0 1
0
1
0
1
Ack 0
Ack 1
1 0
sender
Receiver sender Receiver
0 1

15. Acknowledgement Timer
0
1
2
3Ack 2
Ack 4
3 2 1 0 0 1 2 3
Acknowledgement timer
sender

16. case: 1
Acknowledgement Timer is too big:
• ADVANTAGE :
More packets can be acknowledged at a time at the receiver
side.
• DISADVANTAGE:
Timeout(To) timer runs out at the sender side.

17. Case : 2
Acknowledgement Timer is too small:
• DISADVANTAGE: Acknowledgement will be send by the receiver for
every packets.
• So , Acknowledgement timer should not be too small or too big. It should
be a timer with fixed size.
• Timeout timer must be greater than the Acknowledgement timer.

18. Relation between window size and sequence
number
3 2 1 0
0
0
1
2
3To
3 0 121
0
1
2
3
2 3
Duplicate packets

19. ADVANTAGES
• The sender can send many frames at a time.
• Timer can be set for a group of frames.
• Only one ACK can acknowledge one or more frames.
• Efficiency is more.
• Waiting time is pretty low.

20. Points to be remembered
• Sender window size is N & N value must be greater than 1
• Receiver window size is always 1
• There are two types of acknowledgement in GBN
•Cumulative Acknowledgement
•Independent acknowledgement