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Go back-n protocol
- 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