Lec-23: Go-Back-N ARQ (Automatic Repeat Request) | Data Link layer
Go-Back-N Sliding Window Protocol
This section introduces the concept of the Go-Back-N sliding window protocol and explains its characteristics.
Sliding Window Protocol
- The Go-Back-N protocol is a sliding window protocol.
- Unlike the previous protocol, it has multiple windows instead of just one.
Window Size
- The size of the sender's window is 2^m - 1, where m represents the number of bits used to represent the sequence numbers.
- The size of the receiver's window is always 1.
Sequence Number Representation
- The value of m determines how many bits are required to represent the sequence numbers.
- For example, if m = 2, we can represent sequence numbers from 0 to 3 (00, 01, 10, 11).
Synchronization
- The sender and receiver windows remain synchronized.
- After reaching the maximum sequence number, they wrap around back to zero.
Window Size Limitation
- It is important that the window size is less than 2^m. If it equals 2^m, issues may arise.
Sliding Window Operation
- The sender sends packets within its window size.
- As packets are acknowledged by the receiver, the window slides forward.
Sliding Window Size and Issues
This section discusses the importance of sliding window size and potential issues that can occur when it is not properly set.
Importance of Window Size
- The sender's window size should be less than 2^m to avoid problems.
Example Scenario
- If a sender with a window size of 3 (for m = 2) sends frames numbered as 0,1,and 2,
all acknowledgments for these frames are lost.
Window Size Limitation
- When the window size is equal to 2^m, issues can occur due to lost acknowledgments.
Sliding Window Operation
This section explains how the sliding window operates and how packets are sent and acknowledged.
Sliding Window Process
- The sender's window moves forward as packets are acknowledged.
- The receiver's window remains fixed at a size of 1.
Packet Transmission
- The sender can send multiple frames within its window size.
- The receiver can only accept one frame at a time.
Example Scenario
- As the sender sends frames 0, 1, and 2, the receiver accepts them in order.
- Once frame 0 exits the window, the window moves to include frames 1 and 2.
- This process continues as long as there are more packets to be sent.
Conclusion
The Go-Back-N sliding window protocol is a method for reliable data transmission. It uses multiple windows with specific sizes for both the sender and receiver. The sender's window slides forward as packets are acknowledged by the receiver. It is important to set the window size correctly to avoid issues with lost acknowledgments.
Understanding Frame Loss in Go-Back-N Protocol
In this section, the speaker explains the concept of frame loss in the Go-Back-N protocol and its impact on the transmission process.
Frame Loss and Acknowledgement
- When a frame is sent but its acknowledgement is lost, it creates a problem in the communication.
- The sender waits for a timeout to occur when no acknowledgement is received within a certain time period.
- If no acknowledgement is received, the sender assumes that the frame was lost and retransmits it along with subsequent frames.
Window Size Considerations
- The window size should be less than 2 raised to power m, where m represents the number of bits used for sequence numbers.
- Taking a window size equal to 2 raised to power m minus 1 ensures efficient transmission without accepting duplicate frames.
Out-of-order Packets
- In Go-Back-N protocol, out-of-order packets are not accepted by the receiver.
- The receiver uses cumulative acknowledgements to indicate that all previous frames up to a certain sequence number have been successfully received.
Accepting Out-of-order Packets in Go-Back-N Protocol
This section discusses whether out-of-order packets can be accepted in the Go-Back-N protocol and explains how cumulative acknowledgements are used.
Accepting Out-of-order Packets
- In Go-Back-N protocol, out-of-order packets are not accepted by the receiver.
- The sender sends frames sequentially from 0 to n, where n represents the window size.
Cumulative Acknowledgements
- Cumulative acknowledgements are used in Go-Back-N protocol.
- Instead of sending individual acknowledgements for each frame, cumulative acknowledgements indicate that all previous frames up to a certain sequence number have been successfully received.
Conclusion
The transcript provides an explanation of frame loss in the Go-Back-N protocol and discusses the considerations for window size and accepting out-of-order packets. It also highlights the use of cumulative acknowledgements in the protocol. Understanding these concepts is crucial for studying and implementing reliable data transmission protocols.
New Section
This section discusses the issue of packet loss in the Go-Back-N protocol due to limited receiver window size.
Window Size and Packet Loss
- The sender sent packets 0, 1, 2, and 3. Packet 0 was accepted, but an acknowledgement for packet 1 was lost during transmission.
- Since the receiver's window size is only 1, it accepted packet 0 and is waiting for packet 1. Packets 2 and 3 reached the receiver successfully.
- The receiver will not accept packets 2 and 3 because it is still waiting for packet 1. This limitation is specific to the Go-Back-N protocol.
- In Selective Repeat protocol, out-of-order packets can be accepted since the window moves dynamically.
- In Go-Back-N, if a packet arrives out of order (e.g., packets 2 and 3 before packet 1), they will be discarded by force because the receiver wants only packet number 1.
Resending Packets
- After some time, the sender will resend packet number 1. Once acknowledged, the window will move to the next sequence number (packet number) which is now packet number 2.
- Consequently, packets that were previously discarded (e.g., packets 2 and 3) need to be resent as well.
This problem of discarding out-of-order packets due to limited window size is a characteristic issue in Go-Back-N protocol.
The transcript provided does not specify a language other than English.