Lec-24: Selective Repeat ARQ (Automatic Repeat Request) | Data Link Layer
Selective Repeat
This section discusses the concept of Selective Repeat in sliding window protocols. The sender and receiver windows have the same size, which is determined by 2^m - 1, where m is the sequence number length.
Sender and Receiver Window Size
- The sender and receiver windows have the same size, which is determined by 2^m - 1.
- For example, if m = 3, the sequence number range is 0 to 7, so the sender and receiver window sizes are both 4 (0 to 3).
- The window size should not be greater than 2^m - 1 to avoid problems with lost acknowledgements.
Handling Out-of-Order Packets
- Selective Repeat allows for out-of-order packets to be accepted.
- The receiver window can accept any packet within its window range, regardless of the order in which they were sent.
Avoiding Duplicate Packets
- To avoid accepting duplicate packets, the window size should not be greater than 2^m - 1.
- If a larger window size is used, duplicate packets may be accepted when retransmissions occur.
Conclusion
This section concludes the discussion on Selective Repeat and highlights key points to remember.
Key Points
- The sender and receiver windows in Selective Repeat have the same size.
- The window size should be equal to or smaller than 2^m - 1 to avoid issues with lost acknowledgements.
- Selective Repeat allows for out-of-order packets to be accepted.
- Duplicate packets can be avoided by using an appropriate window size.
NAK - Negative Acknowledgement
This section discusses the concept of negative acknowledgement (NAK).
NAK - Negative Acknowledgement
- A negative acknowledgement (NAK) is a response sent by a receiver to indicate that there was an error or failure in receiving or processing data.
- It is used in communication protocols to inform the sender that the message was not successfully received or processed.
- NAK helps in ensuring reliable and accurate data transmission by allowing the sender to retransmit the message if it receives a NAK response.
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