Positive Acknowledgement with Retransmission (PAR)

Positive Acknowledgement with Retransmission (PAR) is a group of error-control protocols for transmission of data over noisy or unreliable communication networks. These protocols reside in the Data Link Layer and Transport Layer of the OSI model. They provide automatic retransmission of frames that are corrupted or lost during transit. PAR is also called Automatic Repeat Request (ARQ).

PAR protocols are used to provide reliable transmissions over unreliable network services. They are commonly implemented in wireless communication systems like GSM networks, where signal interference and packet loss are frequent challenges.

Working Principle

In PAR protocols, the receiver sends an acknowledgement (ACK) message back to the sender when it receives a frame correctly. If the sender does not receive the acknowledgement within a specified timeout period, it assumes the frame was corrupted or lost during transmission. The sender then retransmits the frame and waits for acknowledgement again. This process continues until successful transmission is confirmed.

Types of ARQ/PAR Protocols

There are three main ARQ protocols used in data communication:

Stop-and-Wait ARQ

Stop-and-Wait ARQ provides unidirectional data transmission with flow control and error control. The sender transmits one frame and waits for acknowledgement before sending the next frame. If no ACK is received within the timeout period, the frame is retransmitted. This protocol is simple but has low efficiency due to the waiting time.

Go-Back-N ARQ

Go-Back-N ARQ allows sending multiple frames using a sliding window approach before receiving acknowledgements. If an error is detected in any frame, all frames starting from the erroneous frame are retransmitted, even if some subsequent frames were received correctly.

Selective Repeat ARQ

Selective Repeat ARQ also uses a sliding window but only retransmits the specific frames that were lost or corrupted. Correctly received frames are buffered at the receiver, making this the most efficient ARQ protocol.

Conclusion

PAR protocols ensure reliable data transmission over unreliable networks through acknowledgements and retransmission mechanisms. The choice between Stop-and-Wait, Go-Back-N, and Selective Repeat depends on the trade-off between implementation complexity and transmission efficiency.