Arq -

is a fundamental error-control mechanism used in telecommunications and data link layers to ensure reliable transmission over unreliable communication links. Unlike Forward Error Correction (FEC) , which allows the receiver to correct errors, ARQ relies on the receiver identifying errors and requesting retransmission of data from the sender. This paper covers the primary types of ARQ protocols, their mechanisms, and performance tradeoffs. 2. Fundamental Mechanisms ARQ protocols operate based on two main principles:

Only the specific lost or corrupted frame is retransmitted, rather than the entire window. Pros: Highly efficient utilization of bandwidth. While Stop-and-Wait is efficient for simple

Higher bandwidth–delay products require larger window sizes (GBN/SR) to maintain high throughput. 5. Hybrid ARQ (HARQ) specifically Type-II Hybrid ARQ

Low throughput on links with high propagation delay, as the channel remains idle while waiting for ACKs. 3.2. Go-Back-N ARQ (GBN) if the error is too great

combines traditional ARQ with Forward Error Correction (FEC) . The receiver attempts to correct small errors using error-correcting codes; if the error is too great, a retransmission is requested. This approach, specifically Type-II Hybrid ARQ , provides a more efficient approach in poor radio conditions, optimizing throughput. 6. Conclusion

The sender can transmit multiple frames (within a window size

ARQ is an indispensable tool for achieving reliable digital communication. While Stop-and-Wait is efficient for simple, low-delay networks, Go-Back-N and Selective Repeat are essential for modern high-speed networks. The hybrid approach (HARQ) offers the best balance of efficiency and reliability in modern wireless scenarios.