retrans

Retrans, short for retransmission, is the act of sending a data unit or signal again after the initial attempt failed or the item was lost. It is a fundamental mechanism for preserving reliability in imperfect channels, typically used when error detection methods indicate corruption or absence of an expected packet.

In digital networks, retransmission is a core element of Automatic Repeat reQuest (ARQ). When a receiver detects a corrupted or missing packet, it requests a retransmission. Retransmissions can be timer-based, triggered when an acknowledgement is not received within a calculated retransmission timeout, or triggered by duplicate acknowledgments that signal a loss (fast retransmit). Common ARQ schemes include Go-Back-N, which resends from the missing packet onward, and Selective Repeat, which resends only the specific affected packets. Protocols such as TCP implement retransmission timers and congestion control; excessive retransmissions can indicate congestion and reduce throughput.

In broadcasting and media law, retransmission consent refers to the authorization and compensation required for a broadcaster’s signal to be carried by cable or satellite providers in a given market. This is a regulatory and commercial process rather than a technical mechanism, but it affects the availability and cost of retransmitted content.

In streaming and content delivery, retransmission may involve re-fetching data segments when errors occur, or using partial retransmission strategies. Retransmissions improve reliability but increase latency and bandwidth usage; many systems couple ARQ with forward error correction to balance reliability and efficiency.