Packet Retry
Packet retry is the resending of a packet that is not acknowledged or that fails the checksum. It hides momentary interference pulses from the user and makes the link look stable; in return, every attempt increases latency and the transmission time consumed. The number of attempts is balanced between these two effects.
In an industrial environment interference is usually not continuous but pulsed: a contactor pulls in, a welding arc forms, an inverter switches. These pulses are on the order of milliseconds and the channel is most often clear on the next attempt.
- Number of attempts: too few and the command is dropped under interference, too many and stopping is delayed when the button is released.
- Wait interval: putting a random delay between attempts resolves the deadlock if two transmitters keep colliding.
- Budget: every retry consumes air time and comes out of the hourly transmission budget.
The critical point is this: the retry mechanism must not become a source of delay for the stop command. In a correct design stopping is also guaranteed by a timeout without waiting for retries; if no valid packet arrives within a defined period the outputs go to a safe state on their own.
Context in which this term is used: Wireless remote control installation: pairing, relay, safety