A communication system having at least one first and a second communication unit. The first communication unit has a counter memory unit which stores a counter value, and is designed such that the occurrence of a first communication event prompts the counter value in the counter memory unit to be changed in a defined first manner. At least the occurrence of a defined reference event is followed by the counter value in the counter memory unit being changed in a defined second manner. In the course of a second defined communication event, the first communication unit transmits the current counter value in the counter memory unit to the second communication unit.