A method of commencement of operation of a communication system such as an Ethernet OAM system enables an endpoint (A) to transmit and receive repetitive connectivity check messages on a link between the endpoint and a remote endpoint (B). A loss of connectivity with the remote endpoint is determined by the absence of received connectivity check messages within a monitoring interval. The commencement of the monitoring interval is delayed until a predetermined number of valid connectivity check messages has been received. The method may be implemented by use of a counter for received connectivity check messages and an additional waiting state in a state machine.