In a method of monitoring the reliability of a data link for a data transmission between a server (2) and a client (8), a control unit (14) is triggered by a trigger signal in such a manner that in response to the arrival of the trigger signal the control unit (14) generates first control data for a predetermined running time which commences anew in response to each trigger signal received. The first control data forms a first control representation (23) on a client display screen (11), which first control representation optically indicates the existence of a reliable data link (4) to a user. The control unit (14) generates, after expiration of the running time, second control data which generates a second control representation (23') on the client display screen (11) which optically indicates the absence of a reliable data link to the user. A control unit (15) periodically transmits a test message to a test unit (18). The test unit (18) returns the incoming test message or another test message to the server (2). The control unit (14) is triggered again only if the returned test message is in order. The period with which the control unit (15) repeats the transmission of the test messages is shorter than the running time during which the control unit (14) generates the first control data.