1. A method for functional testing of a data collection system including: sending sets of messages in first predefined intervals to the data collection system, each of the sets including a first predefined number of messages; and sending batches of said sets in second predefined intervals to the data collection system, each of the batches including a second predefined number of said sets, wherein the first predefined number of messages in each set of messages and the first predefined interval between said sets of messages are defined so that the momentary system load caused by the messages to the data collection system does not cause disturbance to a data collection service provided to clients by the data collection system during the functional testing.