A network includes a network core and a node that operates in accordance with a parameter value and is operable to record performance data during a repeating measurement period. The network includes OSS for instructing the node to change the parameter value. The node includes a processor which implements the change in parameter value always at the same predetermined point in the repeating measurement period, preferably at the beginning of the period. In this way, changes to the parameter value are synchronized with the repeating measurement period. It is therefore straightforward to determine how a change in the parameter value affects the performance data as the performance data gathered during a particular measurement period will have been collected while the parameter had a particular value. The processor may automatically change the parameter repeatedly between a first value and a second value during successive measurement periods.