A method in a network management system for diagnosing a root cause of a fault in a multi-layer communications network, the method comprising the steps of: - receiving a first set of performance measurements representing performance of network elements on at least a first and a second layer in the multi-layer communications network, characterized in that the method further comprising: - representing the first set of performance measurements by input events (e(i)); - localizing the fault by identifying a probable set of the network elements affected by the fault, wherein the probable set of the network elements is identified by: associating a set of the network elements (e(i).elements) to each of the input events (e(i)), wherein a network element, of the set of the network elements (e(i).elements), is associated to a plurality of the input events (e(i)); and identifying at least one network element, of the set of the network elements (e(i).elements) associated to each of the input events (e(i)) representing the first set of performance measurements; - inferring a type of the fault from one or more symptoms incurred from one or more performance measurements in the first set of performance measurements; - determining the root cause by combining information on the probable set of the network elements with the inferred type of fault; and - triggering a second set of performance measurements in addition to measurements included in the first set of performance measurements based on results of the root cause analysis, wherein the second set of performance measurements represents more frequent performance measurements than those given in the first set of performance measurements. - receiving a first set of performance measurements representing performance of network elements on at least a first and a second layer in the multi-layer communications network, characterized in that the method further comprising: - representing the first set of performance measurements by input events (e(i)); - localizing the fault by identifying a probable set of the network elements affected by the fault, wherein the probable set of the network elements is identified by: associating a set of the network elements (e(i).elements) to each of the input events (e(i)), wherein a network element, of the set of the network elements (e(i).elements), is associated to a plurality of the input events (e(i)); and identifying at least one network element, of the set of the network elements (e(i).elements) associated to each of the input events (e(i)) representing the first set of performance measurements; associating a set of the network elements (e(i).elements) to each of the input events (e(i)), wherein a network element, of the set of the network elements (e(i).elements), is associated to a plurality of the input events (e(i)); and identifying at least one network element, of the set of the network elements (e(i).elements) associated to each of the input events (e(i)) representing the first set of performance measurements; - inferring a type of the fault from one or more symptoms incurred from one or more performance measurements in the first set of performance measurements; - determining the root cause by combining information on the probable set of the network elements with the inferred type of fault; and - triggering a second set of performance measurements in addition to measurements included in the first set of performance measurements based on results of the root cause analysis, wherein the second set of performance measurements represents more frequent performance measurements than those given in the first set of performance measurements.