Abstract: The quality of service provided by a connection in transferring data units (19) between first and second points (A, B) across a network (10), is measured by a method involving monitoring the normal connection traffic at the first and second points (A, B) to detect the occurrence of the same events at each point (unit 22). An event is deemed to have occurred at a monitored network point when a predetermined set of criteria concerning one or more data units is satisfied by the connection data-unit traffic at the point concerned. Whenever an event is detected, an event report or "digest" is generated (unit 25), this digest including a signature based on the contents of the data units giving rise to the event. Digests from both monitored network points (A, B) are sent to a correlation unit (30) where digests relating to the occurrence of the same event at the two network points are matched up. The matched digest pairs are then passed to a measurement unit (31) to derive quality of service measurements.

Abstract: Monitoring apparatus is provided for monitoring an on-going input to detect and report events of a predetermined type as defined by a set of criteria stored by the apparatus. Detected events are reported in a report stream the data rate of which increases with the number of such events. The data rate of the report stream is regulated to lie within upper and lower bounds by changing the selectivity of the set of event-selection criteria. This changing of the selectivity of the critera is effected in such a manner that the events detected with more selective criteria constitute a subset of the events that would have been detected with a less selective criteria. In a preferred embodiment, changing the criteria selectivity is done by changing the length of a bit mask.