A base station has a wireless interface, a processor, and a management interface. The processor determines a delay before an attach request was successful, the processor also being arranged to determine a cumulative distribution of the delay values of different attach requests, and a compressed representation of the cumulative distribution. The management interface is used to send the compressed representation to a management system. The cumulative distribution is simpler, having no change in a sign of its gradient, and so can be approximated and represented in compressed form with much less risk of loss of relevant information.