1. A method of controlling usage of network resources with a Policy and Charging Rules Function server “PCRF”, which is in charge of providing control rules and usage limits for usage of network resources by users during a plurality of reporting periods of time, and with a Policy and Charging Enforcement Function server “PCEF”, which is in charge of enforcing the control rules and monitoring the usage of network resources per user during the plurality of reporting periods, the method comprising the steps of: determining at the PCEF an event during a first reporting period for which control rules are required for one or more user traffic flows of a user; requesting control rules from the PCEF to the PCRF, the request including information about usage of network resources by the user during the first reporting period; determining at the PCRF first control rules as a result of a first policy evaluation by taking into account the information about usage of network resources by the user during the first reporting period; determining at the PCRF second control rules as a result of a second policy evaluation by taking into account an initial value for usage of network resources by the user during a second reporting period following the first reporting period; submitting from the PCRF to the PCEF the first and second control rules; enforcing at the PCEF the first control rules received from the PCRF during the first reporting period; and without interrogating the PCRF and upon starting the second reporting period for the user, inactivating the first control rules and enforcing at the PCEF the second control rules received from the PCRF.