The invention relates to an online charging method for always on IP connectivity in a communication system, the method comprises a Diameter router receiving an online charging request from a Diameter client, wherein the online charging request includes subscriber information; the Diameter router finding a corresponding online charging system in a subscriber index database based on the subscriber information; sending a successful acknowledge to the Diameter client and generating a ticket file when the corresponding online charging system does not exist in the subscriber index database or the corresponding online charging system is not available, wherein the successful acknowledge includes a dummy quota specified by the Diameter router; and the ticket file processor reading the ticket file based on a scheduled timing and generating a first online charging event request and sending the first online charging event request to the Diameter router when the corresponding online charging system exists in the subscriber index database or the corresponding online charging system is available.
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