is an old sgsn after a ue using an isr mechanism changes from the 3gpp network mobile to a non 3gpp network the us initiates an attachment procedure through the 3gpp network then a resource release process is performed the method includes the following steps in step h 1 the ue performs a service process in the 3gpp communication network and initiates an attach session negotiation procedure in step h 2 after the attach negotiate is completed a non 3gpp gw or a non 3gpp access ne of the non 3gpp communication network sends a handover complete message to the old mme of the first 3gpp communication network or the old sgsn of the second 3gpp communication network of the 3gpp communication networks the process that the non 3gpp gw or the non 3gpp access ne sends the handover complete message to the old mme of the first 3gpp communication network is the same as the process that the non 3gpp gw or the non 3gpp access ne sends the handover complete message to the old sgsn of the second 3gpp communication network in this embodiment the process that the non 3gpp gw non 3gpp access ne sends the handover complete message to the old mme of the first 3gpp communication network is set as an example for description in step h 3 after receiving the handover complete message the 3gpp communication network performs a session negotiation on the resource release process with a serving gw by sending a delete bearer request message or a delete pdp context request message including first indication information or third indication information so as to release the resources the first.