It should be noted that, in the embodiment of the present invention, description is performed by taking the network side supporting the ISR mechanism as an example That is, in the case that the network side supports the ISR mechanism, the UE performs step 401, where the network side supporting the ISR mechanism refers to both the 2G/3G network and the LTE network support the ISR mechanism. In the embodiment of the present invention, if either the 2G/3G network or the LTE network does not support the ISR mechanism, the network side does not support the ISR mechanism. In the case that the network side does not support the ISR mechanism, step 401 is changed to: when the CS domain and the PS domain of the UE camp on the 2G/3G network, if the UE needs to send or receive data, turn on the LTE radio frequency unit, and the UE initiates a tracking area update request to an MME in the LTE network, so as to make the PS domain of the UE camp on the LTE network.

It should be noted that, in the embodiment of the present invention, an ISR indicator is saved in the UE. The ISR indicator is used to indicate whether the network side supports the ISR mechanism. Therefore, the UE may determine whether the current network side supports the ISR mechanism. A method for obtaining the ISR indicator is specifically: in step 201 in the embodiment shown in FIG. 2, in the process that the UE executes a routing area update process of the PS domain of the 2G/3G network, the MME sends a routing area update feedback message to the UE, where the routing area update feedback message includes the ISR indicator, and the ISR indicator is used to indicate whether the network side supports the ISR mechanism, and the UE saves the ISR indicator.