The UE may configure a DRB by applying the information and transmit an RRCConnectionReconfigurationComplete message to the gNB at step 2e-45. The gNB that has completed the DRB configuration with the UE may transmit an “INITIAL CONTEXT SETUP COMPLETE” message to the MME at step 2e-50. The MME that has received the message may exchange an “S1 BEARER SETUP” message and an “S1 BEARER SETUP RESPONSE” message with an S-GW in order to configure an S1 bearer at steps 2e-055 and 2e-60, respectively. The S1 bearer is a connection for data transmission set up between the S-GW and the gNB, and corresponds to a DRB in a one-to-one manner. When the entire process is completed, the UE may transmit/receive data to/from the gNB through the S-GW at steps 2e-65 and 2e-70, respectively.