Method for establishing/releasing a MAC (medium access control) entity in a wireless communication system and a device therefor

A user equipment (UE) in a wireless communication system comprising a master base station (BS) and a secondary BS, the UE comprising:

• a transmitter and a receiver;
• and
• a processor, operatively coupled to the transmitter and the receiver, that:
• controls the receiver to receive a first Radio Resource Control (RRC) message, for configuration of a new secondary cell group (SCG) associated with the secondary BS, from the master BS via a serving cell of a master cell group (MCG),
• configures a second medium access control (MAC) entity for the SCG based on the first RRC message, when the second MAC entity is not configured for the UE,
• controls the receiver to receive a second RRC message for addition of a new SCG cell to the SCG from the master BS, wherein the processor adds the new SCG cell to the SCG according to the second RRC message,
• wherein the first MAC entity for the MCG associated with the master BS is configured for the UE,
• wherein the second MAC entity is used for one or more secondary cells including the new SCG cell belonging to the SCG, and
• wherein every bearer served by the SCG is mapped to the second MAC entity.