Device and method of handling data transmissions after a handover

A communication device for handling data transmissions after a handover, comprising:

• a storage device, for storing instructions of:
• receiving a first radio resource control (RRC) message configuring a first data radio bearer (DRB) and configuring a mapping that a first flow is mapped to the first DRB, from a first base station (BS), wherein the first RRC message includes a mapping configuration configuring the mapping and a DRB configuration configuring the first DRB;
• transmitting a first plurality of Protocol Data Units (PDUs) of the first flow over the first DRB in response to the first RRC message;
• receiving a second PDU of the first flow or a second flow over a second DRB from the first BS;
• transmitting a third plurality of PDUs of the first flow over the second DRB in response to the second PDU;
• receiving a handover command from the first BS;
• performing a handover to a second BS in response to the handover command;
• and
• transmitting a fourth plurality of PDUs of the first flow over the second DRB to the second BS in response to the handover command, after performing the handover to the second BS;
• and
• a processing circuit, coupled to the storage device, configured to execute the instructions stored in the storage device.

A network comprising a first base station (BS) and a second BS for handling data transmissions, configured to perform the instructions of:

• the first BS transmitting a first radio resource control (RRC) message configuring a first data radio bearer (DRB) and configuring a mapping that a first flow is mapped to the first DRB, to a communication device, wherein the first RRC message includes a mapping configuration configuring the mapping and a DRB configuration configuring the first DRB;
• the first BS communicating a first Protocol Data Unit (PDU) of the first flow over the first DRB with the communication device in response to the first RRC message;
• the first BS determining to transmit a second plurality of PDUs of the first flow or a second flow over a second DRB to the communication device;
• the first BS communicating a third plurality of PDUs of the first flow over the second DRB with the communication device in response to the determination;
• the first BS transmitting a handover command to the communication device to hand over the communication device to the second BS;
• and
• the second BS communicating a fourth plurality of PDUs of the first flow over the first DRB with the communication device according to the mapping, after the communication device hands over to the second BS.