A method for improving a buffer status triggering mechanism of a medium access control, MAC, layer for a mobile device of a wireless communication system, the method comprising: (a) receiving an indication indicating that uplink data becomes available for transmission in a radio link control, RLC, layer or a packet data convergence protocol, PDCP, layer; (b) determining whether any data available for transmission other than the uplink data is included in the transmission buffer of the RLC layer or of the PDCP layer; (c) when data available for transmission other than the uplink data is included in a transmission buffer of the RLC or PDCP layer, determining whether a logical channel corresponding to any data of the uplink data has higher priority than logical channels corresponding to the data available for transmission in the transmission buffer; and (d) when the logical channel corresponding to any data of the uplink data has higher priority than the logical channels corresponding to the data available for transmission in the transmission buffer, triggering a buffer status report, BSR; characterized by: (e) when the uplink data becoming available in the transmission buffer is for a retransmission of first data, and the transmission buffer includes no other data available for transmission, triggering a BSR for the retransmission of the first data; and (f) when the uplink data becoming available in the transmission buffer is for a retransmission of the first data, and the transmission buffer includes other data available for transmission, and a logical channel corresponding to the uplink data has higher priority than a logical channel corresponding to the other data available for transmission, triggering a BSR. (a) receiving an indication indicating that uplink data becomes available for transmission in a radio link control, RLC, layer or a packet data convergence protocol, PDCP, layer; (b) determining whether any data available for transmission other than the uplink data is included in the transmission buffer of the RLC layer or of the PDCP layer; (c) when data available for transmission other than the uplink data is included in a transmission buffer of the RLC or PDCP layer, determining whether a logical channel corresponding to any data of the uplink data has higher priority than logical channels corresponding to the data available for transmission in the transmission buffer; and (d) when the logical channel corresponding to any data of the uplink data has higher priority than the logical channels corresponding to the data available for transmission in the transmission buffer, triggering a buffer status report, BSR; characterized by: (e) when the uplink data becoming available in the transmission buffer is for a retransmission of first data, and the transmission buffer includes no other data available for transmission, triggering a BSR for the retransmission of the first data; and (f) when the uplink data becoming available in the transmission buffer is for a retransmission of the first data, and the transmission buffer includes other data available for transmission, and a logical channel corresponding to the uplink data has higher priority than a logical channel corresponding to the other data available for transmission, triggering a BSR.