LCG ID: The Logical Channel Group ID field identifies the group of logical channel(s) which buffer status is being reported. The length of the field is 2 bits. For NB-IoT, the LCG ID is set to #0.
Buffer Size: The Buffer Size field identifies the total amount of data available across all logical channels of a logical channel group after all MAC PDUs for the TTI have been built. The amount of data is indicated in number of bytes. It shall include all data that is available for transmission in the RLC layer and in the PDCP layer; the definition of what data shall be considered as available for transmission is specified in [3] and [4] respectively. The size of the RLC and MAC headers are not considered in the buffer size computation. The length of this field is 6 bits. If extendedBSR-Sizes is not configured, the values taken by the Buffer Size field are shown in Table 6.1.3.1-1. If extendedBSR-Sizes is configured, the values taken by the Buffer Size field are shown in Table 6.1.3.1-2.
[FIG. 6.1.3.1-1 of 3GPP TS 36.321 V14.2.0, entitled “Short BSR and Truncated BSR MAC control element”, is reproduced as FIG. 5]
[FIG. 6.1.3.1-2 of 3GPP TS 36.321 V14.2.0, entitled “Long BSR MAC control element”, is reproduced as FIG. 6]
[Table 6.1.3.1-1 of 3GPP TS 36.321 V14.2.0, entitled “Buffer size levels for BSR”, is reproduced as FIG. 7]
[Table 6.1.3.1-2 of 3GPP TS 36.321 V14.2.0, entitled “Extended Buffer size levels for BSR”, is reproduced as FIG. 8]
6.1.3.1a Sidelink BSR MAC Control Elements