一个 MAC PDU 由 1 个 MAC 头（MAC header）+ 0 个或多个 MAC SDU + 0 个或多个 MAC CE（Control Element） + 可能存在的 padding 组成。MAC header 由一个或多个 MAC subheader 组成。每个 subheader 对应一个 MAC PDU，或一个MAC CE，或 padding。
The MAC header is of variable size and consists of the following fields:
- LCID: The Logical Channel ID field identifies the logical channel instance of the corresponding MAC SDU or the type of the corresponding MAC control element or padding as described in tables 6.2.1-1, 6.2.1-2 and 6.2.1-4 for the DL-SCH, UL-SCH and MCH respectively. There is one LCID field for each MAC SDU, MAC control element or padding included in the MAC PDU. In addition to that, one or two additional LCID fields are included in the MAC PDU, when single-byte or two-byte padding is required but cannot be achieved by padding at the end of the MAC PDU. A UE of Category 0 [12] shall indicate CCCH using LCID “01011”, otherwise the UE shall indicate CCCH using LCID “00000”. The LCID field size is 5 bits;
- L: The Length field indicates the length of the corresponding MAC SDU or variable-sized MAC control element in bytes. There is one L field per MAC PDU subheader except for the last subheader and subheaders corresponding to fixed-sized MAC control elements. The size of the L field is indicated by the F field and F2 field;
- F: The Format field indicates the size of the Length field as indicated in table 6.2.1-3. There is one F field per MAC PDU subheader except for the last subheader and subheaders corresponding to fixed-sized MAC control elements and except for when F2 is set to 1. The size of the F field is 1 bit. If the F field is included; if the size of the MAC SDU or variable-sized MAC control element is less than 128 bytes, the value of the F field is set to 0, otherwise it is set to 1;
- F2: The Format2 field indicates the size of the Length field as indicated in table 6.2.1-3. There is one F2 field per MAC PDU subheader. The size of the F2 field is 1 bit. If the size of the MAC SDU or variable-sized MAC control element is larger than 32767 bytes, and if the corresponding subheader is not the last subheader, the value of the F2 field is set to 1, otherwise it is set to 0.
- E: The Extension field is a flag indicating if more fields are present in the MAC header or not. The E field is set to “1” to indicate another set of at least R/F2/E/LCID fields. The E field is set to “0” to indicate that either a MAC SDU, a MAC control element or padding starts at the next byte;
- R: Reserved bit, set to “0”.