Figure 2.7 Structure of various MAC subheaders [9]. F: The format field indicates the size of the length field. There is one F field per MAC subheader except for subheaders corresponding to fixed-sized MAC CEs, padding, and MAC SDUs containing uplink CCCH. The size of the F field is 1 bit, where the 8-bit and 16-bit Length fields are indicated by F=0 and F=1 1, respectively. R: Reserved bit, which is set to zero. The MAC subheaders consist of the following fields: E: The extension field is a flag indicating whether the MAC sub-PDU, including the MAC subheader is the last MAC sub-PDU in the MAC PDU. It is set to one to indicate at least another MAC sub-PDU follows; otherwise, it is set to zero. T: The type field is a flag indicating whether the MAC subheader contains a random- access preamble identifier (RAPID) or a backoff indicator (BI). It is set to zero to indi- cate the presence of a BI field in the subheader; otherwise it is set to one to indicate the presence of a RAPID field in the subheader. R: Reserved bit, which is set to zero. BI: The BI field identifies the overload condition in the cell. The size of the BI field is 4 bit. RAPID: The RAPID field identifies the transmitted random-access preamble. The size of the RAPID field is 6 bits. If the RAPID in the MAC subheader of a MAC sub-PDU corresponds to one of the random-access preambles configured for SI request, MAC random-access response (RAR) is not included in the MAC sub-PDU. The MAC sub- header is octet aligned. There are a number of MAC CEs that are specified for the following purposes [9]: Duplication activation/deactivation The duplication activation/deactivation MAC CE consists of one octet that is identi- fied by a MAC PDU subheader with LCID index 56. It has a fixed size and contains eight D-fields, where the Di field indicates the activation/deactivation status of the 206 Chapter 2 PDCP duplication of the ith DRB associated with the RLC entities currently assigned to this MAC entity. SCell activation/deactivation The SCell activation/deactiva