 1 Buffer size 1 Octet 2 UE contention resolution identity Octet 1 Buffer size 2 Octet 3 UE contention resolution identity Octet 2 UE contention resolution identity Octet 3 Buffer size m Octet m+1 UE contention resolution identity Octet 4 Long BSR and long truncated BSR MAC CE UE contention resolution identity Octet 5 UE contention resolution identity Octet 6 PH (Type 1, PCell) Octet 1 UE contention resolution identity MAC CE PCMAX Octet 2 Single entry PHR MAC CE Octet 1 SCell activation/deactivation MAC CE Octet 1 Duplication activation/deactivation MAC CE Figure 2.8 Some MAC control element formats [9]. RAR. A MAC sub-header with backoff indicator consists of five header fields E/T/R/R/BI (see Fig. 2.9). A MAC sub-PDU with backoff indicator is placed at the beginning of the MAC PDU, if included. The 'MAC sub-PDU(s) with RAPID' and 'MAC sub-PDU(s) with RAPID and MAC RAR' can be placed anywhere between MAC sub-PDU with backoff indicator and the padding bits. A MAC sub-header with RAPID consists of three header fields E/T/RAPID. Padding bits are placed at the end of the MAC PDU, if necessary. The presence and the length of the padding bits are based on the transport block size and the size of the MAC sub-PDU(s). The MAC entity is further responsible for distributing IP packets corresponding to each flow across different component carriers or cells, when carrier aggregation is utilized. The carrier aggregation relies on independent processing of the component carriers in the physi- cal layer, including control signaling, scheduling, and HARQ retransmissions. The MAC sublayer makes multi-carrier operation transparent to the upper sublayers/layers. The logical 210 Chapter 2 Octet 1 Timing advance command Octet 1 E/T/R/R/BI MAC subheader Timing advance command Uplink grant Octet 2 Uplink grant Octet 3 RAPID Octet 1 Uplink grant Octet 4 E/T/RAPID MAC subheader Uplink grant Octet 5 Temporary C-RNTI Octet 6 Temporary C-RNTI Octet 7 MAC random-access response E/T/R/R/BI E/T/RAPID E/T/RAPID MAC RAR subheader subheader subh