ated by the eNB entity; - management of the HARQ retransmission mechanism; - generation of the RACH transport channel. 8.4.2. Protocol structure The MAC protocol defines a header for multiplexing RLC frames from different instances and from MAC protocol control elements. In 4G, the global MAC protocol header is made up of a collection of unit headers, each unit header being relative to an RLC frame or a control element. RLC data was multiplexed into the MAC payload. In 5G, the MAC header is placed immediately before RLC data or before MAC control element. Figure 8.19 describes the general MAC frame structure. Each header unit has a size of 2 or 3 bytes depending on the size of the field length. R/F/ LCID Control R/F/ LCID Control R/F/ LCID RLC frame header element header element header MAC header with MAC header with MAC header with RLC frame Control element Control element Control element Octet 1 Octet 2 Figure 8.19. Structure of MAC frame (L in octet) NG-RAN and 5G-NR F (Format): this bit specifies the Length (L) field format, coded on 7 bits (bit value ONE) or 15 bits (bit value ZERO). LCID (Logical Channel Identifier): this field is coded on 5 bits. It indicates the logical channel instance identifier or the type of control element (Tables 8.2-8.4). The identifiers assigned to the DCCH and DTCH logical channels are defined during the connection procedure. Description Value CCCH logical channel identifier 000000 DCCH and DTCH logical channel identifiers 000001-100000 Reserved 100001-101111 ADM Activation/Deactivation 110000-111010 DRX discontinuous reception 111011-111100 Timing Advance (TA) 111101 UE Contention Resolution Identity (CRI) 111110 Filling 11111 Table 8.2. LCID field values for the DL-SCH transport channel Description Value CCCH logical channel identifier 000000 Logical channel identifier 000001-100000 Reserved 100001-110110 Configuration of allocation confirmation 110111 PHR reserve power available (extended) 111000 PHR reserve power available 111001 C-RNTI (Cell Radio Network Temporary Identifie