d coding scheme, and the antenna mapping. By varying the transport format, the MAC sublayer can realize different data rates, which is known as transport format selection. The MAC entity in the UE manages the broadcast channel (BCH), DL-SCH, paging channel (PCH), uplink shared channel (UL-SCH)], and random-access channel (RACH). When the UE is configured with a secondary cell group (SCG), two MAC entities (instantiations) are configured where one of them is associated with master cell group (MCG) and another one is related to the SCG. Different MAC entities in the UE may operate independently, for example, the timers and parameters used in each MAC entity are independently configured. The serving cells, cell radio network temporary identifier (C-RNTI), radio bearers, logical channels, upper and lower layer entities, logical channel groups (LCGs), and HARQ entities are separately mapped to each MAC entity. If the MAC entity is configured with one or more SCells, there are multiple instances of DL-SCH, UL-SCH, and RACH per MAC entity; how- ever, there is one instance of DL-SCH, UL-SCH, and RACH on the special cell (SpCell), one DL-SCH, zero or one UL-SCH, and zero or one RACH for each SCell. If the MAC entity is not configured with any SCell, there is one instance of DL-SCH, UL-SCH, and RACH per MAC entity. Fig. 2.5 illustrates an example structure of the MAC entities when MCG and SCG are configured [9]. In the context of dual connectivity, the SpCell refers to the primary cell of the MCG or the primary SCell of the SCG depending on whether the MAC entity is associated with the MCG or the SCG. Otherwise, the SpCell refers to the PCell. A SpCell supports PUCCH transmission and contention-based random-access pro- cedure and it is always activated [9]. New Radio Access Layer 2/3 Aspects and System Operation 203 The MAC sublayer provides data transfer and radio resource allocation services to the upper layers and at the same time receives certain services from the physical layer which include data transfer, signaling o