 As part of the channel cod- ing operation in the physical layer, a transport block is split into one or more code blocks with channel coding applied to each of the code blocks of up to 8448 bits in order to main- tain a reasonable complexity. In practice and in the presence of burst errors, only a few code blocks in the transport block may be corrupted and the majority of code blocks are correctly received. In order to correctly deliver the transport blocks to the destination MAC sublayer, it is sufficient to only retransmit the erroneous code blocks. Furthermore, to avoid the excessive control signaling overhead due to individual code block addressing by HARQ mechanism, code block groups have been defined. If per-CBG retransmission is configured, feedback is provided per-CBG and only the erroneously received code block groups are retransmitted. The CBG-based retransmissions are transparent to the MAC sublayer and are handled in the physical layer. From MAC sublayer perspective, the transport block is not correctly received until all CBGs are correctly received. It is not possible to mix the CBGs New Radio Access Layer 2/3 Aspects and System Operation 215 belonging to another transport block with retransmissions of CBGs belonging to the incor- rectly received transport block in the same HARQ process [17]. 2.2.2 Radio Link Control Sublayer The RLC sublayer is located between PDCP and MAC sublayers as shown in Fig. 2.1. The RRC sublayer controls and configures the RLC functions. The RLC functions are performed by the RLC entities. For an RLC entity configured at the gNB, there is a peer RLC entity configured at the UE and vice versa. An RLC entity receives/delivers RLC SDUs from/to the upper layer and sends/receives RLC PDUs to/from its peer RLC entity via lower layers. The RLC sublayer can operate in one of the three modes of operation defined as transparent mode, unacknowledged mode, and acknowledged mode. Depending on the mode of opera- tion, the RLC entity controls the usage of error correction, segmentation, 