parent: the transmitter side does not segment, nor add any headers, and the receiver side simply delivers the PDU as is to the upper layers. TM mode is meant to be used for SRBO (see Section 7.7.1). In UM, an RLC entity provides segmentation service to the upper layers. In AM, an RLC entity provides segmentation and error correction services to the upper layers. 7.5.5 Duplication Duplication for URLLC services (see Section 7.4.6) only impacts the handling of RLF in RLC: because the addition of a secondary leg for duplication should not increase the chances of declaring RLF, reaching the maximum number of retransmissions on Note 1: One exception is when a 12-bit SN is used in RLC UM, in which case the header grows by one byte (from one to two) for the first segment. 7 5G Radio Protocols the secondary RLC entity does not trigger RLF. If the secondary leg also triggered RLF, the network would have to be very conservative when configuring additional cells/CGs to minimize the risk failure of the secondary path. Having such a requirement on the configuration of additional resources would limit the usefulness of duplication. MAC Layer 7.6.1 Overview The MAC layer is the lowest sublayer of Layer 2 and is responsible for interfacing with the physical layer. As such, it is the UP layer which had to evolve the most to cope with the new properties of the 5G radio (the sublayers of the UP above MAC being fairly isolated). The main functions of the MAC layer include random access, mapping between logical channels and transport channels, multiplexing and demultiplexing of MAC SDUs (i.e. PDUs), error correction with HARQ, scheduling information reporting, discontinuous reception (DRX), BWP operation, and beam failure detection (BFD) and recovery. The MAC layer also conveys the signaling required for functions like timing advance and recommended bit rate. Lastly, the MAC layer is used to control the selection, activation, and deactivation of features such as PDCP duplication (see Section 7.4.6), and SCells in CA. In the following