nding to state transitions and radio bearer configurations should be performed by the RRC sublayer due to higher signaling reliability requirement. The RLC PDUs are delivered to the MAC sublayer, which multiplexes a num- ber of RLC PDUs and adds a MAC header to form a transport block. It must be noted that in NR, the MAC headers are distributed across the MAC PDU, such that the MAC header corresponding to a particular RLC PDU is located immediately prior to it (see Fig. 2.3). New Radio Access Layer 2/3 Aspects and System Operation IP packet IP packet IP packet Radio bearer x Radio bearer y SDAP SDU SDAP SDU SDAP SDU PDCP SDU PDCP SDU PDCP SDU RLC SDU RLC SDU SDU segment SDU segment MAC SDU MAC SDU MAC SDU MAC SDU MAC PDU MAC PDU PHY SDU (transport block) PHY SDU (transport block) H: Header or subheader Figure 2.3 Layer 2 packet processing [8]. This is different compared to LTE, in which all header information are located at the begin- ning of the MAC PDU. The NR MAC PDUs can be assembled as soon as the RLC PDUs become available; thus there is no need to assemble the full MAC PDU before the header fields can be computed. This reduces the processing time and the overall latency 8,18]. The physical layer provides information transfer services to the MAC and higher layers. The physical layer transport services are described by how and with what characteristics data is transferred over the radio interface. This should be clearly distinguished from the classification of what is transported which relates to the concept of logical channels at the MAC sublayer. It was mentioned earlier that each sublayer in layer 2 radio protocols receives SDUs from the previous layer, processes the information according to the configured functions and parameters of the sublayer, and generates PDUs that are delivered to the next layer. In this process, a unique header or subheader is attached to the SDU by each sublayer. An example is shown in Fig. 2.3, where a transport block is generated by MAC sublayer by means of concatenating two RLC