sublayer to the RLC sublayer are in the form of logical channels. A logical channel is defined by the type of information it carries and it is generally classified either as a control channel, for transmission of control and configuration information, or a traffic channel, for transmission of user data. The MAC sublayer provides services to the physical layer in the form of transport channels (see Fig. 2.4). A transport channel is defined by how and with what characteristics the information is transmitted over the radio interface. The information traversing a transport channel is organized in the form of trans- port blocks. In each physical layer transmission time interval (TTI), one transport block with dynamic size is transmitted over the radio interface to a device. In the case of spatial QoS flows QoS flows QoS flow QoS flow QoS flow handling handling handling Radio bearers Radio bearers Security Security Security Security Security Security RLC channels RLC channels Segmentation Segmentation Segmentation Segmentation Segmentation Segmentation Logical channels Logical channels Scheduling/Priority handling Scheduling Multiplexing UE1 Multiplexing UE, Multiplexing Transport channels Transport channels Downlink Uplink Figure 2.4 NR layer 2 functions in the downlink and uplink [8]. 202 Chapter 2 Upper layers Upper layers MAC-control MAC-control Logical channel prioritization (uplink) Logical channel prioritization (uplink) (De) Multiplexing Control (De) Multiplexing Control Random Random access control access control PCH of BCH of DL-SCH of UL-SCH of RACH of BCH of DL-SCH of UL-SCH of RACH of Lower layer of MCG Lower layer of SCG Figure 2.5 Example MAC structure with two MAC entities [9]. multiplexing when more than four layers are configured, there are two transport blocks per TTI. There is a transport format associated with each transport block, specifying how the trans- port block is transmitted over the radio interface. The transport format includes information about the transport block size, the modulation an