tantiation can support one or more OFDM numerologies, transmission timings as well as mapping restrictions on logical channels. In case of CA, the multi-carrier properties of the physical layer are only exposed to the MAC sublayer. In that case, one HARQ entity is required per serving cell. In both uplink and downlink, there is one independent HARQ entity per serving cell and one transport block is generated per transmission time interval per serv- ing cell in the absence of spatial multiplexing. Each transport block and the associated HARQ retransmissions are mapped to a single serving cell [16]. 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 Schedulingand priority handling Multiplexing Multiplexing UE1 Multiplexing UEn Transport channels Transport channels Uplink layer 2 structure Downlink layer 2 structure Figure 1.66 3GPP NR DL/UL layer-2 protocol structure [ 16] 5G Network Architecture 137 MAC sublayer provides different type of data transfer services through mapping of logical channels to transport channels. Each logical channel type is defined by the type of informa- tion being transferred. Logical channels are classified into two groups of control and traffic channels. In NR, the control channels are used for transport of control-plane information and are of the following types (see Fig. 1.67): Broadcast control channel (BCCH) is a downlink channel for broadcasting system con- trol information. Paging control channel (PCCH) is a downlink channel that transports paging and system information change notifications. Common control channel (CCCH) is a logical channel for transmitting control information between UEs and the network when the UEs have no RRC connection with the network. Dedicated control channel (DCCH) is a point-to-point bidirectional channel