ormation Blocks (SIBs) are mapped on the DL-SCH (see Section 7.7.2); the PCCH is mapped onto the PCH; and the remaining logical channels (CCH, DCCH, and DTCH) are mapped onto the DL-SCH. There are no multicast-related channel defined in 5G specifications in Release 15, nor is there work in progress to define those in Release 16. 7.6.3 Random Access Procedure The random access procedure is used by UEs in the IDLE and INACTIVE states to gain access to the network (see Section 7.7.1) and is used by UEs in the CONNECTED state when no valid PUCCH resources are available, for instance, at handover, after SR failure (see Section 7.6.5) or when beam failure occurs (see Section 7.6.11). A random access procedure can be contention based (CBRA) or contention free (CFRA), the latter being used only when the UE is already known to the network and has preamble(s) allocated. The physical layer details of the random access procedure are covered in Chapter 6. From a MAC sublayer perspective, the main differences compared to LTE are related to beam-based operation and uplink selection: The resources for the random access procedure are always linked to particular beams, and the MAC sublayer in the UE first needs to select beams before transmitting a preamble in the uplink. If beam failure occurs during a random access procedure, beam selection is initiated once more. This implies that a UE may switch from CFRA to CBRA if the contention-free resources of the CFRA procedure are associated with failed beams. When Supplemental Uplink (SUL) is configured, the MAC sublayer in the UE selects the uplink carrier to use for the random access procedure (either NUL or SUL). The selection is done based on the RSRP threshold, and once a carrier is selected, it is used throughout the random access procedure until it either succeeds or fails, in which case another carrier can be selected. 7.6.4 HARQ and Transmissions As in LTE, multiple parallel HARQ processes are maintained in 5G [1], and the MAC sublayer instructs the physical layer whether to f