om-Access Procedure (L2/L3 Aspects) The random-access procedure is triggered by a number of events including initial access from RRC_IDLE state; RRC connection reestablishment procedure, handover, downlink/ uplink data arrival when in RRC_CONNECTED state and if uplink synchronization status is non-synchronized; uplink data arrival while in RRC_CONNECTED state and when there are no available PUCCH resources for SR; SR failure; request by RRC upon synchronous reconfiguration; transition from RRC_INACTIVE state; establishing timing alignment upon SCell addition; request for other SI; and beam failure recovery. Furthermore, the random- access procedure takes two distinct forms: contention-based random access and contention- free random access as shown in Fig. 2.31. For random access in a cell configured with SUL, the network can explicitly signal which carrier to use (uplink or SUL); otherwise, the UE selects the SUL carrier, if the measured quality of the downlink is lower than a broad- cast threshold. Once started, all uplink transmissions of the random-access procedure remain on the selected carrier [8]. The complete description of the random-access procedure includ- ing the physical layer aspects can be found in Chapter 3. Random access preamble Random access preamble assignment Random access response Random access preamble Scheduled transmission Random access response Contention resolution Contention-based random access procedure Contention-free random access procedure Figure 2.31 Contention-based and contention-free random-access procedures [8]. New Radio Access Layer 2/3 Aspects and System Operation 277 2.8 Multi-radio Dual Connectivity (L2/L3 Aspects) MR-DC is a generalization of the intra-E-UTRA dual connectivity. The NG-RAN supports MR-DC operation wherein a UE in RRC_CONNECTED state is configured to utilize radio resources provided by (at least) two distinct schedulers, located in two different NG-RAN nodes connected via a non-ideal backhaul, one providing the NR access and the other pro- viding either E-U