in addition to the high-level split defined in 3GPP, the Open Radio Access Network (O-RAN) Alliance defined the low-level split, in which the gNB-DU, which implements the lower layers of the split, can be further split into two network nodes (this last split, however, is not reflected in the 3GPP architecture model) This is described in detail in Chapter 4. Multi-radio dual connectivity (MR-DC): NG-RAN supports several options of multi- connectivity, in which a single UE may be connected to two different network nodes, one providing NR access and the other one providing either E-UTRA or NR access. Either node can act as the master node (MN), while the other one acts as the secondary node (SN). This is the generalization of the dual connectivity architecture introduced in E-UTRAN, with the main difference being that MN and SN can use different access technologies. This is described in detail in Section 4.3. Slicing: slicing allows MNOs to categorize customers into different tenant types, each having different service requirements, as reflected in a Service Level Agreement (SLA). Slic- ing allows, for example, an MNO to lease parts of resources of their network to a "vertical" (e.g. a factory that may want to use that wireless technology without investing in their own deployment). A network slice always consists of a RAN part and a core network part. This is somewhat similar to DECOR and eDECOR available in E-UTRAN; however, it has better flexibility as both 5GC and NG-RAN have been designed from the beginning with slicing in mind. Integrated access and backhaul (IAB): starting from Release-16, NG-RAN supports IAB functionality, which is roughly equivalent to the relaying support in E-UTRAN. Unlike E-UTRAN, IAB in NG-RAN supports multi-hop backhauling with topology adaptation and redundant links for better performance and resilience to links failures. IAB supports both in-band and out-of-band relaying, among other features. This is described in detail in Section 5.2. Non-terrestrial networks (NTNs): starting from R