elease-17, NG-RAN will also sup- port NTNs (i.e. satellite). This is described in detail in Section 5.3. Virtualization: Even though 3GPP specifications do not explicitly define virtualized NG-RAN, nevertheless it has been designed from the beginning with virtualization in mind. Certain provisions have been made in the definition of all NG-RAN interfaces to allow deployments in virtualized environments. Additionally, for the scenario with split gNB, the CU hosting the high-level protocols can also be virtualized. Furthermore, the implementa- tion for control-user-plane separation in both NG-RAN and 5GC makes it particularly well suited to be used with software defined networks (SDNs). 3 5G System Overview 3.3.2 Network Protocol Stacks Before discussing the functionalities of the NG-RAN interfaces, we first describe the proto- col stacks used by these, specifically: Control-plane protocol stack (used on interfaces NG, Xn, F1, and E1); User-plane protocol stack (used on interfaces NG, Xn, and F1). 3.3.2.1 Control-Plane Protocol Stack All NG-RAN control-plane interfaces (NG, Xn, F1, and E1) use the same control-plane pro- tocol stack (illustrated in Figure 3.3.2) with Stream Control Transmission Protocol (SCTP) (IETF RFC 4960) on top of IP. SCTP is chosen for increased reliability of control-plane mes- sages, which are generally considered more critical than user-plane messages. Even though network interfaces typically use wired transport network, which is assumed to be more reliable compared IPv6 (RFC 2460) with the air interface, the transport network may still be prone to, and/or IPv4 (RFC 791) for example, congestion resulting in packet loss, therefore the usage of a reliable transport is important. SCTP also supports in-sequence Data link layer delivery of control-plane packets and multi-homing, among other Physical layer features. It should be noted that the protocol stack in Figure 3.3.2 does Figure 3.3.2 not specify the transport network, other than saying that it should Control-plane provide IP connectivity