# On the Rollout of Network Slicing in Carrier Networks: A Technology Radar
## 2 Network Slicing: Concept, System Architecture and Deployment
### Network Slice Concept  
A network slice provides a service-tailored connectivity pipe to one or more service applications hosted by the Data Network (DN). Examples of service applications include Immersive Reality (XR) streamers, IoT platforms or V2X backend servers. These applications can be associated with operator services (e.g., communication services) or with third party services. Devices subscribed to one service can establish communication with the service applications through the corresponding network slice, which will provide an enhanced connectivity profile in terms of functionality, performance and/or security [2]. The fact that makes network slicing an E2E concept is that the device-to-application connectivity pipe involves all the technical domains within the operator's managed network, including the RAN, CN and TN domains. The **RAN domain** allows connecting the end devices to the operator's network using a wide variety of access technologies. In this paper, we focus on the Next Generation RAN (NG-RAN) [7; 8]. The NG-RAN consists of multiple gNBs, which provide connectivity towards end devices using 5G New Radio (NR) technology. To take advantage of the benefits that RAN virtualization brings in terms of scalability and centralization, the standards have moved to a new architecture model where a gNB can be logically split into three entities denoted as radio unit (RU), distributed unit (DU) and centralized unit (CU). The NR protocol functions that correspond to each of these entities are determined by the so-called split options. Though there exist up to eight split options available for this gNB decomposition, after a thorough analysis the industry has opted for two: split 2, defined by the 3 GPP and acting as a high layer split; and split 7-2x, defined by the O-RAN Alliance and acting as a low layer split [9]. The figure in Section 2.2 details the partitioning of NR protocol functions into RU, DU and CU, according to these two split options. The **CN domain** allows end devices to send/receive mobile traffic to/from DN hosted applications or the Internet. In 5G, this functionality is provided by the 5G Core (5 GC) [10]. Designed from its inception to be cloud-native, the 5 GC follows a service based architecture (SBA), with the definition of a disaggregated and modular, containerized control plane which is fully decoupled from User Plane Functions (UPFs). Finally, the **TN domain** is in charge of providing infrastructure connectivity between the RU (the entry point to the network for the device) and the DN (where the service applications are hosted). To that end, it makes use of a wide variety of forwarding devices, which are founded on different technologies (e.g., IP/MPLS, optical/DWDM and microwave/backhaul) and connected forming different topologies (e.g., ring, mesh, hub and-spoke), across different aggregation levels. The TN domain sets up the data path across the different RAN and CN functions, by mapping their interfaces into Wide Area Network (WAN) infrastructure resources. According to this mapping, different TN segments can be outlined:  
* Fronthaul segment, scoping the data path between the RU and the DU. This data path implements the O-RAN fronthaul interface (split 7-2x).The control, data, management and synchronization planes of this interface are defined in [11; 12]. * Midhaul segment, which sets up the data path between the DU and the CU. This data path implements the 3 GPP F1 interface (split 2) [13]. * Backhaul segment, established between the CU and the UPF. It covers two 3 GPP interfaces: N3 (CU-to-UPF) and N9 interface (UPF-to-UPF). When the UPF connected to the CU is the anchor UPF, then the N9 interface is not needed [10]. * DN segment, establishing connectivity between the (anchor) UPF [10] and the DN. This segment is the transport level realization of the 3 GPP N6 interface [14]. To make slicing a reality, every technical domain is split into one or more logical network partitions, each referred to as a network slice subnet.