# 5G V2X System-Level Architecture of 5 GCAR Project
## 2 GPP 5G System Architecture  
The 5G system architecture, shown in Figures 1 and 2, has been designed by 3 GPP [6; 7] by means of defining network functions (NFs) instead of network entities, with the further novelty that the control plane (CP) is centered around services instead of interfaces. The latter feature allows each function to offer web-based services, utilizing representational state transfer (REST) APIs, and other functions can access such services through subscription procedures. In the remainder of this Section, we provide an overview of the functionalities of the NFs of the 3 GPP 5G system architecture which has been assumed as a baseline for the architectural work in 5 GCAR. Figure 1: 5G system architecture [6]. The network functions repository function (NRF) is used in order to collect and maintain information of the NF instances which are available at any time in the network. Examples of such information are NF profiles and addresses. The NRF supports NF service discovery. The policy control function (PCF) provides a unified policy framework governing the network behaviour, with tasks such as providing the other CP functions with the policy rules to be enforced. Furthermore, the PCF acts as the front-end interface for other NFs to access subscription information relevant for policy decisions which are stored in the unified data repository (UDR). The session management function (SMF) handles all of the functionalities related to the management of Protocol Data Unit (PDU) Sessions, such as establishment, modification, and release. Additional functionalities include maintaining tunnels between the (R)AN and user plane (UP) nodes, IP addresses allocation, and management. The access and mobility management function (AMF) is the termination point of the non-access stratum (NAS), performing functionalities such as User Equipment (UE) registration management, mobility management, reachability, connection management, user authentication, and access authentication [7]. Furthermore, the AMF handles session management messages coming from the core network towards the RAN. The unified data management (UDM) manages information related to the UE and to the subscriber profile. Among the tasks of the UDM, there are the 3 GPP Authentication and Key Agreement credentials, user identification handling, and storing of data related to registration management of NFs serving the UE. The network data analytics function (NWDAF), provides other NFs with analytics information on the network behaviour, such as load level information on a slice level. The authentication server function (AUSF) implements authentication and security functions. The network slice selection function (NSSF) is an NF introduced in 5G to determine the set of network slices that a certain UE is allowed to access. In addition, the NSSF determines the set of candidate AMFs suitable to serve the UE. The application function (AF) is a CP function interacting with other NFs of the 5G core (5 GC) network. The AF implements procedures such as influence of traffic routing of PDU sessions based on application-specific requirements. According to the deployment configuration, AFs can be trusted or untrusted. In the former case, they may be located within the operator's network and directly connected to the other control plane functions. The latter deployment option is for AFs deployed in a public network. In this case, the AF interacts with the 5 GC via the network exposure function (NEF), which is responsible for exposing network functionalities and capabilities and handles masking of sensitive internal user information before information exposure. The user plane function (UPF) is the only node in UP and implements functionalities of packet routing and forwarding. It is responsible for the enforcement of UP policy rules, and in general QoS enforcement for the UP. The UPF acts as an anchor point for intra/inter-RAT mobility, and an external point of interconnect to a data network for PDU sessions.