## 2 Background on 5G NSA  
The 3 GPP has introduced five architecture options for 5G deployment and divided them into two deployment scenarios: Stand-Alone (SA) and Non-Stand-Alone (NSA). The SA version provides 5G services using solely the New Radio (NR) radio technology, whereas the NSA version provides services using both 4G and 5G radio technologies. The architecture options \"2\" and \"5\" belong to the SA version, while \"3\", \"4\" and \"7\" belong to the NSA version [15]. The standardization of option \"3\" (NSA) and option \"2\" (SA) is already complete, and current deployments are adopting either of these two architecture options [16]. Similar to other mobile network generations, a 5G NSA network architecture (Option 3) can be divided into three components: User Equipment (UE), the Radio Access Network (RAN), and the core network. A simplified overview is shown in Figure 1.  
The UE is a device the end user uses to communicate with the mobile network (e.g., a smartphone). It contains a physical Subscriber Identity Module (SIM) card or an electronic SIM that securely stores the International Mobile Subscriber Identity (IMSI) and the secret long-term symmetric key. The IMSI is a 15-digit number that uniquely identifies a SIM, and its leakage can make a UE prone to tracking or impersonation. The **RAN** consists of two types of BSs: enodeBs and gnodeBs. The **enodeBs** (master nodes) are LTE BSs that are used to exchange control plane and data plane messages with the UE. In a 5G NSA architecture, the enodeB performs functions like paging, over-the-air security, and handovers. The **gnodeBs** (secondary nodes), also referred to as \"en-gNB\" or \"SgNB,\" serve as 5G base stations dedicated to exchanging data plane messages. They work in conjunction with the enodeBs, primarily to significantly enhance the data rate for the UEs. The NSA version of 5G uses the LTE core network known as Evolved Packet Core (EPC). In the following, we provide a brief summary of the main components of the core network. The Mobility Management Entity (MME) is the central control node, which is responsible for authenticating and allocating resources to the UEs when they connect to the network. It also takes care of other important functionalities, such as tracking the UE location at a macro level, bearer activation/deactivating, and ciphering/integrity protection for Non-Access Stratum (NAS) signaling protocols (a set of protocols that run between UE and the MME). The Home Subscriber Service (HSS) is a database that stores user-related data such as IMSIs, shared keys, and subscription data. The Serving Gateway (S-GW) is responsible for routing and forwarding user data packets. It also serves as an anchoring point for handovers. The PDN Gateway (P-GW) serves as a point of interconnection between the core network and the external IP networks (i.e., the Internet and other mobile networks). For MNOs, the 5G architecture option \"3\", the NSA option, is a compromise between adding new capacity for eMBB and saving Capital Expenditure (CapEx) for deploying the 5G core network [17]. In this architecture, the RAN is composed of enodeBs as the Master Node (MN) and gnodeBs as the Secondary Node (SN). The RAN is connected to the 4G core network. MNOs usually use the term 5G NSA to refer to 5G NSA option 3. Therefore,  
Figure 1: The 5G NSA architecture consists of a 4G core network and two types of base stations (BSs). in this paper, any reference to \"5G NSA\" refers to this option. 5G NSA option 3 is further divided into three types, based on the data split method, as shown in the Figure 2.  
From a control plane perspective, in all three options, the UE has control plane connectivity only with the enodeB, while the gnodeB works in conjunction with the enodeB via the X2 interface. Regarding the data plane, option 3 involves a traffic split at enodeB. In this option, the EPC forwards the traffic to only enodeB, which can either transmit it directly to the UE or forward (split data) a part of it to gnodeB for transmission over the NR interface.