# How to Isolate Non-Public Networks in B5G: A Review
## 2 Deployments of NPNs  
A B5G NPN provides refined differentiation of communication services to meet the multifarious demands of vertical industries, and this can be established with resources and infrastructures owned by the NPN itself and by the public system. The resources and infrastructures occupied by a particular NPN are called private parts and the others shared among multiple NPNs are called shared parts. The B5G system employs a modularized architecture, which can meet the various QoS requirements of applications flexibly [25]. With this architecture, the B5G system is composed of a radio access network (RAN) and core network (CN), and the CN mainly consists of the user plane function (UPF) and the control plane (CP). As shown in Figure 2,two kinds of B5G NPNs can be formed with seven deployments. One kind of NPN is the standalone NPN (S-NPN), where none of the B5G system, or only the spectrum, is shared. The two deployments are the isolated S-NPN and the shared spectrum-only S-NPN (see Figure 2a,b), and they are suitable for the application scenarios involving dependent networks with requirements for high isolation, data privacy and low latency, such as the control information transmission of a high-speed train. The other kind of NPN is the public network integrated NPN (PNI-NPN). As shown in Figure 2c-e, with different deployments, more and more parts of a B5G NPN can be shared among different NPNs. For the shared RAN PNI-NPN in Figure 2c, the RAN has been shared. Based on this, the CP of the CN is also used in common in the shared RAN and CP PNI-NPN in Figure 2d. These two deployments of the PNI-NPN can meet the requirements for medium isolation, data privacy and latency since the data plane of the CN is dedicated and can be used for application scenarios focusing on data security, such as remote telemedicine, in which the private data of patients is transmitted. Lastly, for the shared RAN and CN PNI-NPN in Figure 2e, the RAN and the CN are both shared, and this deployment can be used in applicable scenarios with non-sensitive isolation and latency, such as an automation warehousing communication network. Furthermore, there are two local breakout (LBO) NPNs that utilize mobile edge computing data plane (MEC DP) equipment to offload the traffic of the NPN locally, with the MEC DP parsing the data stream from the N3 interface or from the F1 interface; these are called the N3 LBO NPN and the F1 LBO NPN, as shown in Figure 2f,g[26]. The two LBO PNI-NPN deployments have already been used in virtual/augmented reality (AR/VR), for which the cost and the transmission delay can be reduced. In spite of this, the interfaces of the MEC DP are non-standard, which results in a negative impact on security and compatibility. The performance metrics and the application scenarios of the seven NPN deployments are summarized in Table 1.  
Due to the high cost and scarce spectrum of the S-NPN, the PNI-NPN is well-recognized as the dominant B5G NPN [27]. Taking the city of Ritual in Germany as an example, the cost of an NPN equals EUR 1000 + 5 Bt(6a1 + a2), where B MHz is the allocated frequency bandwidth, 1000 is the number of years the NPN is allowed to use the spectrum and a1 and a2 km\({}^{2}\) are a coverage area with a high density of the population and other areas, respectively. However, the spectrum is always scarce. Although some countries have begun to allocate the spectrum to important industrial users [28], it is impossible for all industrial users to have their own spectrum. Therefore, the PNI-NPN is a promising way to solve the above two problems. By sharing the spectrum, RAN and CN among different NPNs, the transmission capacity of one NPN can be made flexible, and the utilization of resources is improved. However, a serious challenge emerges, which is isolating the different NPNs to achieve customized configuration and stable performance [8]. Therefore,  
Figure 2: Deployments of NPNs.