# Designing a Vertical Handover Algorithm for Security-Constrained Applications
## 1 Introduction  
Recently, the number of mobile users (MUs) has exponentially increased. For example, according to the International Telecommunication Union (ITU) estimation in 2022, approximately 5.3 billion people, or 66 percent of the world's population, use the Internet [1]. This represents a 24 percent increase since 2019, with 1.1 billion people estimated to have gone online during that time [1]. Moreover, the MUs demand for services (e.g., voice, video) has been ever-increasing [2, 3]. The primary goal of next generation wireless networks (NGWNs) is to provide high bandwidth at a low cost, while also supporting a variety of mobile applications. This can be effectively accomplished by vertical handover (VHO) [4]. VHO is a process that occurs when an MU moves from the coverage of one RAT to the coverage of another RAT in heterogeneous networks (HetNets) to ensure that the MUs are connected to the best RAT, such as wireless fidelity (Wi-Fi), the global system for mobile communication (GSM), and the universal mobile telecommunications system (UMTS), with a guaranteed quality of service (QoS) [5, 6, 7, 8].  
VHO has three phases as follows: initiation phase; decision phase; and execution phase [9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35], as shown in Figure 1. The first phase is responsible for collecting all required information, namely the network parameters (e.g., coverage and latency), MU's preferences (e.g., security, service cost), and terminal parameters (e.g., speed and battery). The second phase is responsible for evaluating and selecting the best available RAT according to the collected information [36, 37, 38, 39]. The third phase is responsible for establishing and performing a connection to the selected RAT. The VHO decision is mainly based on theinput parameters as follows: received signal strength (RSS); cost; bandwidth; battery status; network load; and network coverage [8]. The final VHO decision is typically made based on QoS such as delay, jitter, and packet loss, perceived by the MU [40].  
However, a number of critical issues still arise when performing seamless vertical handover, such as VHO packet loss, VHO delay, and VHO throughput. Moreover, the security aspect of triggering VHO has not been carefully considered particularly, when an MU's sensitive data are used via payment applications or relevant security applications. For instance, in references [41; 42], the authors use a security-constrained application to countermeasure the attacks that arise when processing the medical images. If the same system is used in a mobile environment, the disruption in the connection may lead to the inefficient use of the algorithms. Therefore, this paper presents a comprehensive performance evaluation for a new secure VHO algorithm, taking into account all these issues, for co-located Wi-Fi and UMTS networks.  
In this paper, we compared our work with the media independent handover (MIH) IEEE 802.21 standard, which facilitates the seamless transfer of connection among heterogeneous networks to enable VHO for security-constrained applications. The MIH standard considers each application with the same priority; therefore, we enhance the working of the MIH standard by assessing the security-constrained applications with high priority. In addition, the proposed system is tested in a networking scenario with a various number of Mus and mobility. The proposed system is experimented with using a various number of handovers and various applications in use. The accuracy of the proposed system shows the efficiency of VHO performance in cases where the MU switches on a security application.  
The rest of the paper is organized as follows: In Section 2, related works are presented. In Section 3, a design of the proposed algorithm is presented. In Section 4, a performance evaluation and results discussion are presented. Finally, a conclusion is given in Section 5.  
Figure 1: Vertical Handover (VHO) phases.