In the late nineties, and at the beginning of the new millennium, wireless networks have evolved from being just a promising technology to become a requirement for everyday activities in developed societies. The transportation facilities have also evolved, offering on-board communication to improve safety and access to infotainment content.
End-user requirements have become technology dependent, meaning that their connectivity needs have increased due to the different requirements for applications running on their portable devices such as tablets, smartphones, laptops or even On-Board Units (OBUs) within vehicles. To fulfill those connectivity requirements while considering different available wireless networks, Vertical Handover (VHO) techniques are required in order to seamlessly and transparently switch between networks without requiring user intervention.

In this thesis we aim at developing scalable and efficient Vertical Handover Decision Algorithms (VHDAs) optimized for Vehicular Network (VN) contexts. In that sense, we have proposed, developed, and tested different VHDAs, based on the facilities available in the current, and probably in the future, wireless and vehicular networks, and combining different techniques, such as computational and mathematical methods, in order to guarantee appropriate connectivity by handing over to the networks that better fit the application and user requirements.

In order to evaluate the surrounding context, we have used different tools to obtain information, such as network availability, status of the network, geolocation of the vehicle, service provider features and user preferences. Based on the information gathered, the VHDAs perform the decision-making process to choose the most suitable candidate network to handover to. Therefore, the information must be gathered in an accurate way, and the decision making process must evaluate it in a fair manner, allowing the OBU to seamlessly disconnect from the old network and connect the new one. The algorithms that we present consider the availability and capacity of candidate Points of Attachment (PoAs), combining different data sources at the OBUs, taking advantage of Global Positioning System (GPS) information, maps, geolocation and navigation information, surrounding context information and routes in order to guarantee the Quality of Service (QoS) and the Quality of Experience (QoE).

To support the development and testing of the VHDAs proposed, we have performed several works including a thorough overview of the VHDAs available in the literature. Moreover, we analyze and present the IEEE 802.21 standard, which was developed in the last couple of years. This standard provides a homogeneous middleware for heterogeneous networks that allows improving the handover processes among different wireless access networks, as well as a service to collect not only network status information, but also service provider information.

We have also extended and developed the Network Simulator (ns-2) and the Seamless and Secure add-on (developed by the National Institute of Standards and Technology (NIST)) to be able to test our VHDA proposals. Additionally, we have tested the performance of different wireless networks, such as Wireless Fidelity (Wi-Fi), Worldwide interoperability for Microwave Access (WiMAX), and Universal Mobile Telecommunications System (UMTS) in order to determine their performance limits, and we tested the viability of a content delivery framework for VNs based on Vehicular Ad-hoc Networks (VANETs).

The proposed algorithms are empowered not only by the IEEE 802.21 standard, but also by the multiple features available on the OBU at the vehicles, such as GPS, high resources (processor and memory), and no power supply constrains. Moreover, the algorithms have been tested under different network conditions within heterogeneous wireless networks such as Wi-Fi, WiMAX, and UMTS. The most promising contribution of our VHDAs is to guarantee the QoS and the continuous connectivity due to the full integration of the heterogeneous wireless networks within the vehicular network context.

The resulting algorithms present novelties concerning heterogeneous networks and the use of the IEEE 802.21 standard. Moreover, advanced geolocation is used to improve the VHDA. The algorithms introduce new concepts about QoS guarantees supported by the combination of geolocation, network, and context information, improving the decision-making process by considering multiple criteria in order to fairly evaluate the candidate networks to switch into. The algorithms are evaluated on well thought-out simulation environments, obtaining results that offer useful insights concerning VHO processes and VHDAs.