Seamless Connectivity Techniques in Vehicular Ad-hoc Networks

Emerging Vehicular Ad-hoc NETworks (VANETs) are representing the preferred network design for Intelligent Transportation Systems (ITS), mainly based on Dedicated Short-Range Communications (DSRC) for Vehicle-to-Vehicle (V2V) communications (Held, 2007). Future vehicles will be fully networked, equipped with on-board computers with multiple Network Interface Cards (NICs) (e.g., Wi-Fi, HSDPA, GPS), and emerging wireless technologies (e.g., IEEE 802.11p, WiMax, LTE). Although V2V is potentially the most viable approach to low-latency short-range vehicular networks, connectivity in VANETs is often not available due to quick topology network changes, random vehicle speed, and traffic density (i.e. sparse, dense, and totally disconnected neighbourhoods) (Chiara et al., 2009). As an alternative, longer-range vehicular connectivity are supported by a Vehicle-to-Infrastructure (V2I) protocol (Held, 2007), which exploits a pre-existing network infrastructure, for communications between vehicles and wireless/cellular access points (referred to Road Side Units, RSUs). As a further benefit, V2I protocols allow access to the Internet and delivery of traditional applications in addition to dedicated applications for ITS, thus making vehicle communications more versatile. Both the paradigms —V2V and V2I— exhibit connectivity problems. Different speed and traffic densities result in low vehicular contact rate, and limit communications via V2V protocol, while V2I communications are reduced, especially in highway scenarios, by the low number of RSUs displaced on the roads. Moreover, V2I limitations are due to particular vehicular applications, and performance is also strictly dependent on the specific wireless technology for the RSUs. It can then become advantageous to adopt hybrid schemes combining V2V and V2I communication into a single protocol and allowing fast migration from V2I to V2V connectivity depending on the operation context (high/low density, high/low velocity). Such mechanism is the so-called Vertical Handover (Pollini, 1996). In this chapter we shall describe the traditional techniques —Vertical Handover algorithms— used for seamless connectivity in heterogeneous wireless network environments, and in particular adopt them in VANETs, where V2V and V2I represent the main communication protocols. Section 2 deals with the basic features of Vertical Handover (VHO) in the general context of a hybrid wireless network environment, and it discusses how decision metrics can affect handover performance (i.e. number of handover