The envisioned heterogeneous wireless systems are expected to integrate multiple access networks over a common IP platform. In such systems, since mobile users expect to achieve the Always Best Connected (ABC) experience, heterogeneous systems have to support the vertical handover of users among different access networks. On the other hand, the field of Multiple Attribute Decision Making (MADM) has proved to be a suitable tool to study the vertical handover process. This chapter presents a survey of MADM algorithms that have been proposed for vertical handover. First, the authors describe the procedures of methods such as SAW, MEW, TOPSIS, ELECTRE, and VIKOR. Then, the chapter compares them by mean of simulations and performance analysis for an heterogeneous system integrated by WLAN, UMTS, and WiMAX networks, when users conduct different applications. The chapter concludes with a summary of open issues and future research directions in the area of vertical handover.
TopIntroduction
The envisioned heterogeneous wireless systems also called Beyond Third Generation (B3G) or Fourth Generation (4G) systems are expected to integrate multiple radio access networks over a common IP (Internet Protocol) platform (Akyildiz, 2004). Networks such as Wireless Local Area Networks (WLANs) based on IEEE 802.11/Wi-Fi standards, Universal Mobile Telecommunications System (UMTS) 3G cellular networks, CDMA 2000 3G cellular networks, Wireless Metropolitan Area Networks (WMANs) based on IEEE 802.16/WiMAX standards, etc., tend to be integrated in the coming years allowing the best connectivity to users’ communication applications anywhere at anytime. This feature is the most important issue in such heterogeneous wireless environment and it is also known as the Always Best Connected (ABC) concept (Gustafsson, 2003). The ABC connectivity requirement must be performed based on several (i.e., multiple) decision factors such as Received Signal Strength (RSS), available bandwidth, service type, monetary cost, network conditions, security, and user preferences.
The current existing homogeneous wireless systems are unable to provide such ubiquitous service availability. Hence, it is desirable to interoperate between heterogeneous and complementary wireless technologies (i.e., 3G cellular, WLAN, WMAN, etc.), that is, mobile users will switch between different radio access technologies to satisfy the ABC requirement. This important mobility process is known as the vertical handover (McNair, 2004), (Nasser, 2006). The last few years, plenty of research efforts have been focussed in this important and challenging mobility process in heterogeneous wireless systems.
The field of Multiple Attribute Decision Making (MADM) (Yoon 1995) has proved to be suitable tool to study and model the vertical handover process. Several MADM methods have been proposed in the literature for vertical handover, methods such as SAW (Simple Additive Weighting) (Zhang, 2004), TOPSIS (Technique for Order Preference by Similarity to Ideal Solution) (Zhang, 2004), MEW (Multiplicative Exponent Weighting) (Stevens-Navarro, 2006), Grey Relational Analysis (GRA) (Song, 2005), ELECTRE (Elimination and Choice Translating Priority) (Bari, 2007), WMC (Weighted Markov Chain) (Ying, 2008), and VIKOR (Gallardo-Medina, 2009). Considerable amount of research to develop and modify MADM methods for vertical handover have been conducted. It is necessary to evaluate their performance under different scenarios in order to provide the best solution for a particular application.