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Wireless networks are passing through a transition phase for the past few years now and this transition is giving a way towards the convergence of all IP-based networks to form the Next Generation Networks (NGNs). With the proliferation of these networks in daily life, users' needs are also increasing and service operators are offering different services to satisfy their customers for a better grade of service and an elevated quality of experience (QoE). However, a single operator cannot fulfill the huge demands of the users especially, if a user is nomadic. In nomadism, a user traverses number of available networks that might contain cellular or wireless data networks, usually known as heterogeneous wireless networks. These networks offer various services from email to live video streaming depending upon their capacity and nature. During this traversing procedure, a user switches among different networks to satisfy his/her needs in terms of quality of service. This process is commonly known as a vertical handover or handoff (VHO) due to the involvement of heterogeneous wireless networks in it. An extensive work has been carried out in this field in order to fulfill user demands for better QoS and QoE. In this paper, we give a detailed state-of-the-art of these existing vertical handover decision mechanisms that aim at providing ubiquitous connectivity to the mobile users. We have categorized these vertical handover measurement and decision schemes on the basis of their employed techniques and parameters. Also, we present a comprehensive summary of their advantages and drawbacks. This paper gives its readers an overview of the active research initiatives in the area of handover decision making process in heterogeneous wireless networks and identifies the challenges behind the seamless services provisioning during mobility.