1 Introduction
The wide deployment of public wireless access points and the prevalence of portable mobile devices allow people to have ubiquitous wireless access to the Internet. According to a study from Juniper Research, it is estimated that by 2018 there will be over 10.5 million Wi-Fi hotspots owned by mobile operators worldwide [1]. It is also expected that the amount of smartphone and tablet data traffic on Wi-Fi networks will increase to more than 115,000 petabytes by 2019 [1] . Compared with 3G/4G services, Wi-Fi access is one user-preferred connectivity option when using popular applications due to its superiority of cost and connectivity.