According to Cisco's forecast , mobile data traffic will grow to 30.6 exabytes per month by 2020, which amounts to a nearly eight-fold increase between 2015 and 2020 globally. Such a huge amount of traffic is putting increasing pressure on the cellular network operators. On the other hand, traditional network expansion methods, such as acquiring more spectrum and upgrading to more advanced communication technologies such as Long Term Evolution (LTE)-Advanced, are often costly and time-consuming. An efficient way to increase the network capacity in a cost-effective and timely manner is to use complementary technologies, such asWi-Fi or small cells, to offload the traffic originally targeted toward the cellular network. In fact, Cisco showed that offloaded mobile data traffic exceeded cellular traffic for the first time in 2015, where 51% of the total mobile data traffic was offloaded to the fixed network through Wi-Fi or femtocell networks . Owing to the popularity of Wi-Fi usage and deployment, we will focus our attention on the offloading of mobile data through Wi-Fi networks for the rest of this chapter.
In general, there are two main approaches to the initiation of Wi-Fi offloading, namely user-initiated and operator-initiated offloading. In the early days of implementation, when Wi-Fi networks were not tightly integrated with cellular networks, user-initiated offloading was the common choice, where the mobile users needed to manually select the network that they intended to use. However, in such a cellular and Wi-Fi coexistence scenario, the cellular operators usually lose their visibility of the users’ activities and thus cannot provide a guaranteed quality of experience (QoE) to users.
In contrast, with the ongoing standardization efforts that we will discuss in the next section, cellular and Wi-Fi networks are becoming increasingly tightly coupled together, so that the performance of the Wi-Fi network is usually within the mobile operator's control. This enables operator-initiated offloading, where the connection manager in a mobile device connects with the mobile operator's server and retrieves the mobile operator's policy to initiate the offloading procedure. In other words, the mobile operators have more control over the users’ network selections and thus their QoE under the operator-initiated offloading.