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With the increase in data traffic, cellular backbone gets more congested day by day. It is estimated that, by 2020, an average smart phone will generate 4.4 GB of traffic per month, which is a fivefold increase over the year 2015 (average of 929 MB per month) (“Cisco Visual Networking Index: Global Mobile Data Traffic Forecast Update (2015-2020),” 2016). Coping up with data demand surge is one of the prime concerns of mobile network operators. Existing network infrastructures can be scaled to satisfy the huge data demand. However, huge investments are involved in building new infrastructure. Usage of Wi-Fi as a supplementary network to offload the cellular traffic is found to be a promising approach. Presently, most of the mobile phones can use cellular radio (GSM/CDMA/W-CDMA) as well as other technology like Wi-Fi. This further favors the idea of offloading. Some mobile operators like AT&T, T-Mobiles and Verizon are already deploying their own public Wi-Fi hotspots to offload data traffic (“AT&T Expands Wi-Fi Hotzone Pilot Project to Additional Cities”, 2010; “Verizon Wi-Fi Connect,” n. d.).
In mobile data offloading, a portion of the data which is targeted to flow through the cellular network (also called as mobile network) is allowed to flow through the WLAN (Wireless Local Area Network). Offloading decision can be network-controlled, mobile-assisted or mobile-controlled (Tripathi, Reed, & Van Landingham, 1998). Today, when the phone detects the availability of wireless network and when Wi-Fi interface is enabled, the user is prompted to log on to Wi-Fi networks using the username and password. This is exactly mobile controlled offloading. Whereas mobile operators are now setting up their own hotspots and building upon specifications such that users who are near to Wi-Fi hotspots are offloaded from cellular to Wi-Fi networks without manual user authentication, using Subscriber Identity Module (SIM) credentials. In this way, congestion in backbone due to increasing data traffic can be reduced. The main advantage of mobile data offloading is as follows: an average data rate that a user can expect from cellular data provider is only 2 Mbps (“Cisco Visual Networking Index: Global Mobile Data Traffic Forecast Update” (2015-2020), 2016). Compared to this, WLAN that complies with the IEEE 802.11 standard can provide higher data rates over 2.5 Mbps (Deshpande, Hou, & Das, 2010). Because of high data rate, the transmission time is reduced in WLAN which helps in saving battery power. Data transfer in WLAN is significantly more power efficient than 3G for all transfer sizes (Balasubramanian, Balasubramanian, & Venkataramani, 2009). Moreover, it is cost effective to build new APs (Access Points) at public spots than building base stations. However, there are challenges in deploying mobile data offloading. Cellular base stations cover large macro areas (in terms of kilometers), whereas IEEE 802.11 based WLAN covers only limited areas (in terms of meters). As a solution, the emerging WLAN standard, IEEE 802.11ah (Sun, Choi, & Choi, 2013), has been developed to target outdoor Wi-Fi for mobile traffic offloading. It operates in sub-1 GHz band and has a maximum transmission range of 1,013 m (Sun et al., 2013), compared to the conventional 802.11 WLANs operating at 2.4 GHz and 5 GHz bands. In summary, operators have an advantage of saving huge capital expenditure and operational expense by implementing data offloading. At the same time, users are connected to high-speed WLANs occasionally, which reduces their transmission time and hence, transmission power.