Greening campus WLANs: Energy-relevant usage and mobility patterns.

The past years have witnessed a significant increase in the number of WLANs deployed in most of the enterprises, campuses and public areas to provide high-speed Internet connectivity. These WLANs typically consist of APs densely installed to assure enough capacity to meet users demand during the peak period of activity. At the same time, it translates into a serious energy wastage during low-utilization periods, when capacity is not needed at the APs. To reduce this wastage, many proposed solutions consist of adapting the active capacity to the actual needs, introducing switching strategies able to turn on and off the APs. The effectiveness and potential benefit of these strategies strongly depend on the user behavior and traffic patterns.In this paper, we focus our analysis on the real usage characteristics of a dense WLAN (such as users' behavior and users' mobility patterns) in a university campus and evaluate potential energy savings and benefits achievable when introducing AP on/off switching strategies. We discuss different strategies, in which decisions are based either on: (1) historical behavior in the campus, or on (2) current AP utilization. In addition, considering the large overlapping coverage available in dense WLANs, we investigate users' mobility patterns to derive further improvements to AP switching strategies. The results show that, due to the repetitiveness of users' patterns and large differences in WLAN usage between days and nights, as well as between weekdays and weekends, large savings of up to 40% can be easily achieved. Moreover, by fine-tuning the strategies in different areas of the campus, additional savings are possible. The deployment of these strategies leads to energy saving and, as a practical consequence, to a remarkable reduction of electricity costs.