# Hardware Mechanism for Energy Saving in WiFi Access Points
## 1 Introduction
### Related Work: Energy saving in Wireless Fidelity (WiFi) Access Points (APs)  
Energy consumption of WiFi APs depends on several parameters [24]: its electric power consumption, number of connected and transmitting WTs, and range of coverage. Several techniques have been used to reduce the energy consumption.  
The reduction of idle pending combining the monitoring of WiFi downlink and low-power wake-up radio is simulated in [25] achieving considerable reduction of energy consumption. We implemented a hardware mechanism but not considering those techniques. WiFi energy profiles in WT acting as AP (tethering) are used to pass the WiFi interface to sleeping state for 90% of communication time without affecting performance [26]. We do not consider tethering; we apply our mechanism to any WiFi AP that could have an ethernet link to Internet.  
Energy harvesting [27] and time division multiplexing [28] techniques in mobile networks optimize energy saving considering the time of communication and the time needed to gain the access to the mobile channels (allocated in different frequencies) detecting inactive mobile terminals. We take advantage of inactive WiFi APs to shorten the battery charging time.  
Deactivation of the WiFi interface of the AP when a long period of inactivity in the network is detected is another technique. In [29] associations of WT are monitored in order for the AP to estimate the probability of new associations of WT in a short period of time. When that estimation detects no associations will be produced in the short-term future, the WiFi interface of the AP are deactivated for a short period of time. For dense WiFi-controlled networks, in [30] two algorithms are proposed to deactivate the WiFi interfaces of a master AP when a successful reassociation of WTs to slaves AP could be achieved during a prefixed amount of time. A communication delay could be achieved with that technique. We do not deactivate the WiFi interface of the AP in order to not delay communications.  
The real time kernel Energy Neutral Operating system (_EnOS_) [31] adapts energy consumption of high-end APs triggering the energy supply from an additional battery. Our hardware mechanism does not need EnOS or other software and can be applied to any AP.  
Our hardware mechanism can be applied to APs used in smart city as part of a wireless mesh network (WMN) based on IEEE802.11s. Some research works suppose the APs in the WMN are an electrical network or electrically powered by a PSE with PoE. In [32] are analyzed some routing characteristics the WTs must preserve in WMN to maintain routing paths when they will be in sleep state (they must support efficiently WiFi beacon communication and synchronization). In [33] the strength pareto evolutionary algorithm for routing is designed to optimize the energy consumption of APs in the WMN. In [34] the influence of idle pending of PSM is analyzed to prevent degradation of quality of service (QoS) and quality of experience (QoE). The work in [35] implemented experimental tests of energy consumption in a WMN with few APs and simulated the behavior for a high number of APs connected to a solar panel. Our mechanism is clear to the kind of electrical supply and it is applied when the WiFi AP will be not communicating user data.  
Our mechanism can be used in applications of smart home like those [36; 37] that controls turning on and off electrical bulbs on a building depending on the amount of people in a room using infrastructure WiFi APs information of WT in the building. Applying our mechanism, energy is mainly saved when no people are in the room.