IN the Internet of Things (IoT), a wide variety of devices, such as entertainment electronics, health appliances, wearable gadgets and industrial sensors, are powered by batteries, and need to wirelessly communicate with each other or some remote IoT gateways through multi-hop communications. To realize this, many wireless technologies can be employed. On one hand, the IEEE 802.15.4 standard (or ZigBee) has been proposed and widely used for home and building automation, smart metering and IoT in general, due to its low-cost and lowpower features. On the other hand, the IEEE 802.11 standard (or Wi-Fi) dominates the present-day consumer electronics fields because of its high data rate and long communication range. Any IoT device that connects to smartphones, tablets, digital cameras, TVs and PCs would benefit from Wi-Fi connectivity for reliability and high throughput. The economic gains of reusing the existing Wi-Fi infrastructure drive and facilitate faster deployment with Wi-Fi than other competing technologies. More importantly, Wi-Fi has the advantage of native compatibility with IP, which is the key for IoT communications [1]. The feasibility of connecting battery powered sensors to the IoT using off-the-shelf Wi-Fi chips has already been demonstrated in [2], [3]. Nevertheless, due to its energyhungry nature, Wi-Fi is often not recommended for short range multi-hop communications in IoT. To improve the energy efficiency of multi-hop networks based on Wi-Fi, numerous protocols have been proposed. Generally, they can be classified into two categories: synchronous approaches and asynchronous approaches. The IEEE 802.11 DCF standard specifies a power saving management (PSM), which is the most widely studied synchronous approach. The basic idea is to let all nodes wake up every certain time interval, called beacon interval or BI, to exchange beacon frames for time synchronization and then ATIM (Adhoc Traffic Indication Message) frames to announce possible incoming data packets to the receivers. To further improve the performance of PSM, lots of protocols [1], [4]–[۶] have been proposed by using some heuristics to adjust the behaviors of PSM (e.g., modifying ATIM frame, rescheduling beacon frame, turning on/off PSM on demand, etc.). The basic idea in asynchronous approaches [7]–[۱۰] is to let the device that has no outgoing packets stay asleep as long as possible, while periodically waking up to send beacon frames to announce its availability for receiving incoming packets. Hence, low latency requires high wakeup frequency and thus more energy.