The paper "Energy Efficiency in Wireless Sensor Networks: a top-down survey" by Tifenn Rault, Abdelmadjid Bouabdallah, and Yacine Challal provides a comprehensive overview of energy efficiency in wireless sensor networks (WSNs). The authors address the challenges of designing sustainable WSNs, where sensors must operate autonomously for extended periods, often in hostile environments. They identify the need to balance application requirements with energy conservation, as different applications have varying needs, such as real-time data delivery, mobility support, and security. The paper begins by categorizing WSN applications into healthcare, industry, transportation, public safety, military, environmental monitoring, and underground/underwater networks, each with specific requirements like scalability, coverage, latency, QoS, security, and robustness. It then reviews existing low-power WSN standards, including IEEE 802.15.4, ZigBee, WirelessHART, ISA100.11a, Bluetooth Low Energy, IEEE 802.15.6, 6LoWPAN, RPL, and MQTT, highlighting their features and limitations. The authors further discuss various energy-saving mechanisms, including radio optimization (modulation, cooperative communications, transmission power control, directional antennas, and cognitive radio), data reduction (aggregation, adaptive sampling, network coding, and data compression), sleep/wakeup schemes (duty cycling, passive wake-up radios, and topology control), and energy-efficient routing (cluster architectures, energy-aware routing metrics, and multipath routing). The paper concludes by emphasizing the need for a holistic approach to energy efficiency in WSN design, considering both application requirements and energy conservation. It aims to provide a systematic framework for WSN designers to select and integrate efficient solutions that meet specific application needs while ensuring sustainability.The paper "Energy Efficiency in Wireless Sensor Networks: a top-down survey" by Tifenn Rault, Abdelmadjid Bouabdallah, and Yacine Challal provides a comprehensive overview of energy efficiency in wireless sensor networks (WSNs). The authors address the challenges of designing sustainable WSNs, where sensors must operate autonomously for extended periods, often in hostile environments. They identify the need to balance application requirements with energy conservation, as different applications have varying needs, such as real-time data delivery, mobility support, and security. The paper begins by categorizing WSN applications into healthcare, industry, transportation, public safety, military, environmental monitoring, and underground/underwater networks, each with specific requirements like scalability, coverage, latency, QoS, security, and robustness. It then reviews existing low-power WSN standards, including IEEE 802.15.4, ZigBee, WirelessHART, ISA100.11a, Bluetooth Low Energy, IEEE 802.15.6, 6LoWPAN, RPL, and MQTT, highlighting their features and limitations. The authors further discuss various energy-saving mechanisms, including radio optimization (modulation, cooperative communications, transmission power control, directional antennas, and cognitive radio), data reduction (aggregation, adaptive sampling, network coding, and data compression), sleep/wakeup schemes (duty cycling, passive wake-up radios, and topology control), and energy-efficient routing (cluster architectures, energy-aware routing metrics, and multipath routing). The paper concludes by emphasizing the need for a holistic approach to energy efficiency in WSN design, considering both application requirements and energy conservation. It aims to provide a systematic framework for WSN designers to select and integrate efficient solutions that meet specific application needs while ensuring sustainability.