# Energy-Efficient Wireless Sensor Networks for Precision Agriculture: A Review
## 2 Wireless Communication Technologies for Agriculture
### ZigBee Wireless Protocol  
ZigBee wireless protocol is considered one of the best candidate technologies for the agriculture and farming domains. Given its low duty cycle, ZigBee is considered appropriate for PA applications, such as irrigation supervision [12], water quality management, and fertilizer and pesticide control, all of which require a cyclic information update. Based on this technology, the sensor nodes in the agricultural field can communicate with the router or coordinator node over a long range (i.e., 100 m) when XBee Series 2 is used. ZigBee can also reduce the communication distance by up to 30 m for indoor conditions (i.e., greenhouses) [13]. The number of deployed sensor and router nodes will be increased to cover the entire field area under surveillance. Some studies have employed Zigbee for PA. For instance, ZigBee was used to investigate the effect of signal strength on node spacing, base station antenna height, and leaf density [14]. In [14], three experiments were conducted in palm orchards in to evaluate a signal propagation model based on the received signal strength indicator (RSSI) of the ZigBee wireless protocol. Through their experiments, the authors concluded that the wireless channel propagation model must be determined before deploying the sensor nodes in palm orchards to obtain robust signal strength characteristics. A path loss model in a mango greenhouse was investigated based on the ZigBee wireless protocol [15].  
A cattle-localization-system-based ZigBee WSN was also applied in a grazing field [16]. Instead of requiring additional hardware, ZigBee relies on link quality indication (LQI) to measure distance. The ZigBee protocol achieves cattle localization at a low power consumption and minimum cost. The key parameters of greenhouse climate (i.e., humidity, temperature, CO\({}_{2}\), and solar radiation) are considered in [17] to ensure the comfortable growth of plants with 22% energy savings and 33% water usage. ZigBee and Global System for Mobile communication/General Packet Radio Service (GSM/GPRS) technologies were used to monitor and control the climate condition of greenhouses [17]. The ZigBee wireless protocol has also been used to monitor animal behavior (e.g., walking, grazing, lying down, and standing) [18]. ZigBee was also adopted in [18] to overcome the high power consumption and unreliability issues in mobile ad hoc WSNs.  
A WSN-based greenhouse that uses ZigBee star topology is merged with artificial intelligence (i.e., fuzzy logic controller) in [19]. The ZigBee wireless protocol is adopted to preserve energy by switching between active and sleep states. Thus, power consumption can be minimized, and the battery lifetime of sensor nodes can be extended. ZigBee is currently applied in smart beehives [20], orange orchards [21], dairy healthcare monitoring of cows in pastures and barns [22], automation in irrigation [23], greenhouse monitoring systems [24; 25], and livestock monitoring [26]. As a universalstandard for WSNs, the ZigBee protocol was used in several agricultural applications because of its low power consumption, low cost, self-forming characteristics, and suitable communication range.