ZigBee technology is separated into two parts. First, ZigBee alliance designates the application layers, defining the network, security and application software layers. Second, IEEE 802.15.4 standard defines the physical and medium access control layers, where access to wireless channel is through employing unslotted/slotted CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) mechanism for channel access and handling guaranteed time slot (GTS) allocation and management. ZigBee-based wireless devices operate in 868 MHz, 915 MHz, and 2.4 GHz frequency bands.