The device comprises a physical layer (PHY), which contains the RF transceiver along with its low-level control mechanism. A MAC sublayer provides access to the physical channel for all types of transfer. The upper layers consist of a network layer, which provides network confi guration, manipulation, and message routing, and an application layer, which provides the intended function of a device. An IEEE 802.2 logical link control (LLC) can access the MAC through the service specifi c convergence sublayer (SSCS).
The PHY (IEEE 802.15.4) provides two services: the PHY data service and PHY management service interfacing to the physical layer management entity (PLME). The PHY data service enables the transmission and reception of PHY protocol data units (PPDUs) across the physical radio channel. The features of the PHY are activation and deactivation of the radio transceiver, energy detection (ED), link quality indication (LQI), channel selection, clear channel assessment (CCA) and transmitting as well as receiving packets across the physical medium. The standard provides two options based on the frequency band. Both are based on direct sequence spread spectrum (DSSS). The data rate is 250 kbps at 2.4 GHz, 40 kbps at 915 MHz, and 20 kbps at 868 MHz. The higher rate at 2.4 GHz is attributed to a higher-order modulation scheme.
Data Link Layer
The data link layer (IEEE 802.15.4) is divided into two sublayers, the MAC and LLC sublayers. The logical link control is standardized in IEEE 802.2 and is common among all IEEE 802 standards. The IEEE 802.15.4 MAC provides services to an IEEE 802.2 type logical link control through the service-specifi c convergence sublayer (SCCS), or a proprietary LLC can access the MAC services directly without going through the SCCS. The SCCS ensures compatibility between different LLC sublayers and allows the MAC to be accessed through a single set of access points.
Network Layer The network layer of Zigbee (IEEE 802.15.4) is responsible for topology construction and maintenance as well as naming and binding services, which include the tasks of addressing, routing, and security. The network layer should be selforganizing and self-maintaining to minimize energy consumption and total cost. IEEE 802.15.4 supports multiple network topologies, including star, peer-to-peer, and cluster tree. The topology is an application design choice.