Ieee 802.15.4 standard and the zigbee technology

The main goal of the ZigBee technology is to enable WSNs composed of large number of nodes to function with reduced energy consumption. Most WSN technologies like Mica
Motes use ZigBee in order to achieve higher lifetime levels for their WSN applications.
The ZigBee network architecture is based on the Open Systems Interconnection (OSI), however exclusively the more important layers were implemented. ZigBee adopts the IEEE 802.15.4 standard, which only defines the lower layers: the physical layer and the MAC layer
(ZigBee, 2010).
The physical layer may operate on two frequencies 868/915MHz or 2.4GHz, with 16 channels and 250Kbps of maximum transmission rate. The IEEE 802.15.4 MAC layer is based on the Carrier Sense Multiple Access with Collision Avoidance (CSMA‐CA) mechanism. Note that ZigBee technology differs from other wireless technologies due to several reasons: lower data transmission rate, lower energy consumption, lower cost, higher self organization and more flexible network topologies (ZigBee, 2010).
The IEEE 802.15.4 standard has been proposed in 2003 and has become a de facto standard for low energy consumption and low data rate transmission networks. The IEEE 802.15.4
MAC protocol supports two kinds of operational modes that can be selected by a central node called Personal Area Network (PAN) coordinator. The two modes are:
 Beaconless mode: where MAC protocol functions are based on a CSMA/CA without beacon packet;
 Beacon mode: where beacons are periodically sent by the PAN coordinator in order to synchronize nodes that are associated with it and to delimit a superframe. During the superframe duration all node transmission must occur. Moreover, during the contention period of this frame the MAC protocol is ruled by the slotted CSMA/CA. The IEEE
802.15.4 with beacon mode can use the synchronization and the contention free period that is based on a guaranteed slot time.
Thus, the ZigBee Alliance is responsible for the ZigBee