The model for battery lifetime is explained in Section 6. Finally, Section 7 summarizes the main conclusions of the paper.2.?Overview of 802.15.4 Medium full article Access ControlMany possible advantages of employing IEEE 802.15.4 are strongly determined by the configuration of the Medium Access Control (MAC) sublayer. In this sense the IEEE 802.15.4 standard distinguishes two classes of nodes: the so-called Full-Function Devices (FFD) and the Reduced-Function Inhibitors,Modulators,Libraries Devices (RFD). FFDs are enabled to perform as network ��Coordinators��. In that case, FFDs are in charge of the communications of a set (or ��cluster��) of nodes (the ��children�� nodes) following a star topology. On the other hand the role of RFD (which is reserved for very simple devices with limited resources) just permits the communication (as ��end�� nodes) with just one FFD acting as its Coordinator.
Besides, the MAC layer of IEEE 802.15.4 enables two alternative operational modes:Under the non beacon-enabled or point-to-point mode, the access control is governed by non-slotted CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance). According to this medium access Inhibitors,Modulators,Libraries protocol, nodes have to sense the radio medium before starting any transmission. If the channel is busy, the transmitting device has to wait a random time (set in Inhibitors,Modulators,Libraries terms of a number of ��backoff�� periods) before listening to the radio again. Otherwise, if the channel is idle, the device can transmit the packet and will have to wait for an acknowledgment message from the reception point.
If the acknowledgment is not received in a predetermined period the node will proceed to retransmit the packet (up to a maximum number of attempts). Similarly, if two nodes begin their Inhibitors,Modulators,Libraries transmission simultaneously or a transmitting node is unaware that the radio medium in the receptor is busy, a packet collision will occur. Collisions strongly degrade the performance of CSMA algorithm as they prevent the packets to be properly received so that they have to be retransmitted (inducing delay or even data losses if retransmissions fail after applying the typical backoff algorithm of CSMA).Under the beacon enabled mode, the Coordinator node periodically broadcasts a special frame (called beacon). Beacons announce the presence of the Coordinator (identifying the corresponding WPAN) and permit the synchronization of the children nodes, so the Coordinator has to broadcast a special frame (a beacon) periodically.
The time between two consecutive beacons of a Coordinator is called the Beacon Interval (BI). The BI, which can vary from 15 ms to 252 s, defines an interval or superframe (whose duration can be lower than BI) which includes a Contention Access AV-951 Period or CAP and a Contention leave a message Free Period (CFP). During the CAP the communications between a Coordinator and its children nodes are governed by slotted CSMA/CA.