802.15.4 : An IEEE standard of Low Rate Personal Area Networks

802.15.4 is an IEEE standard of Low Rate Personal Area Networks. This standard covers Physical and MAC layer of Low Rate Personal Area Networks. Zigbee uses services provided by 802.15.4 and provides network constructions, security features and applications.

Two types of nodes are supported by this standard, FFD and RFD, which stand for Fully Functional Device and Reduced Functional Device. A FFD can act as coordinator or PAN coordinator or as a device. A RFD can only act as device. For more detailed description and understanding, of RFD and FFD one can refer to the standard itself.
A device must always be associated with a coordinator. The device has to communicate everything to the coordinator only. A coordinator can communicate with peer coordinators, and associated devices.

The standard offers two modes of operations; they are beaconed and non beaconed mode. The coordinator of a star network operating in the beaconed mode organizes channel access and data transmission with the help of a super frame structure shown below.



Super Frame structure in 802.15.4


The coordinator starts the superframe with the frame beacon packet, this will contains superframe specifications. The superframe has active and inactive periods. During inactive periods all nodes including the coordinator can sleep. They wake up just before the active period. The active period is divided into 16 slots, the first slot is used for beacon, and rests of slots are divided among the CAP and GTS.
The nodes operate using slotted CSMA-CA during CAP, the nodes can go to sleep mode during CAP if they do not have any data to receive or transmit. The nodes are active during GTS phase in their respective time slots. The coordinator is active throughout the active period.

All nodes send a request for GTS time slot during CAP to the coordinator. A field in the request packet specifies if the time slot is to transmit data to coordinator form the node or vice versa. There is also a field in the request packet, which specifies the number of GTS time slots required by the node. The coordinator specifies the slot allocated to the node using beacon frame.

If the node has been assigned slots to transmit data to the coordinator, then the node transmits data during that slot and it receives an acknowledgement for the same. If a slot is not assigned to the node and if the node wants to transmit data then, the node transmits data during CAP using slotted CSMA-CA and gets acknowledgement form the coordinator.

The coordinator sends any data it needs to send to a particular node, during node’s slot allocated. If it is not able to send during that slot then, the coordinator specifies that a data to be received is pending, in the beacon frame, by specifying the address of the node on which data is pending. The node requests for the same data during CAP using slotted CSMA-CA and the coordinator updates the data to the node.In non beaconed mode of operation, there is no GTS mechanism or beacon frame. All nodes operate in unslotted CSMA-CA mode and transmit and receive data.

CAP stands for contention access period, during this period all the nodes that want to communicate with the coordinator try to access the medium using slotted CSMA-CA protocol, the nodes request for some number of GTS form the coordinator during this CAP. The nodes also may send some data or receive some data during this period form the coordinator, if there was not slot assigned to the nodes. GTS stands for Guaranteed Time Slot, every node may have some slots assigned to them to receive or transmit data during this phase. One can refer to 802.15.4 for more detailed understanding of this standard.