1). Proposed two collision-free beacon frame scheduling schemes
2). Build a synchronized multi-hop cluster-tree network
3). Schedule the generation time offsets of beacon frames issued from different coordinators to completely avoid beacon frame collisions with each other and with data frames
4). The author proposed a mechanism to implement the Time Division Approach and Beacon-Only Period Approach that proposed by the Task Group 15.4b
- The most important technical features of Zigbee are to provide low power consumption and real-time guarantees.
- The beacon-enabled mode offers real-time guarantees by means of the GTS mechanism.
- A cluster-tree network is formed by several coordinators that periodically send beacon frames to the nodes of their cluster, thus providing them synchronization services.
- The cluster-tree model proposes that the network contain more than one coordinator which generate periodic beacon frames to synchronize nodes in their neighborhood. In this case, if these periodic beacon frames are sent in a non organized fashion, they will collide either with each other or with data frames. In case of beacon frame collisions, nodes that wait the periodic beacon frames will loose synchronization with their coordinators and consequently with the network which will prevent them to communicate.
- The Task group 15.4b proposed some basic approaches for avoiding for beacon frame collisions :
a). Time division approach:
Time is divided such that beacon frames and the superframe duration of a given coordinator are sent during the inactive period of its neighbor coordinators. The idea is that each coordinator selects the starting time Beacon_Tx_Offset to transmit its beacon frame. The starting time must be different from the starting times of its neighbor coordinators and their parents.
b). Time division
a time window is considered at the beginning of each superframe for the transmission of beacon frames in a contention-free fashion. Each coordinator chooses a sending time offset by selecting a contention-free time slot (CFTS) such that its beacon frame does not collide with beacon frames sent by its neighbors.
- The Task group 15.4b has identified two types of collisions :
a). Direct beacon frame collisions :
two or more coordinators are in the transmission range of each other and send their beacon frames at approximately the same time.
b). Indirect beacon frame collisions : two or more coordinators can not hear each other, but have overlapped transmission ranges (indirect neighbors) and send their beacon frames at approximately the same time.
- Indirect beacon frame collision avoidance.
The Task group proposed two alternative :
a). The reactive approach.
b). The proactive approach.
Beacon Frame Scheduling Mechanisms for the Time Division Approach:
- The most intuitive idea is to organize beacon frame transmissions in a serial way such that no beacon frame will collide with another even if coordinator are in direct or indirect neighborhood. In addition, to avoid collisions with data frames, a beacon frame must not be sent during the superframe duration of another coordinator.
- The author proposed Superframe Duration Scheduling (SDS)
Beacon Frame Scheduling Mechanism for Beacon-Only Period :
1). The Contention Free-Time slot (CFTS) used by one coordinator is unavailable to its children and their neighbor coordinators.
2). The CFTS of a coordinator must be different from the CFTS of the parent of all its neighbors (coordinators and simple nodes)
3). The CFTS of a coordinator must be different from the CFTS of its neighbor coordinator.
- Collision-Free Beacon Scheduling Mechanisms for IEEE 802.15.4/ZigBee Cluster-Tree Wireless Sensor Networks. By Anis Kouba, Mario Alves, Melek Attia, Anneleen Van Nieuwenhuyse
Note : This resume is created for self-learning only. Author and Publisher hold copyrights
Maafkan daku yang melepaskanmu karena keluguanku dimasa lalu 🙂
Jan 03, 2009
Taipei City – Taiwan Tech (NTUST)
High Speed Network Lab