Slot data routing in cluster wireless sensor networks based on time synchronization of cluster heads

Abstract

Ones of the crucial factors for stable functioning of stand-alone wireless sensor networks (WSNs), widely used nowadays not only in the telecommunications, but also in many other branches (military, medical etc.), are various methods for energy saving which make possible to extend life cycle of WSNs. Main point is reduction of time when the net devices are operated in active mode, that is to say the devices are predominantly kept in energy saving mode (sleep mode). One of options for solution of this task is construction of WSNs on the basis of IEEE 802.15.4/ZigBee standard, which, at functioning cluster-tree WSNs in the synchronized (beacon) mode, provides for possibility of managing the operating cycles of the devices, which allows to reduce duration of the period while transmitter-receiver units are used, without impeding the net operation. But these nets have some serious problems caused by collisions of superframes, in particular - synchronizing frames of the beacons. To solve the problem, the authors suggest applying the method of determined TDMA (Time Division Multiple Access) to schedule receptions and transmissions of superframes at implementing inter-cluster interaction, which allows to prevent superframe collisions, as well as to provide for energy saving not only on account of inactive part of the superframes, but also on account of inactive superslots in the TDMA cycle of WSN. According to the suggested approach, process of inter-cluster interaction in WSNs is carried out within TDMA cycles of fixed duration and structure (the latter subdivided into superslots). During these cycles it occurs a data exchange between cluster heads (CH), mostly for multi-hop routing (retranslation) of data to the coordinator. So, in the suggested TDMA approach, number of active slot of TDMA cycle and number of guaranteed time slot in contention free period (CFP) of the superframe transmitted during this superslot are assigned to each rising and descending connection in the optimum connecting network tree. At this, during the CFP slots, guaranteed TDMA transfers of data are carried out by the superframe source, which is synchronized with it in this moment by the CH device. Application of the suggested TDMA mechanism enables to implement in the networks under consideration an efficient multi-hop rooting at two-addressed packets, in a similar way to wired networks of STP protocol (Spanning Tree Protocol).