Research On Channel Access Mechanism Based On Slotted CSMA/CA Algorithm For ZigBee Networks

With the rapid development of wireless communication technology and computertechnology and the continuous improving of chip process, a simple microchip, with lowcost and low power consumption, integrates many functions such as information collection,data processing and wireless communication,ect. Wireless communications graduallyreplace wire communications. A series of short distance wireless technologies wereproposed，such as Bluetooth，RFID，ZigBee and so on.Among these technologies, ZigBeehas various topologies, possessing the characteristics of flexible networking, low cost andpower consumption. As a wireless communication technology with high reliability, ZigBeeis widely used in the fields of the industries, intelligent furniture, intelligent transportations,military,ect. The IEEE802.15.4working group and ZigBee alliance are jointly committedto the development of ZigBee’s standards. The former formulated the PHY and MAC, andthe later formulated NWK and APL. ZigBee protocols not only lay the solid foundation forthe development and research of ZigBee, but also provide a new development opportunityfor related industries.With the study of ZigBee protocol stacks and the research of a large number ofdomestic and foreign papers on ZigBee channel access mechanism, a conclusion isobtained that CSMA/CA algorithms using simple binary exponential backoff algorithm,without considering current network condition, cause the irrationality of nodes’ backoffslots length adjustment mechanism. Therefore when in large networks with large quantityand dense distribution, using the CSMA/CA algorithm to compet channel will bring falsedetection that the channel is idle. Meanwhile, the CSMA/CA algorithm with multipleparameters not only requires a higher level of the research methods,but also increases thedifficulty to improve the original algorithm.In this paper, the simulations of the CSMA/CA algorithm use NS2.Simulation resultsprove that regulation of BE and NB has basically the same impact to the network. So thereis a correlation between BE and NB. Taylor formula was used to find out the relationbetween BE and NB. So the number of parameters in the CSMA/CA algorithm decreasesfrom3to2. It greatly simplifies the the process of research on the CSMA/CA algorithm, providing convenience for researchers to study and improve on the CSMA/CA algorithm.Based on the correlation among parameters, this paper proposed a novel parameterassociation optimal slot backoff algorithm. This algorithm further gives the relationalexpressions among exponential backoff (BE)，frame length（L）and backoff slot (wi), whichcan optimizes the backoff slots according to the current network status. At the same time, itputs forward the improvement of channel scanning mechanism in original algorithm——astrategy that entering the channel scan with a reasonable probability pCCAwithout repetitioninstead of entering the channel scan immediately. This parameter association optimal slotbackoff algorithm combines optimal slot revising strategy and probabilistic channelscanning strategy. It greatly reduces the probability of false detection.The network delay and throughput of the proposed algorithm was deduced with theMarkov chain model.The relationship between the backoff slots (wi) and channel scanprobability (pCCA) are analyzed with the gnuplot to express the network throughput anddelay. Finally, experiments with NS were conducted comparing the network performancebetween the proposed algorithm and the original algorithm. Simulation results prove thatthe proposed algorithm can reduce the collision probability and delay in the networks,increase the networks’ throughput.