The Research Of Channel Access Mechanism Of ZigBee Protocol MAC Layer

ZigBee technology,a wireless sensor network technology which is integrated with modern computer technology, wireless communication technology, sensor technology, a high degree of multi-disciplinary knowledge network of monitoring data transmission, and the technology has become a hot research field. ZigBee is a low-cost, low power, low-speed, short range wireless network technology, whatever the occasion demands with these characteristics can be applied, so it can be applied in a wide range of applications. Typical applications such as smart buildings, industrial control, wireless sensor networks, home automation, intelligent transportation systems, automotive, energy management, medical and health care, modern agriculture and so on. The ZigBee equipment can be divided into RFD and FFD device from the perspective of complexity, which RFD only as a network terminal device node, while the FFD is also a router device or coordinator node in addition. In ZigBee, the network topology can be divided into three types, star network topology, cluster tree-shaped network topology and peer network topology.Current researches focus on the protocol of ZigBee technology. Especially the router protocol in NWK layer and mechanism of access control in MAC layer are the key in ZigBee. This article about the ZigBee technology execute a extensive and in-depth study on the ZigBee technology, include its current development status, development status of channel access protocol, three types of ZigBee device, three kinds of network topology, network architecture of ZigBee devices, the distributed channels, original language of the network, data services and management services of PHY and MAC layer, structure of data unit, and so on.The article carry out the in-depth research on the key technology—the channel access mechanism of the MAC layer.First mentioned in this article were two important MAC layer frame structure, the structure of super frame and beacon frame. The super frame structure includes the CAP and CFP, the CAP is divided into some time slots, one occupied by the beacon frame number is 0, which is the first time slot. CSMA / CA channel access mechanism is in fact a mechanism in which the nodes compete to occupy the time slots and avoid the collision when the nodes conflict with others. For the command frame and the acknowledgment frame, the device node is not required by CSMA / CA mechanism and can be directly sent.Next, this paper analyzes and interpretes the entire process of CSMA / CA channel access mechanism, as well as the three important parameters like NB, CW, BE, gives the scope,the rules and related fuction of these three parameters change, and points out that now int the existing CSMA / CA channel access mechanism, the selection of BE value decides the back cycle of the nodes in the network when they conflict,the selection of back cycle decides whether the devices in the network can happen the conflict for the second time and then wheter the capability of the whole network can be pulled down. Then, this paper proposes a improved mechanism based on the existing CSMA / CA–Circular queue channel access mechanism of circular queue, by expanding the range BE selection, changing macMinBE as variables, for each network node's equipment address in the network is different from another, differentiating their selection of back cycle can pull down the conflict rate for the second time, and improve the network performance. By sending lunzhuan variable in the beacon frame which is in the super frame, make sure the justification among the nodes in the network. Because the justification is an important parameter to value a protocol whether is good enough.The tail of the article introduced in detail how to carry out the network simulation by using NS2 platform. The environment of simulation is a star topology network which is beacon frame enabled and included 9 nodes. All the data in the network must go through the coordinator, and all the nodes must be in the communication range of the coordinator. The contrast of the data from the simulation showed that the new mechanism is better than the existing one in throughput, start corresponding time and packet loss rate.