Study On Channel Access Mechanism Of IEEE802.15.4 MAC Layer

ZigBee technology is usually used by automation and wireless control and its advantage is low speed, low power consuming and low price. IEEE802.15.4 define the protocol of PHY and MAC layer. ZigBee alliances define the protocol of network layer. The user can develop the application layer according to their need. Along with the ZigBee technology development and the application, researchers pay more attention to the ZigBee technology wireless sensor network research day by day.According to the demand which applies, the ZigBee technology network has two kind of networks topology: Star topology and coordinated topology. In the star network, all nodes only can communicate with the coordinator. Mutually communicate between the nodes is prohibited. But in the peer-to-peer network, between the FFD (full function device) can mutually correspond, each FFD has the route function. RFD (Reduced function devices) only can carry on the correspondence with the nearby FFD. The point-to-point network may structure more complex network architecture, suit to the application of equipment distribution scope broad, such as industry examination and control, cargo stock track, intelligent agriculture and so on. The star network is the structure that my simulation used.This article has first carried on the simple introduction to the ZigBee technology, including ZigBee technology present situation, ZigBee network topology, physics and MAC level designed by IEEE802.15.4 protocol stack, network level of ZigBee protocol stack, service primitive and so on. Through the ZigBee technology with other wireless communication comparisons, the author point out that it is one of most potential technologies in the wireless sensor network.Then, the author introduce the channel access mechanism–CSMA/CA which is used by the MAC layer in IEEE802.15.4. I analyse from the two aspects : the channel time interval assignment and the algorithm realization process. The coordinator of PAN can define its channel time through the superframe structure. Transmission beacon frame is the signal of beginning superframe. The superframe can be divided into the activation part and the non- activation part. The coordinator can communicate with PAN in the part of activation , in non- activation part time, the coordinator enters the condition of low power loss (sleep) . The competition time follow closely the beacon frame on the competition time , all frames that want to be transmited all use the CSMA/CA mechanism to access the channel. On the competition time, the transmission equipment should guarantee its duty (including the receiving of all confirm frame) before frame time interval in the competition time to be completed. If the device cannot realize this point, equipment time will delay transmission until next ultra frame competition time. The order frame of MAC layer always transmits in the competition time. Because the IEEE802.15.4 standard has already defined the conflict detecting mechanism of CSMA/CA in the MAC level, the center node will transmit the new command frame after delaying a random time when it receives the wrong frame of failing. The FFD must be able to transmit and to receive all orders frame, but RFD not to use.In the CSMA/CA algorithm, BE is the withdraw index, invariable BE had decided the waiting time when device want to access the channel. BE initial supposes the value (minimum value) is 3, but the BE maximum value only defines to 5. The main consideration is to avoid long BE affecting the whole performance, the long waiting time could greatly increase the equipment energy consumption. The small scope of BE value may guarantee the device need not spend too much time when it wait for accessing the channel. But it is easy that two devices or more choose the same BE value. This will enable them simultaneously to detect to the idle channel, that will cause the frequent conflict, will cause loses the package and reissues. So this condition has affected the system throughput, delivery ratio, the loss rate. Because of the disadvantage in CSMA/CA, the author has advanced the improvement algorithm-auto-adapted avoidance mechanism. In my algorithm, the node can compatible change its BE value according to the different network condition. The improvement algorithm must be able to improve network overall performance, simultaneously not to increase the network energy consumption. The article mainly introduce the three phases of the improvement algorithm- analyzing phase, decision phase, implementation phase and the additional space within the beacon payload .At last, the author simulate the improvement algorithm and make the contrast with the original CSMA/CA algorithm. The simulation tool is NS2. NS can realize the MAC and the PHY level of IEEE802.15.4 protocol stack. The WPAN which my simulation involves is the star network.The star network is composed of a ZigBee coordinator and certain end devices. Moreover the author defined that the end device can only transmit date to coordinator.The simulation result indicate the application of the improvement algorithm can improve the network throughput, delivery ratio ,reduce drops but the energy consumption is not more than the original CSMA/CA algorithm.