Improving ZigBee Networks Robustness with Multi-channel Capability

Based on IEEE 802.15.4, ZigBee is developed for low-power and low-data-rate wireless communication and it is building up remarkable position for wireless sensor network (WSN). As ZigBee is using the 2.4GHz Industrial, Scientific, and Medical (ISM) unlicensed frequency band, coexistence issues arise as there are also other wireless technologies sharing the same band, such as 802.11b/g WiFi, Bluetooth, cordless phones and even microwave ovens. Due to the low transmission power, ZigBee is potentially vulnerable to the interference introduced by these technologies rather than vice versa. Therefore, it is desirable to improve the robustness of ZigBee networks. As WiFi is widely deployed and often collocated with ZigBee networks in applications, such as hospitals and home buildings, we take WiFi as the main interference source and work on finding solutions to enhance the robustness of ZigBee networks under WiFi as well as other interferences. To improve the robustness of ZigBee networks, a feature called frequency agility is specified in the ZigBee standard. We found, however, some inadequacies in the standard that needs to be improved before the frequency agility can function well in practice as it is supposed to do. A better periodical window method is proposed to improve the detection time to interference. Besides, in case that there is only a part of the whole network suffering from some local interference, it is neither necessary for the whole network to move to a new idle channel because this movement is costly and risky, nor possible to find an idle channel for the whole network to move to. Therefore, we extend the frequency agility function by enabling a single ZigBee network to work on multiple channels. As some local interference appears, the part of the network which is under the interference can move to a new idle channel while maintaining the communication links with the other part of the network which stays on the original channel and the moved part can move back to the original channel when the interference disappears. OPNET simulations shows that our multi-channel solution can significantly improve the robustness of ZigBee networks in a cost-efficient way,