ZigBee無線感測網路之通訊協定與應用設計

Abstract

無線感測網路相關的研究議題得到許多研究單位及學者的關注，近年來ZigBee通訊協定被視為最適用於感測網路的通訊協定，在本篇論文中，我們將提出建構於ZigBee通訊堆疊中之通訊協定與應用。本篇論文包含五個主題，其中前三個主題我們將探討ZigBee網路層通訊協定之設計，內容包含有：ZigBee無線感測網路生成之研究、ZigBee基礎之長鏈狀網路研究以及ZigBee樹狀網路資料傳遞排程。在後兩個主題為兩個可架構於所設計之網路層協定之上的ZigBee無線感測網路應用，分別為：一緊急室內安全監控與逃生系統以及一智慧型燈光控制系統。

在第一個研究主題中，我們研究ZigBee感測網路的生成問題，在ZigBee規範中，一個節點加入到一個網路的條件為：該節點能找到一個父節點能夠給予該節點一ZigBee網路位址，父節點們可利用ZigBee所定義之分散式位址分配法來指定位址給子節點們，這一個位址指定方法相當簡單但是卻限制了一個節點最多可容忍之子節點個數以及整體網路的深度，我們觀察到如果使用ZigBee所定義之網路生成方式，會使得網路位址的使用率偏低，進而造成節點無法連上網路，因此在本論文中，我們提出適用於ZigBe之網路生成方法，我們的目標是為能夠使得網路上的節點能夠自動組態並且形成一個可通訊之網路。

在第二個研究主題中，我們探討一特殊網路-長鏈狀網路，在這一網路中節點們被部署為數個長條狀拓樸，這些長條狀拓樸們連接成一個網路，並且覆蓋整個想要涵蓋之範圍，這一網路看似為一個特例型態網路，但是我們觀察到該網路型態是相當常見於許多無線感測網路之應用中。本論文探討該如何將ZigBee協定用於此一特殊但卻常見之拓樸中。我們從網路層協定來切入討論，發現原有ZigBee定義之位址指定方式與路由方式並不適用於此長鏈狀網路，因此我們的目標為提出簡單又有效之適用於長鏈狀網路之位址指派方法以及路由通訊協定。

在第三個研究主題中，我們研究ZigBee樹狀網路封包排程，在許多無線感測網路的應用中，網路上的節點被要求回報資料給一個資料收集伺服器，早先所提出之資料回報流排程方法皆著重於要節省網路節點電量消耗與降低回報延遲，但是該些方法皆不能適用於ZigBee網路。在考量ZigBee的特性下，本論文將探討該如何排程節點的信標訊框，並將提出信標排程演算法，目標為針對資料流之特性來排定網路節點之運作時間，以達到省電之目的。

在第四個研究主題中，我們提出一套緊急室內安全監控與逃生系統，該系統可建構於上述所提出之通訊協定方法之上，本系統擁有簡易網路建置方式、可靠拓樸重建以及安全導引人員逃生三項特點。在網路部署完成後，我們的節點將運作於省電模式來監控室內環境狀態，而當網路有緊急事件發生時，節點們則轉換至非省電模式以即時監控網路，同時感測器也會依據急難時感測到的資訊來提供安全的逃生路徑協助室內人員逃離災難現場，保障位處於災難現場人員的生命安全。

最後我們設計一室內智慧型燈光控制系統來提供一更便利以及舒適的生活環境，基於人們在從事各種不同之活動於皆有可能需要不同之照明需求，例如閱讀、看電視等。本系統可依據使用者之需求輔以感測器的回報來自動地調控室內的燈光，調控的目標除了要滿足使用者之需求外，亦強調需達到節能之目的。由本論文所提出之通訊協定方法，我們還可以在其上衍生許多之應用，如老人健康照護、緊急救援，戶外河川水位監測等。

ZigBee is a standard which is considered to be suitable for \emph{wireless sensor networks (WSNs)}. In this dissertation, we propose communication protocols and applications based on the ZigBee protocol stack. This dissertation is composed of five works. In the first three works, we put our attention on designing ZigBee-compatible network layer protocols. The first work and second work discuss network formation problems in general ZigBee networks and in a special type of ZigBee network, respectively.Based on the observation that data gathering is a major application of WSNs, in the third work, we design data collection strategies for ZigBee networks. In the last two works, we propose two applications, an emergency guiding and monitoring system and an intelligent light control system, which can operate based on the proposed network layer protocols.

In the first work, we discuss network formation issues in general ZigBee network. According to ZigBee, a device is said to join a network if it can obtain a network address from a parent device. Devices calculate addresses for their child devices by a distributed address assignment scheme. This assignment is easy to implement, but it restricts the number of children of a device and the depth of the network. We observe that if one uses the random formation policy specified in ZigBee, the utilization of the address pool may be very low. Those devices that can not receive network addresses will be isolated from the network and become orphan nodes. In this dissertation, we divide the orphan problem by two subproblems: the bounded-degree-and-depth tree formation (BDDTF) problem and the end-device maximum matching (EDMM) problem. We then propose network formation strategies to relieve the orphan problem. The simulation results show that, compared to the ZigBee network formation strategy, the proposed schemes can effectively reduce the number of orphan devices.

Although WSNs have been extensively researched, its deployment is still a big concern. In the second work, we promote a new concept of long-thin (LT) topology for WSNs, where a network may have a number of linear paths of nodes as backbones connecting to each other. These backbones are to extend the network to the intended coverage areas. At the first glance, a LT WSN only seems to be a special case of numerous WSN topologies. However, we observe, from real deployment experiences, that such a topology is quite general in many applications and deployments. We show that the address assignment and thus the tree routing scheme defined in the original ZigBee specification may work poorly, if not fail, in a LT topology. We then propose simple, yet efficient, address assignment and routing schemes for a LT WSN. Simulation results are reported.

In most WSN applications, sensors are required to report their sensory data to a sink. This operation is defined as convergecast, which means the reverse of broadcast. Existing convergecast solutions have focused on reducing latency and energy consumption. However, a good design should be compliant to standards, in addition to considering these factors. In the third work, we defines a minimum delay beacon scheduling problem for quick convergecast in ZigBee tree-based wireless sensor networks and proves that this problem is NP-complete. Our formulation is compliant with the low-power design of IEEE 802.15.4. We then propose optimal solutions for special cases and heuristic algorithms for general cases. Simulation results show that the proposed algorithms can indeed achieve quick convergecast.

In the fourth work, we show a novel indoor emergency guiding and monitoring system by ZigBee WSN. At normal time, the network is responsible for monitoring the environment in low-power mode. When emergency events are detected, all sensors switch to active mode to deal with these events. And the network can adaptively modify its topology to ensure transportation reliability, quickly identify hazardous regions that should be avoided, and find safe navigation paths that can lead people to exits.

In the last work, we introduce an intelligent light control system, which aims to provide a more convenient and comfortable indoor environment for users. Users are considered to have different requirements when doing different activities. The system can automatically decide illuminations for users by sensors' reports and users' demands. The goal is to satisfy all users and to conserve power. Based on the designed network layer protocols, we can further develop more applications, such as elder health-care application, emergency rescue, river level monitoring, and so on.