無線網路跨層同步規約之設計

Abstract

在無線的網路環境中，僅考慮媒體存取控制層的同步規約，常常疏忽了實體層的實際傳輸機制，進而使得封包碰撞的議題被低估。在我們提出的方法中，我們將這兩層的議題合併再一起考慮，結合了多種實體層的模型讓無線網路的環境可以更真實的被反應，再因應這樣的狀況提出解決方案，讓隨機分布的使用者能更有效率地進行資料的傳輸。 這個方案包含了三個步驟，首先，各個使用者利用載波偵聽多路存取的方式輪流傳送封包，用廣播初步的建立親子關係，目的是要讓傳送封包的使用者能得知鄰近的使用者有哪些。再來，我們期望能在這些局部的子樹中遴選出一個擁有最多子孫的使用者為根結點。在前面兩個步驟，我們會讓所有使用者與時間最快的使用者同步，如此可以使得大家在時間上的差異性不大。最後，從根結點出發，把整個網路中的使用者都納入樹狀結構中，並在此同時完成與根節點的同步。 為了更進一步展現這個方法的優勢，我們在模擬的環節會與水浸時間同步協定比較。除此，單一通道和多個頻率正交的通道也是個模擬的參數，確認了通道數的增加可減少建立樹的時間，也額外的表現了時間和能量之間的取捨關係。

A wireless network based on a wide area deployment of low power and low cost user equipments (UEs) can be found in many applications. Most of these applications require each UE to maintain its clock synchronized with at least neighboring nodes in order to coordinate message exchanges. Most of the known synchronization protocols for wireless networks, however, lack the physical layer (PHY) considerations which causes collisions of message-carrying packets and confusion in determining the time-ow of the designed operations. The lack of PHY considerations also lead to unrealistic assumptions and assessments on the message delivery and related algorithms. The so-called broadcast storm problem is an obvious example. We first present a new wireless broadcast protocol with which a relay node uses a carrier-sense like approach and the received signal to interference plus noise ratio (SINR) to make a relay or unicast decision so that duplicate or redundant message relaying is minimized. We take into account PHY issues such as packet collision and statistical signal propagation and acquisition to obtain realistic performance assessments. Based on the proposed broadcast protocol we propose a three-phase tree-based network timing protocol. The timing synchronization protocol establish timing synchronization among network nodes while building a tree structure for the network. In the first phase, each UE discovers its neighbors and creates local parent-children connections via local broadcasting. Those nodes having children but no parents are designated as local tree roots from which we build local trees using the parent-children relations derived from them. Using a gossip procedure, local roots exchange its offspring size information and reach a consensus on the global root which has the largest family size. After finding a global root, we then enter the final phase and start construct a global tree that connects all nodes. The tree-building process will start at the root node and terminates at the point when the root receives a notification from all childless UEs. Network timing synchronization is obtained in the final tree-building phase as a byproduct. The proposed scheme provides a practical approach which takes PHY issue into account. The transmission resource can be single-channel and multiple orthogonal channels. As expected, the mean network synchronization time performance is improved with additional transmission resources although the energy consumption is not necessarily reduced. We use the flooding time synchronization protocol (FTSP) as a reference for performance comparison.