適用於都會區車載網路的動態覆蓋式群播機制與基於道路之預測式多路徑繞徑協定

Abstract

在車載網路中，多媒體串流及群組通訊為兩種重要且有潛力的服務，而一個高效能的繞徑協定將能顯著地提升來源端至接收端的多媒體串流效能。因此，在本論文中，在應用層我們提出一個動態覆蓋式群播機制來提供實況多媒體串流給群組節點；而在網路層我們提出一個新穎的服務品質感知之基於道路多路徑繞徑協定。我們所提出的這兩個方法都旨在都會區車載網路中提供流暢的多媒體串流服務。 在都會區車載網路的各種應用服務中，提供資訊娛樂服務(infotainment service)已經是可預見的趨勢，而多媒體串流是潛力極高的資訊娛樂服務。我們考慮的情境是在群組車輛間進行實況多媒體串流群播服務，為此我們使用動態應用層覆蓋(dynamic application-layer overlay)方式來達成。由於非群組中的車輛不一定願意合作來協助群播，因此以應用層覆蓋式群播的方式會比其他種類的群播方式，如基於網路編碼之群播以及網路層群播，更具彈性。因此，為了適應都會區車載網路高移動性及障礙物密集的特性，我們提出了一個稱作OMV (Overlay Multicast in VANETs)的高效能動態覆蓋式群播機制來提供多媒體串流。在這個OMV方法中，我們提出兩項策略來增強覆蓋(overlay)的穩定性：(1)滿足服務品質之動態覆蓋及(2)網狀結構之覆蓋。滿足服務品質之動態覆蓋的策略是按照封包遺失率及端到端延遲時間來挑選潛在的新父節點，以優化此覆蓋架構。而網狀結構之覆蓋的策略是允許子節點可擁有多個父節點。我們採用了兩個真實的影音短片來評估及驗證OMV在多障礙物的都會區車載網路中傳送影音的可行性。評估結果顯示，相較於目前在車載網路上最佳的方法—Qadri等人使用靜態網狀覆蓋的研究，平均來說OMV降低了27.1%的封包遺失率、11.7%的端到端延遲時間，但只增加2.1%的額外控制負荷。我們並將樹狀OMV與ALMA作比較，其中ALMA為另一個樹狀覆蓋式群播且是行動隨意網路(MANETs)中的最佳方法，OMV降低了7.1%的封包遺失率及13.1%的端到端延遲時間。此外，針對都會區車載網路中障礙物密集的問題，我們歸納出在不同的道路疏密度下可行之串流速度與群組大小。據我們所知，現有的車載網路文獻中尚未有如何動態地調整應用層群播覆蓋架構，以有效傳送多媒體串流的研究。總結來說，為處理都會區車載網路中高變動的拓樸，我們提出一個能滿足服務品質的策略，使群組節點可切換至能提供較佳服務品質的新父節點。我們所提出的OMV適用於實況多媒體串流，例如緊急實況影音傳輸及同一群組車輛的遊客們實況影音導遊。 至於在網路層中，穩定且效能高的繞徑是車載網路成功的關鍵。現有文獻已經證明基於道路的繞徑方式是非常適用於車載網路。此外，一旦使用中的路徑失敗時，多路徑繞徑方式可提供額外的路徑供切換。然而，現有的多路徑繞徑協定皆為基於節點的繞徑方法，因此並不適用於都會區之車載網路。在本論文中，我們也提出了一個適用於都會區車載網路之新穎的服務品質感知之基於道路多路徑繞徑協定(RMRV)。我們所提出的RMRV能找到多條路徑並且被有效地使用。我們提出了一個新穎的時空平面圖(space-time planar graph)的方法來預測一條路徑中各個路段的連通性，以得到此路徑未來的各個存活時段(life periods)。如此一來，源節點(source node)可動態地切換使用一條目前是連通且延遲時間較短的路徑，我們稱此為基於服務品質之路徑切換方法(QoS path switching)。採用此方法可預先避免封包遺失及可選用潛在延遲時間較短的路徑。模擬結果顯示，相較於另一個代表性的基於道路單路徑之繞徑協定，RBVT-R，我們所提出之RMRV（一次只使用一條路徑來傳送資料）提高了16.6%的封包送達率、降低了35.3%的端到端延遲時間，以及減少了45.8%的額外控制負荷。據我們所知，現有文獻中尚未有適用於車載網路的基於道路之多路徑繞徑協定。

In VANETs (Vehicular Ad Hoc Networks), multimedia streaming and group communication are two important potential services. An efficient routing protocol can significantly enhance source-destination multimedia streaming performance. Therefore, in the application layer we propose a dynamic overlay multicast scheme for live multimedia streaming among a group of nodes, and in the network layer we propose a novel road-based QoS-aware multipath routing protocol. Both the proposed approaches aim to provide smooth multimedia streaming in urban VANETs. For various application layer services, infotainment service has been a foreseeing trend in VANETs, and multimedia streaming has a high potential in VANET infotainment service. We consider the scenario of live multimedia streaming multicast to vehicles of the same group using a dynamic application-layer overlay. Due to the willingness for cooperation of non-group nodes, application-layer overlay multicast is more feasible than other kinds of multicast such as network-coding-based multicast and network-layer multicast. To adapt to high mobility and full of obstacles in urban VANETs, we propose an effective dynamic overlay multicast scheme for multimedia streaming, called OMV (Overlay Multicast in VANETs). The proposed OMV enhances an overlay’s stability with two strategies: (1) QoS-satisfied dynamic overlay and (2) mesh-structure overlay. The QoS-satisfied strategy to adjust the overlay selects potential new parents based on their streams’ packet loss rates and end-to-end delays. The mesh-structure strategy allows a child to have multiple parents. We evaluate the proposed OMV in urban VANETs with obstacles using two real video clips to demonstrate the feasibility of the OMV for real videos. Evaluation results show that comparing the proposed OMV to Qadri et al.’s work, which is a static mesh overlay and is the best method available in VANETs, the packet loss rate is reduced by 27.1% and the end-to-end delay is decreased by 11.7%, with a small control overhead of 2.1%, on average. Comparing the proposed OMV for tree overlays to ALMA, which is for dynamic tree multicast overlays and is also the best method available in MANETs, the packet loss rate is reduced by 7.1% and the end-to-end delay is decreased by 13.1%. In addition, to address the problem of obstacle-prone urban VANETs, we also derive feasible stream rates and overlay sizes for city maps with different road section sizes. To the best of our knowledge, how to organize and dynamically adjust an application layer multicast overlay for live multimedia streaming have not been studied in existing VANET literatures. In summary, to deal with highly dynamic topologies in urban VANETs, we propose a QoS-satisfied strategy for group nodes to switch to new parents that can offer better QoS. The proposed OMV is feasible for live multimedia streaming applications, such as emergency live video transmission and live video tour guides for passengers in different vehicles that belong to the same multicast group. As to the network layer, stable and efficient routing plays a key role for the success of VANETs. Road-based routing has been shown well-suited for urban VANETs, and multipath routing provides alternative routes once the current route fails. However, existing multipath routing protocols are node-based, which are not suitable for urban VANETs. In this dissertation, we propose a novel road-based QoS-aware multipath routing protocol for urban VANETs (RMRV). The proposed RMRV can find multiple paths and intelligently utilize them. We also propose a space-time planar graph approach to predict the connectivity of each road section (RS) in a path, and then derive the path’s future life periods. In this way, the source node may dynamically switch to use a path which is currently connected with shorter delay, which is called QoS path switching. QoS path switching avoids packet loss in advance and chooses the path with potential shorter delay. Simulation results show that comparing to a representative approach, RBVT-R, which is a single-path road-based routing protocol, the packet delivery ratio of the proposed RMRV (only one path used at a time) is 16.6% higher than that of RBVT-R, the average end-to-end delay of RMRV is 35.3% lower than that of RBVT-R, and the control overhead of RMRV is 45.8% lower than that of RBVT-R, on average. To the best of our knowledge, there is no existing road-based multipath routing protocol for VANETs.