臺灣博碩士論文加值系統

下一世代行動網路的潮流是同一台無線裝置中配備一個以上的無線網路裝置，例如雙網手機通常都會配有3G和WLAN介面。使用者在異質網路環境下的行動過程中會遇到水平換手及垂直換手問題。大部分提出的換手的解決方案是以接收到的訊號強度(RSS)為導向，其中以臨界值之訊號強度(RSS with Threshold)(簡稱為RSS-T)與磁滯區間和臨界值之訊號強度(RSS with Hysteresis and Threshold)(簡稱為RSS-HT)的方法較為廣泛使用。但是RSS-T的方法在無線網路的邊緣會造成很多不必要的換手。雖然RSS-HT的機制能減少多餘的換手，但也會產生其他的缺點，例如較高的延遲換手,效能會下降等。本論文中提出異質網路傳輸多媒體串流服務之垂直換手機制。本垂直換手機制包含兩個步驟：(1)利用可適性磁滯區間和臨界值(Adaptive Hysteresis and Threshold)決定較適當的網路介面。(2)利用評價函式(Score Function)選擇能夠提供較好網路品質的WLAN AP。除此之外，在換手的過程中我們也採用MPEG-4 階層式串流技術與影片畫面重傳機制以保持串流傳輸的品質。

The trend of next generation mobile networks is to embed more than one wireless connectivity, e.g., equipped with WLAN and 3G simultaneously. In a heterogeneous network environment, users may suffer horizontal handoffs or vertical handoffs during their movement. Most current handoff solutions are received signal strength (RSS) oriented, in which the RSS with threshold (RSS-T) and the RSS with hysteresis and threshold (RSS-HT) are most popular. However, the RSS-T approach would suffer many unnecessary handoffs at the edge of the wireless networks. Although the RSS-HT mechanism can reduce such unnecessary handoffs, it may still have several drawbacks, such as high handoff delay and low network throughput. This thesis proposes a vertical handoff mechanism for multimedia streaming services in heterogeneous networks. The proposed vertical handoff algorithm employs (1) adaptive hysteresis with threshold to determine which access technique is the better one and (2) a score function for subnet selection. In addition, we adopt the MPEG-4 layered streaming technology and a frame re-transmission mechanism to maintain the multimedia streaming quality during the handoff operation.

1 Introduction 1
2 RelatedWork 4
2.1 Horizontal Handoff . . . .. . . . . . . . . . . . . 5
2.2 Vertical Handoff . . . . . . . . . . . . . . . . . 7
3 Handoff Algorithm 9
3.1 Hysteresis Adaptation . . . . . . . . . . . . . . . 9
3.2 Horizontal Selection . . . . . . . . . . . . . . . 12
3.3 Proposed Algorithm . . . . . . . . . . . . . . . . 14
4 System Architecture 17
4.1 SIP-based Handoff Control . . . . . . . . . . . . 17
4.2 Multimedia Content Delivery . . . . . . . . . . . 22
5 Performance Evaluation 27
5.1 Performance Comparison . . . . . . . . . . . . . . 27
5.2 Handoff Behavior . . . . . . . . . . . . . . . . . 29
5.3 Retransmission Effect . . . . . . . . . . . . . . 32
6 Conclusion 37

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