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第五章 結論與未來研究
壅塞控制是網路管理的重大問題，隨著網路網路普及，網路電話以及視訊實 況轉播等即時網路多媒體應用興起，但是即時的網路應用程式大部分是採用UDP 做為傳輸協定，UDP沒有壅塞控制機制。此研究主要探討的是：如何讓網路電話 使用具有壅塞控制功能的傳輸協定？以及如何在網路壅塞狀況時可以維持較好 的網路電話語音品質？此研究提出的TCP-friendly VoIP By Scalable Codec over DCCP，是利用DCCP偵測網路狀況的封包來回時間以及封包遺失率來判斷網路 壅塞的程度，並設計出細緻的語音層次，讓網路的壅塞時可以依照壅塞程度有更 適切的反應。
本研究透過實際網路的實驗環境中評估以CBR over UDP、Flexible Bit-Rate 以及Scalable Codec三種方式傳輸網路電話封包的效能，結果顯示透過Scalable Codec方法，能有效降低網路電話的封包遺失率，維持通話品質，而且相較於 Flexible Bit-Rate可以有更準確的壅塞偵測以及更合適的壅塞控制反應。研究結 果顯示在網路壅塞最嚴重的情況下可以和一般的CBR配合UDP的封包遺失率達 到40%左右的改善，與Flexible Bit-Rate方法也的達到8%以上的改善，並且整體網 路狀況更為穩定。而在和TCP頻寬競爭實驗中可看出，在頻寬不足的情況下，此 研究提出的方法可以和TCP友善且公平的競爭頻寬。
本研究中僅粗略的讓語音編碼配合DCCP，由於DCCP尚未常見於一般傳輸 層協定，許多DCCP配合僅依照[10]中所定義的壅塞偵測方法做實做，未來將針 對語音編碼器與DCCP做更細微的配合。
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