網路編碼在近幾年被提出來應用在點對點網路檔案分享系統中，其中利用檔案片段的相互隨機編碼組合，來增加網路的檔案分享傳輸效率。在傳統的點對點檔案分享系統中，多半是以隨機或稀有片段優先的方式選擇下載片段，主要想法是為了避免節點之間重複片段或產生稀有片段的問題。因此，對於片段使用隨機網路編碼組合的方式，將能有效的避免此類問題。然而，某些研究中顯示，使用網路編碼機制將會帶來一些額外花費，其包含編碼及解碼所產生的運算花費與延遲問題，以及傳輸片段係數資訊所產生的額外傳輸訊息花費。在考慮這些花費的前提之下，將會降低使用網路編碼機制的整體傳輸效能表現。因此，本篇將提出一個基於網路編碼的非結構化點對點檔案分享系統，並且在考量網路編碼所帶來的花費的前提之下，與其它未使用網路編碼的傳輸機制進行分析與比較。我們的模擬結果顯示，基於網路編碼的檔案分享系統，在使用正確的傳輸協定以及謹慎選擇檔案的切割片段大小及數量前提下，儘管網路編碼在受限制的編碼及解碼運算下執行，仍將優於其它未使用網路編碼機制的檔案分享系統的效能。

Network coding based P2P file sharing system has been proposed in recent years. It uses the random combination of encoded pieces to make the file sharing more effective. Most of the conventional P2P file sharing systems adopts a random or rarest piece first transmission policy to avoid too many duplicated pieces and rare pieces occurring in the network. Network coding can simply solve this problem. However, some of the studies show that network coding brings the overhead of coding/decoding delay and message overhead ,after all the performance of network coding is not better than other schemes that without network coding. Therefore, we propose a network coding based file sharing system over unstructured P2P networks considering the overhead from the network coding. We make comparisons with other schemes without network coding. Our simulation results show that the proposed network coding based file sharing system is better than others, when the network coding scheme can adopt an appropriate transmission protocol design even though network coding is executed under a limited computation resource.

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