由於行動通訊、無線網路、無線資料網路、衛星技術的急遽擴展，可以讓行動用戶們在任何地點、時間都有存取資訊的能力，在這種環境中的重要構成要素就是：伺服端的廣播能力。資料都是從伺服端上取得，並且是被廣播到空中的，行動用戶持續地監聽，並且擷取他們感興趣的資料。 在一般的行動環境中，行動用戶透過行動支援中心（MSSs）從資料庫伺服器中擷取資料項目，靠著在推式傳遞下載這些項目，以及在拉式傳遞請求這些項目；或者是同時使用這二種頻道的型態來存取它們（混合）。不論是哪一種資料傳遞方法，快取（caching）是一種改善行動用戶資料擷取效能的主要關鍵技術。逐漸茁壯健全的端對端（peer to peer）技術帶來了我們稱為合作式快取(COCA,cooperative caching），在這種快取中，行動用戶可以從他們的鄰近用戶端（neighboring peers）存取資料項目，由此，增加了一種新的資料快取特色。 本論文提出一種快取方法，也是運用到行動用戶端可以從他們的鄰近用戶端存取資料項目，應用 COCA 的觀念，採用雙雜湊演算法，產生快取標記(cache signature)，利用快取標記來判斷鄰近端有無符合的資料，從實驗結果可以看出合作式中快取加入快取標記之後，確實明顯看出合作式關係可以提升資料快取的命中率，以及降低資料的誤判率。 The rapidly expanding technology of cellular communications, wireless LAN, wireless data networks and satellite will give mobile users capability of accessing information anywhere and anytime. One of the important components of this environment is the server with broadcasting capabilities. Data are fetched from the server and been broadcast to the air. Mobile users keep listening within the air and catch those data that interest them. In a conventional mobile environment, mobile clients retrieve data items from database servers via mobile support stations, by down loading the items over scalable broadcast channels (push-based), requesting them over shared point to point channels (pull-based), or making use of both types of channels to retrieve them (hybrid).Caching is a key technique for improving data retrieval performance of mobile clients, regardless of the data delivery mechanism. The emergence of robust peer-to-peer technologies now brings to reality what we call ＂cooperative caching＂ in which mobile clients can access data items from the cache in their neighboring peers, thereby Adding a new dimension for data caching. In this paper, we propose a mechanism which adopts double-hash algorithm to get a cache signature, and we make use of this cache signature to determine whether the data on neighboring peers correspond with our request or not. After our simulation results, we find out when we use cache signatures in the cooperation cache method, it can increase cache hit ratio and reduce miss ratio.
