隨著無線技術接受度的增加，把資訊很有效率地傳送給客戶端是一個很重要的問題。跟客戶端相比較的話，資訊的伺服器端有較多的可用頻寬，所以我們所考慮的環境是不對稱的。於此情況下，在伺服器端，應該週期性地來廣播資訊，這樣的系統已經被提出。在行動計算的背景下，Acharya 等人已經提出一個使用週期散播的架構，稱為廣播磁碟(Broadcast Disks)。使用廣播磁碟可以建立一個階層式結構，其中最高層次含有較少項目並且以較高頻率來廣播它，之後的層次含有越來越多的項目並且以越來越少的頻率來廣播它。透過這個方式，人們可以在高優先資料和低優先項目的存取時間中做一個取捨，其中存取時間指的是從客戶端送出一個詢問直到在廣播頻道中收到他(她)有興趣的資料之間的時間。一個廣播排程明確說明了每一個資料分頁在何時和在哪裡被傳送出去。（註：廣播資料的最小邏輯單元稱為一個資料分頁，而其內容是從資料項目中產生的。廣播一個資料分頁所需的時間稱為一個時間槽縫。）然而，基於Acharya等人的方法，如果在磁碟上之資料量無法很平均地被分成某些區塊，則一些廣播的槽縫位置就會沒有被使用到而導致頻寬的浪費和存取時間的增加。在這篇論文裡，我們提出了兩個方法來改善這個問題，一個是以二進位數字為基礎的方法，另一個是利用彌補空槽的方法。在以二進位數字為基礎的方法中，其廣播頻率必須限定為2的階層，也就是1、2、4、8…等；而在彌補空槽的方法中，其廣播頻率卻沒有限制。從效能分析和模擬測試中，我們顯示出我們所提出的兩個方法比Acharya等人的方法產生出比較小的一個廣播週期（也就是較短的廣播週期）和較短的平均存取時間。此外，我們的第一個方法（二進位數字法）比第二個方法（彌補法）需要較短的廣播週期和平均存取時間；但法一的廣播頻率必須有所限制。所以兩個方法各有其優點與適用性，並且均可避免頻寬的浪費而且可以減少伺服器端等待的時間。

With the increasing acceptance of wireless technology, mechanisms to efficiently transmit information to wireless clients are of interest. The environment under consideration is asymmetric in that the information server has much more bandwidth available, as compared to the clients. It has been proposed that in such systems, the server should broadcast the information periodically. Acharya et al. have proposed the use of a periodic dissemination architecture in the context of mobile systems, called Broadcast Disks. Using Broadcast Disks can construct a memory hierarchy in which the highest level contains a few items and broadcasts them with high frequency while subsequent levels contain more and more items and broadcast them with less and less frequency. In this way, one can establish a trade-off between access time for high-priority data and that of the low-priority items, where access time means that the time elapsed from the moment a client submits a query to the receipt of data of his (her) interest on the broadcast channel. A broadcast schedule specifies when and where each data page is to be transmitted. (Note that the smallest logical unit of the broadcast data is called a data page which is made up by data items. The time required to broadcast a data page is referred to as a time slot.) However, based on Acharya et al.'s algorithm, some broadcast slots may be unused, which resulting in the waste of bandwidth and the increase of access time, if it is not possible to evenly divide the number of broadcast pages assigned on a disk into the required number of chunks. (Note that each disk is split into a sequence of smaller units called chunks.) Therefore, in this thesis, we propose two efficient broadcast programs in which no empty slots is wasted. The first one is the binary-number-based approach and the second one is the complementary approach. In the binary-number-based approach, the broadcast frequency must be restricted to a value of 2^n , n≧0, i.e. 1, 2, 4, 8..., etc; while in the complementary approach, there is no restriction on the broadcast frequency. From our performance analysis and simulation, we show that both of our proposed two approaches generate a small number of slots in one broadcast cycle (i.e., a shorter broadcast cycle) and shorter mean access time than Acharya et al. algorithm. Moreover, our first approach (the binary-number-based approach) requires a smaller number of slots in one broadcast cycle and shorter mean access time than the second approach (the complementary approach); however, there is some restriction on the chosen frequency in the first approach. Therefore, each of our proposed approaches has its own advantages and applicable domains, and both of them can avoid the wasteness of bandwidth and reduce the waiting time of clients.

TABLE OF CONTENTS
LIST OF FIGURES............................... iv
LIST OF TABLES................................ vii
ABSTRACT................................... xi
1. Introduction................................. 1
1.1 Mobile Environments........................... 1
1.2 Asymmetric Communications...................... 3
1.3 Broadcast Programs........................... 6
1.4 Motivations................................ 12
1.5 Organization of the Thesis........................ 13
2. A Survey................................... 14
2.1 Acharya et al.'s Broadcast Disks..................... 14
2.2 Query Sets-based Broadcast Programs................. 17
2.3 Multiple Channels-based Broadcast Programs............. 19
2.4 Real-time Broadcasting Algorithms................... 20
3. A Binary-Number-based Approach................... 25
3.1 Examples of the Empty-Slot Problem.................. 25
3.2 Assumptions................................ 26
3.3 The Algorithm.............................. 28
3.4 Correctness................................ 51
3.5 Combine*................................. 59
3.6 Discussion................................. 63
4. A Complementary Approach....................... 66
4.1 The Algorithm.............................. 66
5. Performance Study............................. 78
5.1 The Performance Model......................... 78
5.2 Performance Analysis........................... 80
5.3 Simulation Results............................ 84
5.3.1 The Performance of the Binary-Number-based Approach... 85
5.3.2 The Performance of the Complementary Approach...... 94
5.3.3 BNB vs. Complementary..................... 101
6. Conclusion.................................. 110
6.1 Summary................................. 110
6.2 Future Research Directions........................ 112
BIBLIOGRAPHY................................ 113

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