未來的行動通訊系統，將會是由多種不同網路系統整合成的多階層式網路。
因此在多階層式網路架構下，如何提出一個有效的網路選擇機制(Network Selection Strategy)，
讓使用者能在需要的時候交遞到合適的網路是相當重要的議題。
在本論文中，我們將\cite{She06}中提出的以接收信號強度(RSS)，接收信號強度變動值(VRSS)，
頻寬(Bandwidth)等因素做為費用函數(Cost Function)輸入參數的機制當作參照，
另外提出了三個網路選擇機制。
首先我們在第一個網路選擇機制中，增加了使用者可選擇網路的數量，
讓使用者可以依照計算出的費用值(Cost Value)高低依序做使用網路的選擇,
有效降低了系統的阻塞機率(Blocking Probability)。
之後加入了使用者速度(Velocity)做為選擇網路的影響因素之一，設計出了第二個網路選擇機制,
依照速度的快慢做網路資源的分配。
最後在第三個網路選擇機制中,我們加入了交遞須達到的費用值改善門檻限制,
有效達到了降低系統垂直交遞率(Vertical Handoff Rate)的效果。
由模擬的數據可以顯示我們所提出的網路選擇機制與現有的機制比較，
阻塞機率的降低能使整體系統效能有較佳的表現。

The form of a hierarchical network composed of various networks will be expected to be
used for the future mobile communication system. Therefore, how to come up with an
effective network selection strategy for the hierarchical network to allow a handoff
user to move to an appropriate network is the issue to be discussed in this thesis.
Mainly based received signal strength (RSS), variation of RSS (VRSS), and bandwidth
as input parameters to a cost function, we design three network selection strategies
in this thesis. The first network selection strategy allows multiple choices of networks
based on cost values to decrease the blocking probability in the system. Further
considering the user's velocity in choosing a network, the second network selection
strategy is formed. Finally, a threshold related to the improvement of the cost value
is further added to have the third network selection strategy to greatly decrease the
vertical handoff rate. By simulation, the three network selection strategies can be
proven effective in terms of blocking probability, vertical handoff rate, and etc.,
thus improving the overall system performance.

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