由於VoIP服務對於傳輸品質要求較高，所以要在無線網路上提供此類服務需要有QoS機制。本文主旨即在探討無線區域網路下提供VoIP等類服務時所需要的QoS保證機制。我們專注在各邊緣網路的邊緣路由器或存取節點上的呼叫允入控制(Call Admission Control, CAC)。傳統無線網路的CAC方法雖然會保留固定的頻寬資源讓換手呼叫使用以提高整體使用者滿意度，但是那些方法卻存在無法根據網路狀況改變保留比例的缺點。其他的CAC方法則需要額外的資訊交換，如此將使得複雜度增加，並且需要過多的運算時間。 為了改善上述這些問題，我們提出一種以Diffserv為基礎並作用在邊緣無線區域網路上的CAC機制。此機制可根據頻寬使用情況動態分配呼叫傳輸頻寬，另外當無線存取點(access points)可用頻寬不夠提供給呼叫要求時，我們也提出動態傳送速率調適方法，來達到增加最大網路利用率和減少呼叫被拒絕的目的。 我們的模擬證明了此新機制較傳統方法可以降低約15%的呼叫阻絕率。因此利用我們的機制將提供較好的具品質服務保證的無線VoIP服務。 As the VoIP services require high quality of transmission, it is necessary to provide QoS mechanisms in wireless networks to enable VoIP services. In this thesis we proposed such a QoS mechanism; we in particular investigate the Call Admission Control (CAC) mechanisms which work on the edge routers or access points. To provide satisfactory services, traditional CAC approaches in wireless networks reserved fix percentage of the total bandwidth for the handoff calls. They have the shortcoming of fixed bandwidth reservation that is unable to adapt varying network conditions. Other related works though can be adaptive, but they need extra information exchanges and computing time. To solve these shortcomings we proposed a Diffserv based CAC mechanism, which woks on the edge networks. This mechanism can dynamically allocate bandwidth according to the bandwidth occupancy. It also adapts the transit rate of ongoing calls to reduce the reject probabilities. Our simulations show that our scheme can perform 15% lower blocking probabilities than traditional schemes do. Using our scheme will provide better VoIP service with QoS in wireless environments.
