虛擬私有網路（Virtual Private Network，簡稱 VPN）提供了企業用戶一個安全且成本低廉的網路連結環境，而隨著無線網路越來越普及，企業用戶的使用者隨時隨地地連接上網路已成為一種趨勢，接著就是如何將虛擬私有網路與移動式網路環境（Mobile Environment）做結合。除了要能夠任意且安全地存取企業網路，QoS 也是個重要的課題，在現有的 IP VPN 網路環境中，大多使用差別式服務（Differentiated Service，簡稱 DiffServ）來提供網路與網路之間的 QoS 服務，較難保證端點對端點（end-to-end）的 QoS 之 VPN 服務。 本文提出了雙層式 VPN 頻寬分配策略，將 VPN 的頻寬分成 VPN 層（VPN level）與呼叫層（Call level），VPN 層的頻寬分配保證了一群 VPN 用戶在網路中可以使用之頻寬，而呼叫層的頻寬分配則讓 VPN 內的使用者共享一個 VPN 層分配的頻寬。另外本文特別針對在 VPN 使用者 handoff 之前做有線及無線的頻寬保留，以確保 VPN 使用者可以在網路中無縫（seamless）的移動，享有一致的 QoS 服務。 由模擬結果可發現，本文所提出之雙層式 VPN 頻寬分配策略能有效地降低呼叫層服務呼叫時的阻絕率，而預先保留的機制則可使得使用者 handoff 過程時的封包遺失率降低。 Virtual Private Networks (VPN) support enterprises a secure and low-priced network environment. And as the wireless networks become more and more popular, more and more enterprise users can access the Internet anywhere, anytime using wireless technology. What comes after is the combination of VPN and the mobile environments. To achieve the mobile VPN environments, besides secure and on demand accessing the enterprise’s network, quality of service (QoS) is also an important issue. In IP VPN networks nowadays, differentiated service (DiffServ) is adopted to provide QoS network to network. But such scheme can’t guarantee end-to-end QoS of VPN services. The thesis proposes a two-level VPN bandwidth allocation strategy. VPN services are classified into VPN level and Call level. Bandwidth allocation of VPN level guarantee services of a group of VPN users, and the bandwidth allocation of call level let the users share the bandwidth allocated to the VPN. Besides, the proposed strategy also reserves the bandwidth of wired links or wireless channels before a user handoffs to ensure that VPN users can receive seamless services while roaming in the networks with consistent QoS. In the simulation results, we found that the proposed two-level VPN bandwidth allocation strategy can lower down the call blocking rate of call level services effectively, and the pre-reservation scheme can lower down the packet loss rate while users handoff.
