利用MPLS技術支援GPRS傳輸網路的QoS

Abstract

目前General Radio Packet Service (GPRS) 的核心網路是屬於利用IP繞送的私有網路 (private network)。GPRS網路使用IP協定來傳輸信息封包與資料封包，但其IP定址及繞徑機制卻獨立於外部的網際網路 (Internet)。GPRS網路對於行動台而言，如同一個連接外部網際網路的one-hop default router。行動台的資料封包必須先經由GPRS Tunneling Protocol (GTP) 封裝，然後再經由GPRS核心網路內的IP-tunnels繞送。而IP-GTP-IP tunneling的機制產生了三項重大的缺點：(1)較長的封包標頭，(2)較長的封包繞送延遲，(3)外部網際網路與GPRS核心網路之間的即時連線服務品質 (Quality of Service, QoS) 協調的困難度較高。 為了解決GPRS網路因IP-GTP-IP tunneling機制所產生的缺點，我們提出一個架構，將Multi-Protocol Label Switching (MPLS) 協定加入GPRS的的protocol stack，稱為MiGPRS。MiGPRS使用標籤交換 (label switching) 的機制替代IP routing，於GPRS核心網路內傳輸資料封包。由於GPRS核心網路通常為一私有管理的網路，所以我們可以在每對GGSN-SGSN 之間預先建立label switched path (LSP)，使用更有效率的label switching來替代IP routing。利用label switching方式，MiGPRS可以加速封包的傳輸速度以及縮短封包標頭長度，因而增加系統的capacity。此外；我們使用label stacking機制，將GPRS transmission plane中的GPRS Tunneling Protocol (GTP) 層移除。利用具有較短長度的MPLS label來代替GTP header，MiGPRS可以更進一步地縮減封包標頭的長度。 為了在MiGPRS網路中提供QoS服務，前述在每對GGSN-SGSN之間的pre-configured LSPs為constraint-based routed LSPs (CR-LSPs)。由於CR-LSPs本身具有的connection-oriented特性，我們可以使用CR-LSPs來提供Differentiated Service (Diff-Serv)以支援GPRS網路與Internet之間的點對點服務品質控制 (End-to-End QoS Control) 傳輸。此外；MiGPRS也在每對SGSN-SGSN之間預建handoff CR-LSPs來支援inter-SGSN順暢換手 (smooth handoff) 。當行動台在進行inter-SGSN handoff時，這些handoff CR-LSPs可以被使用來讓舊的SGSN轉傳 (forward) in-flight packets到新的SGSN。然而當行動台進行換手時，我們必須保留足夠的頻寬給handoff CR-LSPs，以維持行動台即時連線的服務品質。因此；我們提出一個分析模型來計算行動台的inter-SGSN換手率，並根據分析的inter-SGSN換手率，提出一個handoff CR-LSPs的頻寬估計模型，以計算handoff CR-LSPs所需的頻寬。最後，我們進行模擬來分析handoff model的精準性以及顯示MiGPRS的效能。模擬的結果驗證MiGPRS在平均封包延遲、封包丟失率和連線成功率等都有優越的表現。

The core network of General Packet Radio Service (GPRS) is an IP-based private network. It adopts IP protocol in transporting both signal and data packets but its IP addressing and routing is independent of that in the external Internet. The GPRS core network appears to the GPRS mobile stations (MSs) as a one-hop default router to the external Internet. All IP data packets of GPRS MSs are first encapsulated in GPRS Tunneling Protocol (GTP) and then routed through the IP-tunnels in the GPRS core network. This IP-GTP-IP tunneling mechanism results in three significant drawbacks: (1) larger data packet header, (2) longer IP routing delay, and (3) more difficult to provide QoS guaranteed services and support identical QoS agreements across the GPRS core network and Internet. In order to overcome above three deficiencies of the GPRS core networks, we propose a protocol stack that embeds Multi-Protocol Label Switching (MPLS) into the protocol stack of GPRS, henceforth referred as MiGPRS. MiGPRS adopts label switching, instead of IP routing, mechanism to transfer data packets in the GPRS core network. Because the GPRS core network is normally constructed as a privately controlled administration domain, we can pre-configure a label switched path (LSP) between each GGSN-SGSN pair to replace IP routing with a more efficient label switching. With label switching, MiGPRS can speed up packet transmission and shorten packet header length, and thus increase system capacity. Furthermore, we also adopt label stacking mechanism to eliminate the GPRS Tunneling Protocol (GTP) layer from the GPRS transmission plane. By replacing the GTP header with a shorter MPLS label, MiGPRS can further reduce header lengths of data packets. In order to support QoS guaranteed services in MiGPRS, the pre-configured LSPs between each GGSN-SGSN pair are constraint-based routed LSPs (CR-LSPs). Because of the connection-oriented characteristic of CR-LSPs, we could use CR-LSPs to provide Differentiated Services (Diff-Serv) for end-to-end QoS control across the GPRS core network and Internet. Besides, MiGPRS also pre-configure handoff CR-LSPs between each SGSN-SGSN pair to support inter-SGSN smooth handoff. These handoff CR-LSPs could be used by the old SGSN to forward in-flight packets to the new SGSN when an MS performing an inter-SGSN handoff. However, we need to reserve sufficient bandwidth for the handoff CR-LSPs so that we can retain the QoS requirement of MSs’ real-time streams, when MSs are performing inter-SGSN handoffs. Therefore we first present an analytic model to calculate the inter-SGSN handoff rate of MSs and then propose a bandwidth estimation model for handoff CR-LSPs according to the inter-SGSN handoff rate obtained from the analytic model. Finally, we conduct simulation to analyze the accuracy of the handoff model and show the effectiveness of MiGPRS. Simulation results show that MiGPRS is very effective in terms of mean packet delays and packet dropping ratios.