Proposal Architecture For Quality of Service Provisioning Within Inter-domain IP Multimedia Subsystem Context

The Third Generation Partnership Project (3GPP) has adapted the Internet-Protocol Multimedia Subsystem (IMS) as a service delivery platform for the Next Generation Networks (NGN). IMS offers multimedia services over Internet Protocol (IP) based infrastructure. These services require a mechanism to manage Quality-of-Service (QoS). The current 3GPP QoS standards are based on Policy Based QoS Management architecture which provisions QoS in a single IMS domain, but users may be located in different IMS domains, or roaming between deferent IMS domains. This raise issues regarding management of QoS across IMS domains. In this paper we will identify these issues in the current IMS Policy Based QoS Management architecture and propose a mechanism for Inter-domain QoS management based on the IMS platform.


I. INTRODUCTION
A new Era of rich multimedia communication services is emerging. Users will have the ability to communicate anytime, anywhere, using any device. New services and applications will demand a new level of network bandwidth, processing power and responsiveness. The increasing demand for bandwidth is a result of new multimedia rich services and variable Quality-of-Service (QoS) requirements. The IP Multimedia Subsystem (IMS) is a prime candidate as a service delivery platform for Next Generation Networks (NGN), as it addresses the main characteristics of the NGN as defined by the International Telecommunication Union (ITU) [1]. provided independently of end-user access technologies, and seamlessly integrates access domains with an all IP core [2]. Another advantage provided by the IMS is the ability to negotiate the QoS levels of each multimedia stream which constitute the session. While the standardization process has made great progress, specifications for QoS management still have open areas for further studies; most IMS deployments do not support policy controlled resource management, or do some in a scaled down manner. One such open area is policy provisioning for inter-domain IMS QoS provisioning. As multimedia sessions may traverse a sequence of heterogeneous administrative domains in NGN, the policy control system should be able to guarantee QoS resources across all involved domains in order to provide the user with a satisfactory Quality of Experience (QoE) level. In order to achieve end-to-end (e2e) QoS, it is necessary to maintain a level of QoS all along the path from the source Terminal Equipment (TE) to the destination TE across various administrative domains. As each domain has its own mechanisms and policies to support QoS, the end-to-end service level of a communication is affected by the QoS technologies of the domains along the communicationpath. The provisioning of e2e QoS assumes the negotiation of a mutually acceptable Service Level Agreement (SLA) between these domains. In general, an SLA is a formal, negotiated contract between two parties that establishes committed levels of network service performance and responsiveness. The two parties may be a consumer and an operator, or two operators where one takes the customer-role, buying services from another service provider.
In the heterogeneous NGN infrastructure, access to the services may be achieved via different kinds of wireless-wireline access technologies including Wireless Local Area Network (WLAN), Digital Subscriber Line (xDSL) and Universal Terrestrial Radio Access Network (UTRAN). The current QoS mechanisms defined by 3GPP do not meet the requirements of such a heterogeneous network [3]. In the paradigm of NGN, a multimedia session may pass through different technologies and administrative domains, in this existing architecture there is no mechanism for different domains to exchange QoS policies and network limitations dynamically and efficiently as the SLA.
In this paper we introduce the current policy control framework defined by the 3GPP,  The remainder of this paper is organized as follows. Section II reviews background and related work; in particular the standardized relevant work in the literature is presented.
Section III analyses the aforementioned limitations of the current system in depth, and presents the proposed inter-domain policy provisioning architecture. Section IV presents the testbed implementations and the obtained results. Finally Section V concludes the paper.

II. BACKGROUND AND RELATED WORK
The Third Generation Partnership Project (3GPP) is the defining standards organization for the IMS; to control admission and resources and provision flow based charging they define the Policy Control and Charging (PCC) architecture [4]. This architecture combines the functionality of the Service Based Local Policy and the Flow Based Charging architectures.
The PCC architecture has since been adopted and extended by ETSI TISPAN in their Resource and Admission Control Subsystem (RACS) [5], and the ITU-T in their Resource and Admission Control Function (RACF) [6]. While there are subtle differences between these architectures, there are no significant conflicts [7]. For simplicity we utilize 3GPP terminology throughout this work.
The functional requirements for policy control include gating and QoS control; admission control is performed on a per service data flow basis and a policy rule is defined for each flow that defines the QoS requirements. The policy rule must be enforced in the transport level; this process is access network specific and has been fully defined for UMTS only. and using pre-stored policies performs admission control [8]. An AF can be any IP service element and is not limited to IMS. The PCRF creates a policy rule that defines the session and how it should be treated and sends it to the PCEF in the transport layer. The PCEF resides on the physical network devices in the transport layer. This element receives policy rules from the PCRF via the Diameter S7 interface, and enforces them of the network devices. This interface extends the previous Gx interface and allows the PCRF to interact and enforce policy rules across a greater number of access technologies and QoS models. It remains to be seen how this agnostic enforcement will be implemented as the S7 interface is still under development.
An important addition is the S9 interface that facilitates inter-domain communication between PCRFs in neighboring domains [9]. The interface is based on the Diameter protocol and allows PCRFs to request resources from neighboring domains ensuring end to end connectivity. This interaction leaves a number of critical areas open for third party creativity and flexibility. In particular the exchange of SLAs is not standardized. Fig. 1 shows the 3GPP Policy Control and Charging Architecture.
There are numerous works focused on Inter-domain QoS for the Future Internet, however little has been done in the practical environment. Mechanisms to map out topologies and discover resource control elements have been proposed [10], a test-bed implementation of this functionality was designed and implemented, enforcing policy rules on proprietary routing equipment.
The AQUILA project proposes an inter-domain resource reservation protocol that extends the Border Gateway Reservation Protocol (BGRP) [11]. This architecture can be implemented on standard routing equipment, but requires modifications to the transport layer equipment; furthermore potentially sensitive topology information is exchanged.
The EuQoS project exhibits end to end QoS support across heterogeneous networks [12].
They implement cross layer inter-domain mechanisms and propose extensions to the Border Gateway Protocol to select and advertise QoS.
While these works propose novel mechanisms to guarantee end to end QoS on the Future Internet, they are not specific to the IMS/PCC framework and replicate many standardized mechanisms. Additionally they require transport layer modification and do not address the issues of inter-domain SLA interactions. An SLA Broker has been introduced, as a service provider to allow SLA exchange between domains in a scalable manner.
The working mechanisms and protocol interfaces of the SLA Broker element were not fully defined, nor were the negotiated parameters. This work extends the concepts of the SLA Broker [13], and addresses open areas including a complete description of the framework functions and the negotiation of SLA parameters.  Subsequently, required end-to-end QoS levels at the network layer will need to be enforced by negotiating SLAs between IMS domains involved in the session. Since SLAs are typically bilateral agreements between neighboring IMS domains, the QoS sessions that cross at least two IMS domains may become unpredictable. Additionally in practice IMS administrative domains are reluctant to make major modifications to up-and-running networks in production.
These observations raise many questions regarding Inter-domain QoS: In the current QoS 3GPP specifications, there is no specified relationship or agreement between IMS domains to make specific guarantees on reachability, availability, accounting, or network performancethis is left as network operator specific. Moreover, managing QoS in the Inter-domain environment has two different aspects: one related to the management of QoS in the "Access Network" (AN) and the other related to the management of QoS in the Core Network (CN). In [13] we had discussed the limitations of the current system in details.
One of the main deployment challenges the IMS faces is how to allow diverse IMS domains to interwork with underlying resources, and to manage and provide end-to-end QoS in a scalable manner. This task requires coordination between the IMS domains which are involved in the session initiation and between end users as shown in Fig. 2. In [13] we    This service information is conveyed to the PCRF using the Diameter protocol over Gq interface according to 3GPP standards. The PCRF consults its Policy Repository for prestored policies (SLAs) which describe the agreement between the domains involved in the session. In this stage the PCRF makes a decision whether or not to allow the session to proceed, and assigns a QoS class in the acceptance case. At this point the SIP INVITE message will be forwarded to the destination domain using the CSCFs at the destination domain which will configure its own PCRF. This procedure configures the QoS parameters and pushes them to the PCEF entities at the bearer level after the called party has accepted the session. In order to make a decision about the QoS level assigned to the session, the PCRF at each side may need to send a request to the IDPR inquiring about the other domains' SLAs which are involved in the same session.

IV. TEST BED IMPLEMENTATION AND RESULTS
The proposed architecture was implemented in a proof of concept testbed. Free and Open Source software is used throughout the implementation. The Open Source IMS (OSIMS) Playground [14] from FOKUS Institute is used to implement the IMS core.
This software package provides implementations of the Proxy/Interrogating/-Serving-Call Session Control Functions (CSCF) and the Home Subscriber Server (HSS); it is based on 3GPP standards, widely used and provides a reliable testing environment. To represent IMS terminals the UCT IMS Client [16] were used in this research because it supports the multimedia session service as shown in Fig. 5. The OSIMS Core and UCT QoS framework elements were run on a single PC Dual Core 3.0 GHz based on Linux Ubuntu 8.10 system. The end user terminals were run on PCs that had the same previous characteristics.
All PCs in the test bed are connected via a 100Mbps LAN as shown in Fig. 6. All results in this paper are average over 4 test runs. To evaluation the proposed architecture the real time multimedia session has been chosen, because managing of QoS for the real time multimedia session service is more sensitive than other services. To illustrate the exact behavior of Inter-domain QoS provisioning of the proposed architecture, a performance comparison has been presented in this paper for a multimedia session with and without QoS provisioning.      Fig. 9 plots, this increase is related to the increase in the overhead and can be decreased in the real commercial environment using distributed architecture. Comparing the session setup delays when QoS provisions in a single domain (intra-domain) as Fig. 9 plots, the session setup delay in inter-domain environment does not become large. It remains within acceptable range.    Fig.9 Inter and Intra domain session setup delay.

V.CONCLUSION
This paper has presented our research work on provisioning e2e QoS for Inter-domain IMS. The limitations of the policy control framework defined by the 3GPP were discussed, particularly regarding Inter-domain administration.
In this paper, a mechanism for Inter-domain QoS management based on the IMS platform has been proposed that connects to the charging rules function. The proposed architecture was implemented in the form of a standards based test-bed where it was subjected to realistic use case scenarios. The proposed architecture effectively provisions QoS under varying background traffic. The increased traffic overhead and session setup delay was observed to be in an acceptable range for 4 simultaneous session requests.