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TopI. Introduction
Internet Protocol Multimedia Subsystem (IMS) is defined as access independent IP-based service control architecture, aimed to provide all kinds of multimedia services (Poikselka & Mayer, 2008). The requirement for the IMS in the conjunction with the underlying IP connectivity access network and transport network (IP-CAN) is to provide quality of service. Quality of service (QoS) is used to differentiate multimedia offering from traditional Internet services which in most cases do not provide QoS.
Via the IMS, the user equipment negotiates its capabilities and QoS parameters during session setup or modification, using Session Initiation Protocol (SIP) (3GPP TS 24.229 v9.2.0, 2009). The capability to authorize and control the usage of QoS resources, intended for IMS media, based on SIP signaling, is called IP policy control. The IP policy control can increase utilization of bearer resources by parameter control over the access and transport networks. To ensure coherent charging between IP-CAN and IMS, the IP policy control is harmonized with charging control, and the overall interaction between the IMS and the IP-CAN network is called Policy and Charging Control (PCC) (3GPP TS 23.203 v9.3.0, 2009). The PCC encompasses the following high level functions for IP-CANs (e.g., GPRS, WLAN, Fixed broadband, etc.): flow based charging, including charging control and online credit control, and policy control e.g. gating control, QoS control, user plane event reporting and network initiated IP-CAN bearer establishment.
One of the design principles of IMS is separation of applications from generic service control functionality and provision of open interfaces. The open access to network functions allows third party applications to invoke communication functions. Instead of using control protocols, the applications access network resources via application programming interfaces (APIs). Defined as service architecture for mobile networks Open Service Access (OSA) provides APIs for a palette of network functions such as call control, data session control, mobility, messaging etc.
The main goal of the paper is to assess the support of existing APIs for access to PCC functions and to present an approach to design OSA compliant APIs for QoS management.
After this introduction, some related work is discussed in section II. The IMS PCC architecture is presented in section III. The requirements for open access to QoS management based on the PCC framework are summarized in section IV. The standardized capabilities for open access to QoS management are evaluated in section V. In section VI, we describe how OSA compliant interfaces can be designed having in mind the identified requirements. The interface implementation requires mapping of interface’s methods onto SIP and Diameter protocol messages, and it is described in section VII. The interface usage is illustrated by typical use cases. In section VIII, we model the interface behavior by means of state machines and in section IX, we prove formally the behavioral equivalence of the proposed state machines with those, supported by network protocols. Finally, in section X, the paper contribution is summarized.