2. Signaling and Handoff Rates at the Policy Control Function (PCF)

2. Signaling and Handoff Rates at the Policy Control Function (PCF)

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526 IEEE COMMUNICATIONS LETTERS, VOL. 12, NO. 7, JULY 2008 Signaling and Handoff Rates at the Policy Control Function (PCF) in IP Multimedia Subsystem (IMS) Said Zaghloul, Wolfgang Bziuk, and Admela Jukan Abstract —Within the IP Multimedia Subsystem (IMS), a next generation wireless service oriented framework, the Policy Control Function (PCF) authorizes and controls the users’ f ows in the IP network and enables roaming partnerships with other operators. To perform these tasks, the PCF servers should be designed to accommodate varying incoming signaling rates which depend on both the service request and the handoff rates. The handoff rate is a complex function of the number of access gateways, service duration, and mobility. In this paper, we develop for the F rst time an analytical model of the signaling and handoff rates at the policy server by considering all these factors. Our results show that increasing the number of gateways can result in non-linear increase in the signaling rate. Index Terms —Handoff rate, roaming, transient Markov chains, analytical mobility model. I. INTRODUCTION P OLICY Control Functions (PCFs) within the IP Multi- media Subsystem (IMS) control the Quality of Service (QoS) level a service can acquire from the network and the roaming service provisioning among operators [1]. In IMS, the PCF server controls a group of IP access gateways (AGW), be it the GPRS Gateway Support Node (GGSN) in 3GPP or the Packet Data Support Node (PDSN) in 3GPP2 systems (see Fig.1). The AGWs are the IP gateways for multiple Radio Network Controllers (RNCs) which serve a group of base stations. When a session is initiated or when a user moves from one AGW area to another, the QoS signaling is initiated from the serving AGW towards the PCF (link L1). For roaming requests, the foreign PCF queries the home PCF for the user’s QoS pro f le to complete the authorization process (link L2). A key factor to consider when extending the network coverage (i.e., adding more AGWs), is the signaling rate at the PCF as poor system designs may result in PCF overloading and hence delayed authorization. Assessing the signaling rate at the PCF is not trivial since it is directly related to the inter- AGW handoff rate. The inter-gateway handoff rate, however, is a function of the arrival rates for all active sessions; these not only include the active sessions which are starting from within the home network (referred to as on-net traf f c) but also the already established sessions coming from the neighboring networks (referred to as off-net traf f c). Here, both on-net and off-net traf f c may incorporate home (i.e. own) and foreign (i.e., roaming) subscriber requests. The handoff rate also depends on the number of gateways (AGWs) in the network, the session duration, and the residence time within an AGW. While prior important research on the handoff process mod-
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2. Signaling and Handoff Rates at the Policy Control Function (PCF)

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