186 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 5, NO. 1,JANUARY 2006 Caching in I-CSCF of UMTS IP Multimedia Subsystem Yi-Bing Lin, Fellow, IEEE, and Meng-Hsun Tsai, Student Member, IEEE Abstract—The IP multimedia core network subsystem (IMS) provides multimedia services for Universal Mobile Telecommu- nications System (UMTS). In IMS, any incoming call will first arrive at the interrogating call session control function (I-CSCF). The I-CSCF queries the home subscriber server (HSS) to iden- tify the serving CSCF (S-CSCF) of the called mobile user. The S-CSCF then sets up the call to the called mobile user. This paper investigates the performance of the IMS incoming call setup. We also propose cache schemes with fault tolerance to speed up the incoming-call-setup process. Our study indicates that the I-CSCF cache can significantly reduce the incoming-call-setup delay, and checkpointing can effectively enhance the availability of I-CSCF. Index Terms—Call session control function (CSCF), general packet radio service (GPRS), home subscriber server (HSS), IP multimedia core network subsystem (IMS), registration, universal mobile telecommunications system (UMTS). I. I NTRODUCTION U NIVERSAL Mobile Telecommunications System (UMTS) is one of the major standards for the third- generation (3G) mobile telecommunications. In UMTS, the IP multimedia core network subsystem (IMS) provides multimedia services by utilizing the Session Initiation Protocol (SIP) [4]. Fig. 1 illustrates a simplified UMTS network architecture (the reader is referred to [1], [3], [5], and [6] for the detailed descriptions). This architecture consists of a radio access network, the general packet radio service (GPRS) core network and the IMS network. The GPRS core network connects to the IMS network through gateway GPRS support nodes (GGSNs). The home subscriber server (HSS) is the master database containing all user-related subscription information. Both the GPRS and the IMS networks access the HSS for mobility management and session management. A mobile user utilizes a mobile station (MS) or user equipment (UE) to access IMS services. To provide a data session for the UE, a connection between the UE and the GGSN is established. This connection is specified by a Packet Data Protocol (PDP) context. The PDP context must be activated before a UE Manuscript received October 6, 2003; revised September 25, 2004; accepted September 25, 2004. The editor coordinating the review of this pa- per and approving it for publication is Y. Fang. This paper was sponsored in part by National Science Council (NSC) Excellence project NSC93- 2752-E-0090005-PAE, by NSC 93-2213-E-009-100, by National Telecommu- nication Development Program (NTP) Voice over IP (VoIP) Project under Grant NSC 92-2219-E-009-032, by IIS/ACADEMIA Sinica, and by the In- dustrial Technology Research Institute (ITRI)/National Chiao Tung University (NCTU) Joint Research Center. The authors are with the Department of Computer Science and Information Engineering, National Chiao Tung University, Hsinchu 30050, Taiwan, R.O.C. (e-mail: liny@csie.nctu.edu.tw; tsaimh@csie.nctu.edu.tw). Digital Object Identifier 10.1109/TWC.2005.858332 Fig. 1. UMTS network architecture. can access the IMS network. The IMS user data traffic is transported through the media gateways (MGWs). The IMS signaling is carried out by proxy call session control function (P-CSCF), interrogating CSCF (I-CSCF), and serving CSCF (S-CSCF). The I-CSCF determines how to route incoming calls to the S-CSCF and then to the destination UEs. That is, the I-CSCF is the contact point for the IMS network of the destination UE, which may be used to hide the configuration, capacity, and topology of the IMS network from the outside world. When a UE attaches to the GPRS/IMS network and performs PDP context activation, a P-CSCF is assigned to the UE. The P-CSCF contains limited address translation functions to forward the requests to the I-CSCF. Authorization for bearer resources in the network (where the UE visits) is performed by the P-CSCF. By exercising the IMS registration, to be described in Section II, an S-CSCF is assigned to serve the UE. This S-CSCF supports the signaling interactions with the UE for call setup and supplementary-services control (e.g., service request and authentication). This paper investigates the performance of the IMS incoming call setup. Specifically, we propose cache schemes with fault tolerance to speed up the incoming-call-setup process. II. IMS REGISTRATION AND CALL SETUP This section describes the registration and the incoming-call- setup procedures for UMTS IMS. We first elaborate on the basic scheme proposed in 3G Partnership Project (3GPP) 23.228 [3]. Then, we propose a cache scheme and two checkpoint schemes that speed up the incoming-call-setup process. A. The Basic Scheme Suppose that a UE already obtained the IP connectivity through the PDP context activation, and has performed at least one IMS registration. The UE may issue reregistration due to, 1536-1276/$20.00 © 2006 IEEE