Network-based Handover for IMS Service Continuity Wei-Kuo Chiang and Chun-Te Li Department of Computer Science & Information Engineering National Chung Cheng University Chiayi 621, Taiwan, R.O.C. wkchiang@cs.ccu.edu.tw Abstract—The IP Multimedia Subsystem (IMS) has been selected as a telecommunication industrial standard for the signal processing among heterogeneous access networks, and it is also responsible for mobility management. However, the mobility of the user equipment (UE) may disrupt or even intermittently disconnect an ongoing session, which heavily affects the satisfaction of the users. Therefore, how to reduce the service disruption time is an important issue. This paper proposed a network-based handover scheme, abbreviated as NHO, in IMS-based network. When a UE moves to a new network and completes the attach procedure during a mid-session, the access gateway (e.g. PDN-GW in LTE) will send a SIP PUBLISH message with secure context transfer request to the newly attached P-CSCF. Then the new P-CSCF will accomplish the secure context transfer to get the context information of the UE, and then perform the IMS registration procedure and the IMS session setup procedure on behalf of the UE in parallel. The novelty of NHO is that it only requires minor modifications to the attach procedure, and enhances MME, Serving-GW, PDN-GW, HSS, and P-CSCF slightly. Analytical results based on queuing theory show that NHO can shorten the handover delay as compared with current alternatives. Keywords—IP multimedia subsystem; service continuity; network-based handover; queuing delay I. INTRODUCTION The Next Generation Network (NGN), a network that provides packet-based services and information, is gaining more significance in the all-IP world. 3GPP has defined IP Multimedia Subsystem (IMS) [1] to process the session signaling that provide telecommunication services and multimedia applications in the NGN. With the development of different wireless access technologies such as WLAN, Universal Mobile Telecommunications System (UMTS), Worldwide Interoperability for Microwave Access (WiMAX), and Long Term Evolution (LTE), many worthy of investigating issues such as mobility management appears. To achieve seamless handover in the heterogeneous access networks, IMS is responsible for handling the mobility problem. Obviously, the IMS-based service continuity for the UE has become an important issue. 3GPP has also defined service continuity specifications [2][3] for supporting mobility among these different access technologies. A Service Centralization and Continuity Application Server (SCC AS) has been specified as a standard scheme to solve the service continuity problem and provide seamless handover. The SCC AS works as a Session Initial Protocol (SIP) Back-to-Back User Agent (B2BUA) and follows the third party call control (3PCC) architecture. If a UE handovers to a different access network and connects to a new IP-Connectivity Access Network (IP-CAN) during a mid-call, it must wait for the completion of re-registration, and then sends a SIP re-INVITE request with a Session Transfer Identifier/Session Transfer Number (STI/STN) to the SCC AS for re-establishing the ongoing session since a valid security association is a precondition for any communication session establishment. This process will lead to considerable handover delay; therefore, we intend to investigate possible approaches to improve this drawback. The rest of the paper is organized as follows: In section II, we briefly introduce the existing handover schemes. Section III will present the proposed scheme. In section IV, we discuss the handover delays of the existing handover schemes. Finally, we give a conclusion in section V. II. RELATED WORK This section will introduce existing handover schemes, such as MSM, CSC (including CSC*), and DSC. A. Mid-Session Macro-Mobility (MSM) The standard IMS registration procedure (SCC) requires 4 messages from/to the UE and the session establishment procedure requires 11 messages. These procedures may lead to long disruption time at the application layer, which is unacceptable for the real-time services. The MSM with context transfer mechanism was proposed to reduce the number of SIP messages transmitted during handover [4]. The context transfer means that the new P-CSCF attached by the UE obtains the context of the UE from the old P-CSCF which was derived from the previous SIP REGISTER and INVITE messages. B. IMS-based Centralized Service Continuity (CSC) The CSC treats handover as a service in the IMS network [5]. Its architecture and handover procedures are based on service invocation. To shorten the IMS registration procedure during the handover, E-IMS AKA with one pass authentication [6] is adapted to the variant of CSC, called CSC*. The E-IMS This work was sponsored in part by National Science Council of Taiwan under grant NSC 101-2221-E-194-024-MY2. 978-1-4799-4441-5/14/$31.00 ©2014 IEEE