394 IEEE COMMUNICATIONS LETTERS, VOL. 4, NO. 12, DECEMBER 2000 Modeling Mis-Routing Calls Due to User Mobility in Wireless VoIP Ai-Chun Pang, Phone Lin, and Yi-Bing Lin Abstract—This letter proposes an analytic model to study the mis-routing problem caused by user mobility in a wireless VoIP system. We derive the probability that there are mis-routed calls when a subscriber moves from IP network to the GSM network. Our study indicates that if the user residence times in GSM loca- tion areas, inter-call origination times and the inter-call delivery times are of the same order, then the mis-routing effect can not be ignored. Index Terms—GSM, mis-routing, user mobility, wireless VoIP. I. INTRODUCTION S UPPORTING telephony services over IP network is a promising trend in telecommunication business. Telephony services over IP network or the so called voice over IP (VoIP) can provide subscribers low cost telephony services. Thus, in- corporating VoIP services into the existing telecommunication systems is essential. In particular, integrating mobile phone services with VoIP becomes an important issue, which has been intensively studied [1], [2]. Telecommunications and Internet Protocol Harmonization over Network (TIPHON) specifies the mechanism (i.e., a me- diation gatekeeper) to provide the service control functions for convergence of IP networks, mobile networks, fixed wireless network, and the public switched telephone network (PSTN). Several scenarios are defined in TIPHON to illustrate different ways of integrating IP and mobile networks. In this paper, we use GSM as an example to describe a TIPHON mobile/IP integration, where the mobile signaling protocol is GSM MAP described in [3]. This TIPHON scenario is called iGSM [4] and its architecture is illustrated in Fig. 1. In this figure, iGSM gateway performs the conversion between GSM MAP and H.323 family proto- cols, and iGSM VLR (Gatekeeper) records location information as the visitor location register (VLR) in GSM network. iGSM supports user mobility for GSM subscribers to access VoIP ser- vices. By using various types of terminals, user mobility allows a user to move around the service area without losing contact with the system. That is, a GSM subscriber is able to keep the same identity to receive the service even if he/she changes the Manuscript received July 21, 2000. The associate editor coordinating the re- view of this letter and approving it for publication was Prof. I. S. Venieris. This work was supported in part by MOE Program of Excellence Research under Contract 89-E-FA04-4, CCL/ITRI under Contract 2-10B, FarEastone, NSC, and the Lee and MTI Center for Networking Research, NCTU. The authors are with the Department of Computer Science and Information Engineering, National Chiao Tung University, Hsinchu, Taiwan, R.O.C. (e-mail: liny@csie.nctu.edu.tw). Publisher Item Identifier S 1089-7798(00)11514-5. Fig. 1. The iGSM architecture. terminal type from the GSM mobile station (MS) to the H.323 terminal. In Fig. 1, the incoming calls (call deliveries) are routed to the GSM MS when the person is in the GSM network (using GSM MS). On the other hand, the calls are routed to the H.323 terminal when the person is in the IP network (using H.323 ter- minal). We note that to make iGSM work, GSM periodic regis- tration must be disabled for the iGSM subscribers. Otherwise, if an iGSM subscriber forgets to turn off the GSM MS when the subscriber switches to an H.323 terminal, the registration record of the H.323 terminal will be over-written by the periodic reg- istration operation of the GSM MS. To support user mobility, the subscriber needs to explicitly perform registration to inform the system which location area (LA) the MS resides when the subscriber changes the terminal. If the subscriber forgets to take this action, call deliveries to the subscriber may be mis-routed. This problem exists for all ap- proaches based on the concept of universal personal telecommu- nications [5]. The mis-routing problem occurs in the following scenario. Step I [Fig. 2(a)]: The subscriber is in the GSM LA A and the home location register (HLR) indi- cates that the person is in LA A. The sub- scriber then moves to the IP network (LA B) without turning off the GSM MS. Step II [Fig. 2(b)]: The subscriber registers to IP network. After registration, the HLR record is mod- ified and ’s record in VLR A is removed. Step III [Fig. 2(c)]: The subscriber moves back to the GSM MS at LA A. Since the GSM MS is still on, the subscriber does not notice that an 1089–7798/00$10.00 © 2000 IEEE