IEEE Wireless Communications • June 2005 8 1536-1284/05/$20.00 © 2005 IEEE Node-B AP UMTS roamer UTRA MS UT netw (UTR T OWARD S EAMLESS I NTERNETWORKING OF W IRELESS LAN AND C ELLULAR N ETWORKS INTRODUCTION The interworking between third-generation (3G) cellular and wireless local area networks (WLANs) has been considered as a suitable and viable evolution path toward the next gen- eration of wireless networks. Yet this inter- working raises considerable challenges, especially when we demand seamless continuity of multimedia sessions across the two networks. To deal with these challenges, several 3G/WLAN interworking requirements must be identified and fulfilled. Typically, the 3G/WLAN interworking requirements are specified and categorized in terms of several usage scenarios [1, 2]. For example, a common usage scenario is when a 3G subscriber is admitted to a WLAN environ- ment by reusing his/her regular 3G credentials, and then obtains IP connectivity service (e.g., access to the Internet). In this case, the inter- working requirements include support of 3G- based access control, signaling between the WLAN and 3G network for authentication, authorization, and accounting (AAA) purposes, and so on. Other scenarios can call for more demanding interworking requirements. We may envision, for instance, a scenario in which a 3G subscriber initiates a video session in his/her home 3G network and subsequently transits to a WLAN environment wherein the video session is continued seamlessly, without any noticeable change in quality of service (QoS). In this case, not only is 3G-based access control required, but also access to 3G-based services is needed over the WLAN network, which in turn calls for appropriate routing enforcement mechanisms. More important, however, there is need for QoS consistency across 3G and WLAN, which appears to be not very straightforward given the different QoS features offered by these net- works. Indeed, WLANs were initially specified without paying much attention to QoS aspects, aimed primarily at simple and cost-effective designs. Even with the recent IEEE 802.11e [3] developments, WLAN QoS still exhibits several deficiencies with respect to 3G QoS (this is fur- ther discussed later). On the contrary, 3G cellu- lar networks were built with multimedia applications in mind, and trade simplicity and cost for inherently providing enhanced QoS in wide-area environments. Our main interest in this article is to examine APOSTOLIS K. SALKINTZIS, MOTOROLA GERASIMOS DIMITRIADIS, ARISTOTLE UNIVERSITY OF THESSALONIKI DIMITRIS SKYRIANOGLOU AND NIKOS P ASSAS, UNIVERSITY OF A THENS NIOVI P AVLIDOU, ARISTOTLE UNIVERSITY OF THESSALONIKI ABSTRACT This article addresses several challenges relat- ed to the evolution toward seamless interwork- ing of wireless LAN and 3G cellular networks. The main objective is to evaluate the conditions and restrictions under which seamless continuity of video sessions across the two networks is fea- sible. For this purpose, we formulate a number of practical interworking scenarios, where UMTS subscribers with ongoing real-time video sessions hand over to WLAN, and we study the feasibility of seamless continuity by means of simulation. We particularly quantify the maximum number of UMTS subscribers that can be admitted to the WLAN, subject to maintaining the same level of UMTS QoS and respecting the WLAN policies. Our results indicate that the WLAN can support seamless continuity of video sessions for only a limited number of UMTS subscribers, which depends on the applied WLAN policy, access parameters, and QoS requirements. In addition to this study, we do address several other issues that are equally important to seam- less session continuity, such as the QoS discrep- ancies across UMTS and WLAN, the vertical handover details, and various means of access control and differentiation between regular WLAN data users and UMTS subscribers. The framework for discussing these issues is created by considering a practical UMTS/WLAN inter- working architecture. S EAMLESS C ONTINUITY OF R EAL -T IME V IDEO ACROSS UMTS AND WLAN N ETWORKS : C HALLENGES AND P ERFORMANCE E VALUATION The authors address several challenges related to the evolution toward seamless interworking of wireless LAN and 3G cellular networks. Their main objective is to evaluate the conditions and restrictions under which seamless continuity of video sessions across the two networks is feasible.