IP-Based Overlay Signaling for Seamless Service Roaming in Heterogeneous Networks Kai Daniel, Thang Tran and Christian Wietfeld Communication Networks Institute Dortmund University of Technology 44227 Dortmund, Germany Email: {Kai.Daniel, Thang.Tran, Christian.Wietfeld}@uni-dortmund.de Abstract—Despite recent advancements in cellular mobile net- work architectures, full coverage for seamless handoffs is still an open issue. Moreover, cost efficient calls for the end users in urban regions with very high coverage can be improved. In many areas overlapping networks like WLAN, GSM and 3G provide alternative wireless links. Ideally, the user should be connected to the most attractive network any time according to predefined priorities using an automatic handover. In this paper, we present a framework for vertical handoff with dual- mode (cellular/WLAN) devices by using the conference bridging functionality of a PBX gateway. The communication between client and server will be realized via an overlay signaling channel. Based on the work done in [1] we extend and integrate some important services in the proposed framework: The ICAS (Individual Communication Address Space) feature allows users to be reachable by an alias name without revealing his real phone number. Our proposed Presence Enabled Phonebook shows the system presence and status information of the users’ contacts. Beyond the illustration of the framework, the quality and reliability of voice and overlay channel were analyzed in terms of a vertical handoff. I. I NTRODUCTION Mobile and everywhere communication is still lacking from high costs for the end users. Although many multimode devices are available on the market and many users are often equipped with both circuit and packet switched connectivity, the continuous and transparent use for the user is still not applicable. Either Voice-over-IP (VoIP) or circuit switched clients are used in the present time, but those are rarely jointly deployed. The overall coverage of wireless networks like WLAN, GSM and (W)CDMA still grows with the introduction of Mo- bileWiMAX (IEEE 802.16e) and LTE (Long Term Evolution). Hence, new protocols for seamless vertical handoffs become an emerging problem. Consequently, there is a need for a sim- ple multi-technology enabling protocol that does not depend on any configuration in the existing network architecture. The involvement of a managed software PBX is an practicable solution. Telephone numbers are still dominantly used for addressing in circuit switched telephone networks. This concept has its origin in the very early days of telephony, but is not user- friendly any more. Beside the problem of memorizing a lot of telephone numbers, there is a lack of privacy protection for the owner as well. In contrast to published e-mail addresses the telephone number can hardly be renewed. By means of CLIR (Calling Line Identification Restriction) the caller party is able to hide its identity but not vice versa. Several advanced addressing concepts exist [8][7] and outline requirements but have, as far as we know, not resulted in real implementations yet. For many end users and enterprises the handling of telephone numbers is still a pain especially in two exemplary situations: 1) In case of incoming calls, a Caller ID Presentation (CLIP) based on a telephone number, which is not registered locally in the device’s phone book, offers no useful information about the caller’s identity or name. 2) In case of a telephone number change (e.g. by a network operator or residence change), end users have to inform their communication partners manually about their new number. To make the handling of telephone numbers more user-friendly we introduced an Individual Communication Address Space (ICAS) concept in [1], which applies user-defined plain text (alias) addresses. Based on this concept we now present a prototype integration of the proposed service framework, that is based on an overlay signaling channel. II. RELATED WORK Approaches considering Layer 2 handover protocols mostly have handover performance and connectivity loss problems in common. Several concepts exist allowing for seamless vertical handoffs by supporting multi-technology processing, which means managing diverse access technologies. Service providers could enable subscribers to roam and handover between public cellular networks and private unlicensed wireless networks using dual-mode mobile handsets [6] by deploying Universal Mobile Access (UMA) technology in the existing network infrastructure. But the UMA standard is restricted to a dual mode operation and not useful for a multi-technology handoff between several available carriers within the users’ operation area. For a practical implementation, commercial network operators need to complement their network architecture with an additional UNC (UMA network controller). Another approach in handover management is the idea of deploying Media Independent Handover (MIH) with IEEE 802.21, based on a Layer 2–Layer 3 configuration [10],