1 Abstract— This paper describes the system architecture for a terminal-based location service enabler in the 3 rd Generation (3G) IP Multimedia Subsystem (IMS). A location service enabler provides data about the geographical position of mobile termi- nals to requesting clients. Since location data can be regarded as a type of presence information, we propose an architecture that builds on the IMS presence specifications. We show that several mechanisms for processing presence like notification handling, access control and privacy management apply to location data as well. It is therefore possible to reuse a large part of the IMS presence infrastructure. Based on the observation that the processing of trigger condi- tions for location information and creating notification messages should be done in the terminal itself, we propose a terminal- based location enabler. The terminal uses a built-in method to determine its geographical position such as a Global Positioning System (GPS) module. We argue that the proposed architecture scales better and is more accurate, efficient and cost effective than current network-based location enablers. I. INTRODUCTION Location Based Services (LBS) provide added value mainly by using the physical position of mobile users. Loca- tion data may consist of plain geographical coordinates, ac- cess point cell IDs, civil location in form of postal addresses or more abstract definitions like ‘in the office’, ‘at home’. Examples of services are a map showing the user’s current location or changing the handling of incoming calls when the user enters a specified area. Service enablers, defined to expose network functionality to external service providers, are becoming the cornerstones of modern service architectures defined by the Parlay Group, the Open Mobile Alliance (OMA) or in the IP Multimedia Subsystem (IMS). A Location Service Enabler is a functional entity in the network enabling value-added services to query the current position of a user or to request a trigger when a specified area is entered or left. Current standardization in the 3 rd Generation Partnership Project (3GPP) [18][19] concen- trates on Location Service Enablers that use core network components to provide location data of a user’s terminal. The starting point of our handset-based architecture is the observation that location information should be processed at its source, at the user terminal itself. For advanced notifica- tion-based location processing, we show that our distributed, handset-based architecture scales better, is more accurate, efficient and cost effective. Location information provides considerable value to in- formation and communication services. On the other hand, users are concerned about revealing their position data to oth- ers, especially to untrusted third party applications. Further- more, most countries have legal restrictions that regulate processing of personal data and the protection of privacy in electronic communications. It is of utmost importance that the users can control who gets access to their location data and that the transport in the network of such sensitive data is pro- tected by strong security mechanisms. The target of our service is the IP Multimedia Subsystem (IMS) [21], an overlay network on top of the UMTS packet service (GPRS). It has been specified by the 3GPP in the last years and is currently in the roll-out phase. The IMS makes heavy use of the Session Initiation Protocol (SIP) and its ex- tensions and defines several service enablers such as pres- ence, instant messaging, push to talk, etc. Whereas most UMTS applications can be realized without the IMS the specified mechanisms promise a uniform, standardized way of handling quality of service, charging, roaming and integration of different services. Our analysis of the IMS presence system shows that the re- quirements regarding access authorization, encryption and privacy for presence are indeed identical to those for location. The concept of presence was introduced in instant messaging systems. Presence is information about the online status of other users. A watcher (user) can subscribe to notifications about the state of another user. The watched user is called ‘presentity’. Classical presence information is defined as the willingness of the presentity to communicate. Presence and geographical location have semantic differ- ences: presence attributes, defined by the Internet Engineering Task Force’s (IETF) under “rich presence” (RPID, [4]) such as location type, activity, sphere, etc. can each take a small number of values, whereas geographical location has a con- tinuous range of values, limited only by the location accuracy. This requires different handling of subscriptions: whereas for presence all watchers subscribe to changes that occur to a subset of attributes, for location each watcher application may define different criteria for triggering events. Hence, publish- A Terminal-Based Location Service Enabler for the IP Multimedia Subsystem Rudolf Pailer Florian Wegscheider, Sandford Bessler mobilkom austria AG & Co KG Telecommunications Research Center Vienna (ftw.) Obere Donaustrasse 29, A-1020 Vienna, Austria Donau City Straße 1, A-1220 Vienna, Austria mailto:r.pailer@mobilkom.at mailto:{wegscheider, bessler}@ftw.at