Presence and event notification in UMTS IP multimedia subsystem

Presence service is defined as a service of the IP multimedia subsystem (IMS) in the current UMTS specification. It is responsible for distribution of presence information not only of a user, but also of a service, supporting notifications after different kinds of asynchronous events. This paper discusses presence service in IMS with the particular focus on its subscription mechanism. According to this mechanism, subscriptions are time limited. Users have to periodically subscribe for presence information in order to stay subscribed over a longer time. This behaviour periodically generates signalling traffic on the radio interface that is the most expensive one in the UMTS architecture. This paper proposes a mechanism that optimizes this traffic significantly. It introduces a hard state subscription mechanism over the radio interface moving the soft state subscription mechanism in the core network.