On Analysing Cost for Optimizing the Watcher Subscription Time in the IMS Presence Service

IMS (IP Multimedia Subsystem) is the technology that will merge the Internet (packet switching) with the cellular world (circuit switching). Presence is one of the basic services which is likely to become omnipresent in IMS (IP Multimedia Subsystem). It is the service that allows a user to be informed about the reachability, availability, and willingness of communication of another user. The flow of messages will be massive for large amount of publishers and watchers joining an IMS system, because of the security architecture of the IMS. Although the IETF (Internet Engineering Task Force) engineers have proposed several solutions to reduce the signalling overhead to facilitate the presence service, the heavy traffic flows have been compromised with several factors like real time view and information segregation etc. The life time of a watcher subscription has not received any attention so far. The constant time set (both short and long) may create bottleneck because of excessive message flow in the network. In this paper, we propose a mathematical model to analyse the system-performance of the IMS presence service during heavy traffic. The model derives the cost functions that are based on the real parameters of the Presence Server. An algorithm is demonstrated based on the derived model to set up an optimal life time for the Presence Server to assign to its joining IMS watchers during busy traffic. Simulation results have been shown that provide useful insight into the system behaviour. Keywords-SIP, IP Multimedia Subsystem, Presence Service, Cost, Optimal life time (keywords)