Improvement Of Session Timer Mecahanism Over Wireless Environment

The lack of keep-alive mechanism in SIP has been confirmed a defect when implementing VoIP with wireless connection. Session Timer was proposed as compensation, which does this by sending UPDATE as refresh message periodically. The interval of two consequent messages is negotiated by request INVITE establishing this session.In this mechanism, the interval is a static value, when the status of wireless link degenerated; the performance of system is worsened. The main purpose of this paper is to propose a new mechanism suit the need of VoIP over wireless based on Session Timer.Our research focused on three parts displayed as blow:The first: analyze the defect of Session Timer, construct an analysis model. Analyze the relationship between system performances and refresh interval. And draw a conclusion: the time system found disconnection directly related to refresh interval.The second: propose a new mechanism based on Session Timer. Adjust refresh interval according to ratio of losing frame. So system can find disconnection of quickly when the status of connection degenerated. Additionally, we define the algorithm of when sending refresh request.The third: develop sip module in NS-2, simulate our mechanism based on it and analyze the simulate result.