An effective calibration of VOIP internet telephony performance using VPN between PAC and PNS

In this paper, we have created VoIP terminals that use virtual private network (VPN) to eliminate security risks due to the complexity of protocols involved and network equipments used. Point-to-point tunneling protocol (PPTP) is used on the session initiation protocol (SIP) stack, which is enabled by Internet telephony terminal to test its performance and feasibility. To measure the performance of call quality, we have built the experiment environment and observed that the delay caused by encapsulation between internet and VoIP telephones is under 8 ms at most. The major delay is assumed the time required to capsulate the packet for tunneling of VPN. Because the difference of average delay time is under 4 ms~5 ms, the difference of call quality between VoIP and VoIP telephone adopting VPN is negligible. We have concluded that the capsulation process between PPTP access concentrator (PAC) and PPTP network server (PNS) is the major factor influencing the network load by changing the number of fames in a packet during communication. Also, we have concluded that the most suitable frame numbers is two or three by adding the frame numbers in a packet to obtain the suitable frames in a packet and setting up end to end delay to be 160 ms~170 ms. We have suggested, developed, and evaluated an internet telephony system that implements secure user authentication and data transmission. Through measuring the delay time between a conventional VoIP telephone and the proposed system, the quality of call made through the proposed system and its´ feasibility are discussed.