[Users] using of openser

Hi, I would like to know the typical design to use openser in order to increase the performance. Since openser is a proxy server and it can be used together with another sip server to provide more function such as asterisk. As I have tested using sipp in a normal PC, only openser can handle about 60 concurrent calls. I have no idea where is the bottleneck. I have searched that someone will use openser with asterisk to provide a complete (nearly) solution in voip system. I want to have more information about that combination, such as the design, performance after integration, etc. Does anyone can give me some idea if you are using such combination? unplug