With every communication client, there are a set of basic features that need to be supported to make the client user friendly. Normally, it is taken for granted that these features will come along with any kind of communication software or hardware that the end users purchase. These include call hold/resume, call transfer/forward, call screening, call ignore, mute, redial, and so on. We can integrate these either in the frontend logic, which is written in JSP/HTML, or towards the SIP Application Layer, which is written in the form of JAIN-SLEE or SIP Servlets.

An Application Server is used to introduce call-control logic in a normal call-flow scenario. The following diagram depicts the role of Application Server, managing the call-control logic as well as components such as Registrar and Proxy Servers:

A SIP request that is sent from a SIP or WebRTC agent makes its way to the SCSCF after passing through PCSCF. At SCSCF, it is sent to the Application Server...