SIP is short for Session Initiation Protocol, and refers to the application layer that handles communication between IP devices, be they VoIP handsets, cell phones are computers. It mimics much of the same functions that traditional phone lines and switches perform, but does it better in just about every way.
Simply put, this protocol connects and manages two or more devices communicating with each other. It is a middleman and only facilitates and maintains a session. To accomplish this, it provides several services. It first determines the location of the device making the call, and verifies that the party receiving the call is willing to connect to the caller.
Once this is established, SIP sets up the parameters and media channels required to complete the connection. Next, it rings the receiving party to answer once these parameters have been determined. Finally, and assuming the person or persons pick up, this protocol will modify the session parameters should anything change during the conversation, such as transferring or ending the communication.
To facilitate all of this, the system needs to operate through a set of servers all aimed at putting together the link. Let’s say Bob is trying to reach Jim’s VoIP handset with his cell phone. Bob punches in Jim’s number and attempts to make the call. While he’s waiting, a proxy server looks up Jim’s information on his behalf through a registrar server. This registrar houses a database that can be used to look up Jim’s device location. With this, the proxy contacts another proxy that signals Jim’s cell on Bob’s behalf. Once Jim picks up, the protocol will continue to monitor the link until it changes or the phone call ends.