Session Initiation Protocol - SIP

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    A telephone call on a VOIP,  Voice Over Internet Protocol,  System will rely on a SIP,  Session Initiation Protocol to establish the connection between the 2 or more, calling parties.



    A Basic Sip Session will look as follows, 

    The calling device (Desk phone, Computer, etc) send a Invite to the destination (person you are calling).

          If the device is available and able to accept a call (Answers the call) , it will send a 200 OK back.

    The calling device will send a ACK (Acknowledgement).  The  RTP  Media stream will then begin ( Voice i.e. your conversation and or Video) and   continue until the call is finished.

    At which time the parting hanging up first, will send a BYE.  The other party will send a 200 OK back.

    This will terminate the session.

    If a BYE is not received, or acknowledged with a 200 OK, even though both parties have hung up, their phones respectively. The session may remain on the Voice servers or PBX, consuming license or lines into the business.

    A call trace of this conversation would look like below, 

    Tom calls Joe

    Tom  ---> Invite  sip:[email protected]

    Joe    ---> to Tom   200 ok

    Tom --->    ACK   ---> to Joe

    Media stream begins     < -------> 

    Tom hangs up   ---- BYE  --->  sent to Joe

    Joe sends      200 OK ---> to Tom 

    Call is ended