Volte call flow: Made simple

Dear Folks, today we will discuss about one of the basic things in Volte, which is Volte call flow.

The Normal Volte call flow is look like below:

We will discuss each step one by one:

Step 1: SIP Invite: Direction:- A Party---B Party
Calling party(A) informs IMS network and Called party(B) about New call. Session Description protocol(SDP) is used for carrying information such as Bandwidth and codec.

Step 2: 100 Trying: Direction:- B Party----A party
100 Trying is the response to the calling party to stop the retransmission of SIP invite. Every Node in IMS send the 100 trying message.

Step 3: 183 session in progress: Direction:- B Party----A Party
Called party informs about codec supported in response of SDP. Dedicated bearer(QCI=1) are created at both ends i.e. Calling end as well as called end.

Step 4: SIP PRACK(Provisional Response Acknowledgment):  Direction:-A Party----B Party
It is a provisional response acknowledgment to 183 session in progress message received. Calling party uses this PRACK to communicate final selected Voice codec.

Step 5: SIP 200 OK (PRACK): Direction:- B Party----A Party
Called party accepts final selected Voice codec by Calling party.

Step 6: SIP Update: Direction:- A Party----B Party
Calling party confirms the Resources to voice call.

Step 7: SIP 200 OK (Update):- Direction:- B Party----A Party
Called party confirms the resources to voice call.

Step 8: 180 Ringing:- Direction:- B Party----A Party
All pre conditions are satisfied now and called party alerts the Calling party about ringing of call.

Step 9: SIP 200 OK(INVITE): Direction:- B Party----A Party
Finally called party Answers the call.