SIP (Session Initiation Protocol) Call Flow

Hi All,

        We have already discussed the basics of SIP in our last post. Here we would like to share the SIP call flow. Here we have also included PSTNs, so that the reader can co-relate the message of SIP and ISUP.

Scenario:
   
     A Number wants to call  B Number which is catered by PSTN B.

• PSTN A to MSS X protocol used in ISUP.
• MSS X to MSS Y protocol used in SIP.
• MSS Y to PSTN B protocol is ISUP.

1) After receiving IAM  from PSTN A with called party number as B number  MSS X after number analysis detects the B number has to be routed to MSS Y which is connected by SIP. MSS X send a INVITE message.
Major components of INVITE are "Called Party", "Calling Party", "Bearer information", "Codec".

2) MSS Y receives INVITE and responds with 100 (Trying) message.
This response indicates that the request has been received by the next-hop server and that some unspecified action is being taken on behalf of this call (for example, a database is being consulted). At the same time after number analysis its send IAM to PSTN B.

3) MSS Y send 183 message to MSS X. This message is called as Session progress indicated that session is in progress. In response to 183 MSS A sends PRACK. PRACK (PRovisional ACKnowledgement) is like any another request within a dialog.
** PRACK is response for 1XX mesages
    ACK is response for 2XX messages.

4) After analysis of B number PSTN B sends ACM with "Called Party status indicator = no indication" to MSS Y. MSS Y sends 200 message to MSS X in turn MSS X forwards ACM message to PSTN A.

5) When B number starts ringing PSTN B send CPG message with "Called Party status indicator = Subscriber free". Indicates that subscriber is free and ringing.
MSS Y send 180 ringing message to MSS X.

Which is communicated to PSTN A in CPG message and A number can hear a ringtone.

6) B number answers the call in response PSTN B send ANM message to MSS Y. MSS Y send 200(With ANM) message to MSS X. MSS X responds with ACK , also forwards ANM message to PSTN A. At this point of time speech path is through.

7) After conversation A number releases the call. REL is send from PSTN A to MSS X in response MSS X sends BYE message to MSS Y. MSS Y send the REL message to PSTN B.

8) PSTN B release the resources and respond with RLC message to MSS Y. MSS Y relays the 200(with RLC) message to MSS X. RLC is then forwarded to PSTN A. This complete the release of all resources used for call.

Comments and Appreciations are most welcome.

ChEEeeeEEeeeErs!!!
Telecom Tigers Team
telecomtigers@gmail.com
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