Integrated Services Digital Network (ISDN) User Part (ISUP) to Session Initiation Protocol (SIP) Mapping

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RFC 3398                  ISUP to SIP Mapping              December 2002


7. SIP to ISUP Mapping

7.1 SIP to ISUP Call flows

   The following call flows illustrate the order of messages in typical
   success and error cases when setting up a call initiated from the SIP
   network.  "100 Trying" acknowledgements to INVITE requests are not
   displayed below although they are required in many architectures.

   In these diagrams, all call signaling (SIP, ISUP) is going to and
   from the MGC; media handling (e.g., audio cut-through, trunk freeing)
   is being performed by the MG, under the control of the MGC.  For the
   purpose of simplicity, these are shown as a single node, labeled
   "MGC/MG."

7.1.1 En-bloc Call Setup (no auto-answer)

       SIP                       MGC/MG                       PSTN
        1|---------INVITE---------->|                          |
         |<----------100------------|                          |
         |                          |------------IAM---------->|2
         |                          |<=========Audio===========|
         |                          |<-----------ACM-----------|3
        4|<----------18x------------|                          |
         |<=========Audio===========|                          |
         |                          |<-----------CPG-----------|5
        6|<----------18x------------|                          |
         |                          |<-----------ANM-----------|7
         |                          |<=========Audio==========>|
        8|<----------200------------|                          |
         |<=========Audio==========>|                          |
        9|-----------ACK----------->|                          |

   1.  When a SIP user wishes to begin a session with a PSTN user, the
       SIP node issues an INVITE request.

   2.  Upon receipt of an INVITE request, the gateway maps it to an IAM
       message and sends it to the ISUP network.

   3.  The remote ISUP node indicates that the address is sufficient to
       set up a call by sending back an ACM message.

   4.  The "called party status" code in the ACM message is mapped to a
       SIP provisional response (as described in Section 7.2.5 and
       Section 7.2.6) and returned to the SIP node.  This response may
       contain SDP to establish an early media stream (as shown in the
       diagram).  If no SDP is present, the audio will be established in
       both directions after step 8.



Camarillo, et. al.          Standards Track                    [Page 11]