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


   3.  The INVITE message will continue to be sent to the SIP node each
       time the timer T1 expires.  The SIP standard specifies that
       INVITE transmission will be performed 7 times if no response is
       received.  Since SIP T1 starts at 1/2 second or more and doubles
       each time it is retransmitted, it will be at least a minute
       before SIP times out the INVITE request; since SIP T1 is allowed
       to be larger than 500 ms initially, it is possible that 7 x SIP
       T1 will be longer than ISUP T11 + ISUP T9.

   4.  When T11 expires, an ACM message will be sent to the ISUP node to
       prevent the call from being torn down by the remote node's ISUP
       T7.  This ACM contains a 'Called Party Status' value of 'no
       indication.'

   5.  When ISUP T9 in the remote PSTN node expires, it will send a REL.

   6.  Upon receipt of the REL, the gateway will send an RLC to
       acknowledge.

   7.  The REL will trigger a CANCEL request, which gets sent to the SIP
       node.

8.1.5 SIP Error Response

       SIP                       MGC/MG                       PSTN
         |                          |<-----------IAM-----------|1
         |                          |==========Audio==========>|
        2|<--------INVITE-----------|                          |
        3|-----------4xx+---------->|                          |
        4|<----------ACK------------|                          |
         |             ** MG Releases PSTN Trunk **            |
         |                          |------------REL---------->|5
         |                          |<-----------RLC-----------|6

   1.  When a PSTN user wishes to begin a session with a SIP user, the
       PSTN network generates an IAM message towards the gateway.

   2.  Upon receipt of the IAM message, the gateway generates an INVITE
       message, and sends it to an appropriate SIP node based on called
       number analysis.

   3.  The SIP node indicates an error condition by replying with a
       response with a code of 400 or greater.

   4.  The gateway sends an ACK message to acknowledge receipt of the
       INVITE final response.





Camarillo, et. al.          Standards Track                    [Page 35]