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


   Protocol error

   ISUP Cause value                        SIP response
   ----------------                        ------------
   102 recovery of timer expiry            504 Gateway timeout
   111 protocol error                      500 Server internal error

   Interworking

   ISUP Cause value                        SIP response
   ----------------                        ------------
   127 interworking unspecified            500 Server internal error

7.2.5 Early ACM received

   An ACM message is sent in certain situations to indicate that the
   call is in progress in order to satisfy ISUP timers, rather than to
   signify that the callee is being alerted.  This occurs for example in
   mobile networks, where roaming can delay call setup significantly.
   The early ACM is sent before the user is alerted to reset T7 and
   start T9.  An ACM is considered an 'early ACM' if the Called Party's
   Status Indicator is set to 00 (no indication).

   After sending an early ACM, the ISUP network can be expected to
   indicate the further progress of the call by sending CPGs.

   When an early ACM is received the gateway SHOULD send a 183 Session
   Progress response (see [1]) to the SIP network.  In SIP bridging
   situations (where encapsulated ISUP was contained in the INVITE that
   initiated this call) the early ACM SHOULD also be included in the
   response body.

   Note that sending 183 before a gateway has confirmation that the
   address is complete (ACM) creates known problems in SIP bridging
   cases, and it SHOULD NOT therefore be sent.

7.2.6 ACM received

   Most commonly, on receipt of an ACM a provisional response (in the
   18x class) SHOULD be sent to the SIP network.  If the INVITE that
   initiated this session contained legitimate and comprehensible
   encapsulated ISUP, then the ACM received by the gateway SHOULD be
   encapsulated in the provisional response.

   If the ACM contains a Backward Call Indicators parameter with a value
   of 'subscriber free', the gateway SHOULD send a '180 Ringing'
   response.  When a 180 is sent, it is assumed, in the absence of any
   early media extension, that any necessary ringback tones will be



Camarillo, et. al.          Standards Track                    [Page 27]