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


   When there is interworking with some legacy networks, it is possible
   for an ISUP switch to receive an ANM immediately after an early ACM
   (without CPG or any other backwards messaging), or without receiving
   any ACM at all (when an automaton answers the call).  In this
   situation the SIP user will never have received a 18x provisional
   response, and consequently they will not hear any kind of ringtone
   before the callee answers.  This may result in some clipping of the
   initial forward media from the caller (since forward media
   transmission cannot commence until SDP has been acquired from the
   destination).  In ISDN (see [12]) this is solved by connecting the
   voice path backwards before sending the IAM.

7.2.8 Timer T9 Expires

   The expiry of this timer (which is not used in all networks)
   signifies that an ANM has not arrived a significant period of time
   after alerting began (with the transmission of an ACM) for this call.
   Usually, this means that the callee's terminal has been alerted for
   many rings but has not been answered.  It may also occur in
   interworking cases when the network is playing a status announcement
   (such as one indicating that a number is not in service) that has
   cycled several times.  Whatever the cause of the protracted
   incomplete call, when this timer expires the call MUST be released.
   All of the gateway resources related to the media path SHOULD be
   released.  A '480 Temporarily Unavailable' response code SHOULD be
   sent to the SIP network, and an REL message with cause value 19 (no
   answer from the user) SHOULD be sent to the ISUP network.  The PSTN
   can be expected to respond with an RLC and the SIP network to respond
   with an ACK indicating that the release sequence has been completed.

7.2.9 CPG Received

   A CPG is a provisional message that can indicate progress, alerting
   or in-band information.  If a CPG suggests that in-band information
   is available, the gateway SHOULD begin to transmit early media and
   cut through the unidirectional backwards media path.















Camarillo, et. al.          Standards Track                    [Page 29]