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


11.2 Blocking messages

   There are two kinds of blocking messages: maintenance messages or
   hardware-failure messages.  Maintenance blocking messages indicate
   that the circuit is to be blocked for any subsequent calls, but these
   messages do not affect any ongoing call.  This allows circuits to be
   gradually quiesced and taken out of service for maintenance.

   Hardware-oriented blocking messages have to be treated as reset
   messages.  They generally are sent only when a hardware failure has
   occurred.  Media transmission for all calls in progress on these
   circuits would be affected by this hardware condition, and therefore
   all calls must be released immediately.

   BLO is always maintenance oriented and it is answered by the gateway
   with a Blocking ACK Message (BLA) when the circuit is blocked - this
   requires no corresponding SIP actions.  Circuit Group Blocking (CGB)
   messages have a "type indicator" inside the Circuit Group Supervision
   Message Type Indicator.  It indicates if the CGB is maintenance or
   hardware failure oriented.  If the CGB results from a hardware
   failure, then each call in progress in the affected range of circuits
   MUST be terminated immediately as if a REL had been received,
   following the procedures in Section 10.  CGBs MUST be answered with
   CGBAs.

11.3  Continuity Checks

   A continuity check is a test performed on a circuit that involves the
   reflection of a tone generated at the originating switch by a
   loopback at the destination switch.  Two variants of the continuity
   check appear in ISUP: the implicit continuity check request within an
   IAM (in which case the continuity check takes place as a precondition
   before call setup begins), and the explicit continuity check signaled
   by a Continuity Check Request (CCR) message.  PSTN gateways in
   regions that support continuity checking generally SHOULD have some
   way of accommodating these tests (if they hope to be fielded by
   providers that interconnect with any major carrier).

   When a CCR is received by a PSTN-SIP gateway, the gateway SHOULD NOT
   send any corresponding SIP messages; the scope of the continuity
   check applies only to the PSTN trunks, not to any IP media paths
   beyond the gateway.  CCR messages also do not designate any called
   party number, or any other way to determine what SIP user agent
   server should be reached.

   When an IAM with the Continuity Check Indicator flag set within the
   NCI parameter is received, the gateway MUST process the continuity
   check before sending an INVITE message (and proceeding normally with



Camarillo, et. al.          Standards Track                    [Page 53]