4. When the TPS has allocated SIP bearer resources and received an
ISUP Address Complete Message (subscriber free) from the
terminating PSTN switch, it sends a SIP 180 RINGING
provisional response, which includes the following:
• SDP answer with bearer session description for TPS end of
IP bearer,
• SIP Allow header indicating support for these methods:
INVITE, ACK, BYE, CANCEL, UPDATE, INFO,
OPTIONS, PRACK,
• SIP Require header indicating that the call will use reliable
provisional responses (indicated by ″100rel″ parameter), and
• SIP Rseq header with a sequence number specific to this
provisional response. (The CSeq number for the INVITE
transaction is the same for all provisional and final
responses, so an additional sequence number is required to
uniquely identify one provisional response for reliable
transport.)
Because the OPS and TPS have agreed to require reliable
provisional responses, the TPS will retransmit the 180 RINGING
until it receives a PRACK message acknowledging receipt by the
OPS.
5. When the TPS receives a PRACK request containing a SIP RAck
header with the RSeq of the 180 RINGING, it stops
retransmitting the 180 RINGING message and sends a 200 OK
PRACK to acknowledge receipt of the PRACK. Because the
transport layer is UDP, the OPS will retransmit the PRACK until
it receives the 200 OK PRACK. If the TPS receives
retransmissions of the PRACK, it will respond with a
retransmission of the 200 OK PRACK.
Note: The TPS will not send any more provisional responses,
and will not accept any more requests from the OPS, until the
180/PRACK/200 OK PRACK exchange has been completed.
6. When the PSTN Gateway TPS receives an ISUP ANM from the
terminating PSTN switch, it sends a 200 OK INVITE. The TPS
retransmits the 200 OK INVITE until it receives an ACK from
the OPS. At this point, the call is in the talking state.
Message Flows
Call Flow
....................................................................................................................................................................................................................................
3-24
Lucent Technologies 235-200-118
Issue 3.02B, March 2007
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