message exchange. The options that might lead to additional
messages being exchanged before the call is answered include
support for SIP preconditions procedures (allowed only when the
transport layer is SCTP), and support for reliable provisional
responses (PRACK).
8. If PRACK procedures are supported by both the OPS and the
TPS, the TPS will retransmit the 18X provisional response until a
PRACK is received to acknowledge it, or until the call is
answered. This retransmission of provisional responses is
performed by the SIP layer independent of transport layer, over
either SCTP or UDP. When PRACK procedures are enabled, the
TPS will not send any additional provisional responses, and will
not accept any new requests, until the PRACK is received to
acknowledge the last provisional response. If PRACK procedures
are not supported by both ends, the TPS will not retransmit the
18X response when the transport layer is SCTP, and will only
retransmit the 18X response as a result of receiving a
retransmitted INVITE request from the OPS when the transport
layer is UDP.
9. When an ISUP message is received indicating that the call has
been answered at the other end of the ISUP trunk in the PSTN,
the TPS formats and sends a 200 OK INVITE final response.
The TPS retransmits the final response, independent of transport
layer, until it receives an ACK from the OPS.
10. After the 200 OK INVITE is sent and the ACK is received, the
call proceeds to the ″talking″ state, and call setup is complete.
SIP Call Tear Down
This section describes tearing down of a SIP call, from a high-level
view.
Originating Packet Switch:
From the viewpoint of a 5ESS
®
switch OPS, when the calling party in
the PSTN goes on-hook:
1. The OPS receives an ISUP REL message from the switch at the
other end of the ISUP trunk in the PSTN.
2. The OPS formulates a SIP BYE messages and forwards it to the
TPS. The OPS also tears down the call and releases resources
dedicated to the call.
Terminating Packet Switch:
High-Level Call Flow
Call Flow
....................................................................................................................................................................................................................................
3-12
Lucent Technologies 235-200-118
Issue 3.02B, March 2007
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