SIP TPS Call Setup
The following is a high-level view of call setup from the viewpoint of
a 5ESS
®
switch TPS:
1. The 5ESS
®
switch TPS receives a SIP INVITE from a far switch.
2. If the transport layer is UDP, the TPS immediately responds with
a ″100 TRYING″ message to inform the OPS that the INVITE
has been received, so that INVITE retransmissions may cease. If
retransmitted INVITEs are received, the TPS responds by
retransmitting the latest INVITE response sent.
3. The TPS extracts the call signaling data from the SIP headers
and/or the encapsulated ISUP IAM (if any) in the SIP INVITE,
according to the provisioned SIP-to-PSTN mapping rules.
4. The TPS selects and allocates an IP address and port on an
OFI-IP to be the bearer for the call.
5. The TPS uses the call signaling data to route the call to an
outgoing ISUP trunk. The call signaling data extracted from the
SIP message is included in an ISUP IAM to be sent the PSTN
switch at the far end of the ISUP trunk.
6. The TPS formats and sends a provisional response to the OPS.
The exact response, such as ″183 SESSION PROGRESS″ or
″180 RINGING″, depends on the nature of the call, the methods
and procedures supported by the OPS as indicated in the
incoming INVITE, and the methods and procedures supported by
the TPS, according to the SIP parameters provisioned for the
packet group over which the INVITE was received. The response
includes the bearer session information determined by the TPS,
and may or may not include an encapsulated ISUP response from
the switch at the far end of the ISUP trunk group in the PSTN,
depending on whether the received INVITE included
encapsulated ISUP.
7. There may, or may not, be additional SIP messages exchanged
between the OPS and TPS before the call is answered, depending
on the nature of the call and the provisioned options for SIP
methods and procedures allowed and/or supported at each end,
which are communicated and agreed upon between the OPS and
TPS by parameters in SIP headers in the INVITE/18X SIP
High-Level Call Flow
Call Flow
....................................................................................................................................................................................................................................
235-200-118
Issue 3.02B, March 2007
Lucent Technologies
3-11
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