High-Level Call Flow
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SIP OPS Call Setup
This section describes the steps for setting up a SIP call on a 5ESS
®
switch OPS from a high-level view. It presumes that the incoming
circuit call to be terminated over a SIP packet group is an ISUP trunk
call, although other circuit originations can terminate to SIP as well.
1. An ISUP IAM message from the far switch in the PSTN arrives
at the 5ESS
®
switch OPS. Call processing determines that the
incoming ISUP call should be routed out over the IP network to
another switch.
2. The OPS selects and allocates an IP address and port on an
OFI-IP to be the bearer for the call.
3. The OPS formulates a SIP INVITE message. The SIP INVITE
message requests that the TPS allocate resources for the call.
This message includes:
• the IP address of OPS signaling point,
• the IP address of TPS signaling point,
• a description of the requested bearer session, including the
IP address and port number for the OPS bearer resource,
• information about the SIP methods and procedures supported
by SIP at the OPS, and
• call information from the ISUP IAM, such as the called and
calling party information.
Note: The call information from the IAM is translated into
the SIP headers according to the PSTN-to-SIP mapping rules
provisioned for the selected SIP packet group, and the ISUP
IAM may be encapsulated in the SIP message or not,
according to the provisioning of the ISUP encapsulation
parameter for the packet group.
4. The OPS sends the SIP INVITE to the TPS, and waits for a
response. If the transport layer is UDP, the SIP layer at the OPS
retransmits the INVITE until a response is received. If the
transport layer is SCTP, which is a reliable, connection-oriented
protocol, the SIP layer does not retransmit the INVITE.
Call Flow
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235-200-118
Issue 3.02B, March 2007
Lucent Technologies
3-9
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