Call Flow Overview
....................................................................................................................................................................................................................................
When the 5ESS
®
switch determines a call must be routed to another
switch, the call processing programs in the 5ESS
®
switch determine a
path to route the call. If the switch selects an IP network path, call
processing programs use Session Initiation Protocol (SIP) signaling to
establish the call. The switch converts incoming in-band or
out-of-band signaling messages to SIP messages so Public Switch
Telephone Network (PSTN) calls can traverse an IP network. In this
situation, the switch is functioning as an Originating Packet Switch
(OPS) at the boundary between the PSTN and the IP network.
At the far-end switch, the call is typically completed to a time
division multiplexing (TDM) circuits. When a call is completed to a
TDM circuit at the terminating 5ESS
®
switch, the appropriate PSTN
outgoing signaling message is formulated from the received SIP
message.
The 5ESS
®
switch can also terminate calls that arrive from an IP
network, in which case it converts incoming SIP signaling message to
in-band or out-of-band signaling in order to complete the call on a
circuit path in the PSTN. In this situation, the switch is functioning as
a Terminating Packet Switch (TPS) at the boundary between the
PSTN and the IP network.
Many call flows are possible, depending on the following factors:
• network architecture,
• function of the 5ESS
®
switch in the network,
• SIP protocol options supported by other SIP-enabled switches
which the 5ESS
®
switch connects via a SIP packet group, and the
function of those far-end switches in the PSTN and/or IP
network,
• transport layer used to carry the SIP messages in the IP network,
• provisioned mapping rules for signaling message conversion
between PSTN and SIP protocols for the packet group on the
5ESS
®
switch, and
• SIP protocol options provisioned for the packet group on the
5ESS
®
switch.
Call Flow
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3-2
Lucent Technologies 235-200-118
Issue 3.02B, March 2007
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