5.4a Overview of session flow procedures

23.2283GPPIP Multimedia Subsystem (IMS)Release 18Stage 2TS

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5.4a.1 End-to-End session flow procedures

This clause contains the overview description and list of individual procedures for the end-to-end session flows.

For an IP Multi-Media Subsystem session, the session flow procedures are shown in the following diagram.

Figure 5.9: Overview of Session Flow Sections

The following procedures are defined:

For the origination sequences:

– (MO#1) Mobile origination, roaming , see clause 5.6.1;

– (MO#2) Mobile origination, home, see clause 5.6.2;

– (PSTN-O) PSTN origination, see clause 5.6.3;

– (NI-O) Non-IMS network origination (external SIP client), see clause 5.6.4;

– (AS-O) Application Server origination, see clause 5.6.5.

For the termination sequences:

– (MT#1) Mobile termination, roaming, see clause 5.7.1;

– (MT#2) Mobile termination, home, see clause 5.7.2;

– (MT#3) Mobile termination, CS Domain roaming, see clause 5.7.2a;

– (PSTN-T) PSTN termination, see clause 5.7.3;

– (NI-T) Non-IMS network termination (external SIP client), see clause 5.7.4;

– (AS-T#1) PSI based Application Server termination, direct, see clause 5.7.5;

– (AS-T#2) PSI based Application Server termination, indirect, see clause 5.7.6;

– (AS-T#3) PSI based Application Server termination, direct, using DNS, see clause 5.7.7;

– (AS-T#4) PUI based Application Server termination, indirect, see clause 5.7.8.

For Serving‑CSCF/MGCF-to-Serving‑CSCF/MGCF sequences:

– (S-S#1) Session origination and termination are served by different network operators, see clause 5.5.1;

– (S-S#2) Session origination and termination are served by the same operator, see clause 5.5.2;

– (S-S#3) Session origination with PSTN termination in the same network as the S‑CSCF, see clause 5.5.3;

– (S-S#4) Session origination with PSTN termination in a different network to the S‑CSCF, see clause 5.5.4.

The media being offered and acknowledged to can take multiple negotiation steps or only one negotiation may be used. In these flows, a minimum of two negotiations has been shown. But the subsequent responses may not carry any media information and just confirm the initial media set agreement.

For example, for a non-roaming user initiating a session to another non-roaming user, each a subscriber of the same network operator, it is possible to construct a complete end-to-end session flow from the following procedures:

– (MO#2) Mobile origination, home,

– (S-S#2) Single network operator,

– (MT#2) Mobile termination, home.

There are a large number of end-to-end session flows defined by these procedures. They are built from combinations of origination, serving to serving, and termination procedures, as determined from the following table. For each row of the table, any one of the listed origination procedures can be combined with any one of the serving-serving procedures, which can be combined with any one of the termination procedures.

Service control can occur at any point during a session, based on the filter criteria.

Note that the flows show service control only for the initial INVITE for originating and terminating party as an example.

The flows assume precondition mechanism is used, but as shown in clause 5.7a, a UE may originate a session without using preconditions.

Table 5.2: Combinations of session procedures

Origination Procedure (pick one)	Serving‑CSCF-to-Serving‑CSCF Procedure (pick one)	Termination Procedure (pick one)
MO#1 Mobile origination, roaming, home control of services (2). MO#2 Mobile origination, located in home service area. PSTN-O PSTN origination. AS-O Application Server origination NI-O Non-IMS network origination	S-S#1 Different network operators performing origination and termination, with home control of termination (2).	MT#1 Mobile termination, roaming, home control of services(2). MT#2 Mobile termination, located in home service area. MT#3 Mobile termination, CS Domain roaming. AS-T#1,2,3,4 Application Server terminations NI-T Non-IMS network termination
MO#1 Mobile origination, roaming, home control of services (2). MO#2 Mobile origination, located in home service area. AS-O Application Server origination	S-S#2 Single network operator performing origination and termination, with home control of termination.	MT#1 Mobile termination, roaming, home control of services(2). MT#2 Mobile termination, located in home service area. MT#3 Mobile termination, CS Domain roaming. AS-T#1,2,3,4 Application Server terminations
MO#1 Mobile origination, roaming, home control of services (2). MO#2 Mobile origination, located in home service area. AS-O Application Server origination	S-S#3 PSTN termination in the same network as the S‑CSCF.	PSTN-T PSTN termination.
MO#1 Mobile origination, roaming, home control of services (2). MO#2 Mobile origination, located in home service area. AS-O Application Server origination	S-S#4 PSTN termination in different network than the S‑CSCF	PSTN-T PSTN termination.

5.4a.2 Transit network session flow procedures

In addition to the combinations of flows constructed from the above scenarios, elements of an IMS network may be used by an operator in support of transit network scenarios. Figure 5.9a shows session flow combinations for transit network scenarios.

Figure 5.9a: Overview of Session Flow Sections for transit scenarios

Table 5.2a: Combinations of IMS transit network procedures

Origination Procedure

(pick one)

IMS Transit Network Procedure

Termination Procedure

(pick one)

MO#1 Mobile origination,
roaming, home control of
services (2).

MO#2 Mobile origination,
located in home service
area.

PSTN-O PSTN origination.

NI-O Non-IMS network
origination

I-T IMS Transit Network

MT#1 Mobile termination,
roaming, home control of
services(2).

MT#2 Mobile termination,
located in home service
area.

MT#3 Mobile termination, CS
Domain roaming.

PSTN-T PSTN termination.

NI-T Non-IMS network
termination