Tech-invite3GPPspecsSIPRFCs
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Content for  TS 23.228  Word version:  16.6.0

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5.5  Serving-CSCF/MGCF to serving-CSCF/MGCF proceduresWord‑p. 106

5.5.0  General |R6|

This clause presents the detailed application level flows to define the procedures for Serving-CSCF/MGCF to Serving-CSCF/MGCF.
In the IM CN subsystem the MGCF is considered as a SIP endpoint. It translates ISUP/BICC messages of the PSTN side to SIP signalling of the IM CN subsystem side and vice-versa. It should also be noted that the MGCF does not invoke Service Control.
This clause contains four session flow procedures, showing variations on the signalling path between the Serving-CSCF that handles session origination, and the Serving-CSCF that handles session termination. This signalling path depends on:
  • whether the originator and destination are served by the same network operator,
  • whether the network operators have chosen to hide their internal configuration.
The Serving-CSCF handling session origination performs an analysis of the destination address, and determines whether it is a subscriber of the same network operator or a different operator.
If the analysis of the destination address determined that it belongs to a subscriber of a different operator, the request is forwarded) to a well-known entry point in the destination operator's network, the I-CSCF. The I-CSCF queries the HSS for current location information. The I-CSCF then forwards the request to the S-CSCF. If the analysis of the destination address determines that it belongs to a subscriber of the same operator, the S-CSCF passes the request to a local I-CSCF, who queries the HSS for current location information. The I-CSCF then forwards the request to the S-CSCF.
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5.5.1  (S-S#1) Different network operators performing origination and termination

The Serving-CSCF handling session origination performs an analysis of the destination address, and determines that it belongs to a subscriber of a different operator. The request is therefore forwarded to a well-known entry point in the destination operator's network, the I-CSCF. The I-CSCF queries the HSS for current location information, and finds the user either located in the home service area, or roaming. The I-CSCF therefore forwards the request to the S-CSCF serving the destination user.
Refer to Table 5.2 in clause 5.4a to see which origination sequences share this common S-S procedure. In addition the text below clarifies the role of the" Originating Network".
MO#1
Mobile origination, roaming. The "Originating Network" of S-S#1 is therefore a visited network.
MO#2
Mobile origination, home. The "Originating Network" of S-S#1 is therefore the home network.
PSTN-O
PSTN origination. The "Originating Network" of S-S#1 is the PSTN network. The elements of Figure 5.16 replace all elements of the Originating network and Originating Home Network in Figure 5.10.
AS-O
Application Server origination. The" Originating Network" of S-S#1 is the home network. The element labelled S-CSCF#1 corresponds to the S-SCSF in Figure 5.16b.
NI-O
Non-IMS network origination. The external SIP client of Figure 5.16b replaces all elements of the Originating network and Originating Home Network in Figure 5.10. There may be other non-IMS SIP servers on the path that are not shown.
Refer to Table 5.2 in clause 5.4a to see which termination sequences share this common S-S procedure. In addition the text below clarifies the role of the" Terminating Network".
MT#1
Mobile termination, roaming. The "Terminating Network" of S-S#1 is a visited network.
MT#2
Mobile termination, located in home service area. The "Terminating Network" of S-S#1 is the home network.
MT#3
Mobile termination, CS Domain roaming. The "Terminating Network" of S-S#1 is a CS domain network.
AS-T#1,2,3,4
Application Server termination. The elements of the corresponding AS-T termination Figure (5.7.5, 5.7.6, 5.7.7, and 5.7.8) replace all elements of the Terminating Home Network and Terminating Network off Figure 5.10.
NI-T
Non-IMS network terminations. The external SIP client of figure5.19a replaces all elements of the Terminating Home Network and Terminating Network in Figure 5.10. There may be other non-IMS SIP servers on the path that are not shown.
(not reproduced yet)
Figure 5.10: Serving to serving procedure - different operators
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Procedure S-S#1 is as follows:
Step 1.
The SIP INVITE request is sent from the UE to S-CSCF#1 by the procedures of the originating flow. This message should contain the initial media description offer in the SDP.
Step 2.
S-CSCF#1 invokes whatever service logic is appropriate for this session attempt.
Step 3.
S-CSCF#1 performs an analysis of the destination address, and determines the network operator to whom the subscriber belongs. For S-S#1, this flow is an inter-operator message to the I-CSCF entry point for the terminating user. S-CSCF#1 forwards the INVITE request directly to I-CSCF#2, the well-known entry point into the terminating user's network
Step 4.
I-CSCF#2 (at the border of the terminating user's network) shall query the HSS for current location information.
Step 5.
HSS responds with the address of the current Serving-CSCF for the terminating user.
Step 6.
I-CSCF#2 forwards the INVITE request to the S-CSCF (S-CSCF#2) that will handle the session termination.
Step 7.
S-CSCF#2 invokes whatever service logic is appropriate for this session setup attempt
Step 8.
The sequence continues with the message flows determined by the termination procedure.
Step 9.
The media stream capabilities of the destination are returned along the signalling path, as per the termination procedure.
Step 10.
S-CSCF#2 forwards the SDP to I-CSCF#2
Step 11.
I-CSCF#2 forwards the SDP to S-CSCF#1.
Step 12.
S-CSCF#1 forwards the SDP to the originator, as per the originating procedure.
Step 13.
The originator decides on the offered set of media streams, confirms receipt of the Offer Response with a Response Confirmation, and forwards this information to S-CSCF#1 by the origination procedures. The Response Confirmation may also contain SDP. This may be the same SDP as in the Offer Response received in Step 12 or a subset.
Step 14-15.
S-CSCF#1 forwards the offered SDP to S-CSCF#2.
Step 16.
S-CSCF#2 forwards the offered SDP to the terminating endpoint, as per the termination procedure
Step 17-20.
The terminating end point acknowledges the offer with answered SDP and passes through the session path to the originating end point.
Step 21-24.
Originating end point acknowledges successful resource reservation and the message is forwarded to the terminating end point.
Step 25-28.
Terminating end point acknowledges the response and this message is sent to the originating end point through the established session path.
Step 29-32.
Terminating end point then generates ringing and this message is sent to the originating end point through the established session path.
Step 33-36.
Terminating end point then sends 200 OK via the established session path to the originating end point.
Step 37-40.
Originating end point acknowledges the establishment of the session and sends to the terminating end point via the established session path.
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5.5.2  (S-S#2) Single network operator performing origination and terminationWord‑p. 108
The Serving-CSCF handling session origination performs an analysis of the destination address, and determines that it belongs to a subscriber of the same operator. The request is therefore forwarded to a local I-CSCF. The I-CSCF queries the HSS for current location information, and finds the user either located in the home service area, or roaming. The I-CSCF therefore forwards the request to the S-CSCF serving the destination user.
Refer to Table 5.2 in sub clause 5.4a to see which origination sequences share this common S-S procedure. In addition the text below clarifies the role of the" Originating Network".
MO#1
Mobile origination, roaming. The "Originating Network" of S-S#2 is therefore a visited network.
MO#2
Mobile origination, home. The "Originating Network" of S-S#2 is therefore the home network.
AS-O
Application Server origination. The" Originating Network" of S-S#1 is the home network. The element labelled S-CSCF#1 corresponds to the S-CSCF in Figure 5.16b.
Refer to Table 5.2 in clause 5.4a to see which termination sequences share this common S-S procedure. In addition the text below clarifies the role of the" Terminating Network".
MT#1
Mobile termination, roaming,. The "Terminating Network" of S-S#2 is a visited network.
MT#2
Mobile termination, home. The "Terminating Network" of S-S#2 is the home network.
MT#3
Mobile termination, CS Domain roaming. The "Terminating Network" of S-S#2 is a CS domain network.
AS-T#1,2,3,4
Application Server termination. The elements of the corresponding AS-T termination Figure (clause 5.7.5, clause 5.7.6, clause 5.7.7, and clause 5.7.8) replace all elements of the Terminating Home Network and Terminating Network off Figure 5.11.
(not reproduced yet)
Figure 5.11: Serving to serving procedure - same operator
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Procedure S-S#2 is as follows:
Step 1.
The SIP INVITE request is sent from the UE to S-CSCF#1 by the procedures of the originating flow. This message should contain the initial media description offer in the SDP.
Step 2.
S-CSCF#1 invokes whatever service logic is appropriate for this session setup attempt
Step 3.
S-CSCF#1 performs an analysis of the destination address, and determines the network operator to whom the subscriber belongs. Since it is local, the request is passed to a local I-CSCF.
Step 4.
I-CSCF shall query the HSS for current location information.
Step 5.
HSS responds with the address of the current Serving-CSCF for the terminating user.
Step 6.
I-CSCF forwards the INVITE request to the S-CSCF (S-CSCF#2) that will handle the session termination.
Step 7.
S-CSCF#2 invokes whatever service logic is appropriate for this session setup attempt
Step 8.
The sequence continues with the message flows determined by the termination procedure.
Step 9-12.
The terminating end point responds with an answer to the offered SDP and this message is passed along the established session path.
Step 13-16.
The originator decides on the offered set of media streams, confirms receipt of the Offer Response with a Response Confirmation, and forwards this information to S-CSCF#1 by the origination procedures. This message is forwarded via the established session path to the terminating end point. The Response Confirmation may also contain SDP. This may be the same SDP as in the Offer Response received in Step 12 or a subset.
Step 17-20.
Terminating end point responds to the offered SDP and the response if forwarded to the originating end point via the established session path.
Step 21-24.
Originating end point sends successful resource reservation information towards the terminating end point via the established session path.
Step 25-28.
Terminating end point sends successful resource reservation acknowledgement towards the originating end point via the established session path
Step 29-32.
Terminating end point sends ringing message toward the originating end point via the established session path.
Step 33-36.
The SIP final response, 200-OK, is sent by the terminating endpoint over the signalling path. This is typically generated when the user has accepted the incoming session setup attempt. The message is sent to S-CSCF#2 per the termination procedure.
Step 37-40.
The originating endpoint sends the final acknowledgement to S-CSCF#1 by the origination procedures and it is then sent over the signalling path to the terminating end point.
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5.5.3  (S-S#3) Session origination with PSTN termination in the same network as the S-CSCF.Word‑p. 111
The Serving-CSCF handling session origination performs an analysis of the destination address, and determines, with support of applications or other databases, that the session is destined to the PSTN. The request is therefore forwarded to a local BGCF. The BGCF determines that the MGCF should be in the same network, and selects a MGCF in that network. The request is then forwarded to the MGCF.
Refer to Table 5.2 in sub clause 5.4a to see which origination sequences share this common S-S procedure. In addition the text below clarifies the role of the" Originating Network".
MO#1
Mobile origination, roaming. The "Originating Network" of S-S#3 is therefore a visited network.
MO#2
Mobile origination, located in home service area. The "Originating Network" of S-S#3 is therefore the home network.
AS-O
Application Server origination. The" Originating Network" of S-S#1 is the home network. The element labelled S-CSCF corresponds to the S-CSCF in Figure 5.16b.
Refer to Table 5.2 in clause 5.4a to see which termination sequences share this common S-S procedure. In addition the text below clarifies the role of the "Terminating Network".
PSTN-T
PSTN termination. This occurs when the MGCF is selected to be in the same network as the S-CSCF.
(not reproduced yet)
Figure 5.12: Serving to PSTN procedure - same operator
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Procedure S-S#3 is as follows:
Step 1.
The SIP INVITE request is sent from the UE to S-CSCF#1 by the procedures of the originating flow. This message should contain the initial media description offer in the SDP.
Step 2.
S-CSCF#1 invokes whatever service logic is appropriate for this session setup attempt
Step 3.
S-CSCF#1 performs an analysis of the destination address. From the analysis of the destination address, S-CSCF#1 determines that this is for the PSTN, and passes the request to the BGCF.
Step 4.
The BGCF determines that the MGCF shall be in the same network, and hence proceeds to select an appropriate MGCF. The SIP INVITE request is forwarded to the MGCF. The PSTN terminating information flows are then followed.
Step 5-7.
The media stream capabilities of the destination are returned along the signalling path, as per the PSTN termination procedure.
Step 8.
The originator decides the offered set of media streams, confirms receipt of the Offer Response with a Response Confirmation, and forwards this information to S-CSCF#1 by the origination procedures. The Response Confirmation may also contain SDP. This may be the same SDP as in the Offer Response received in Step 7 or a subset.
Step 9-10.
S-CSCF#1 forwards the offered SDP to the terminating endpoint as per the PSTN terminating procedures via the established session path.
Step 11-13.
The terminating end point answers to the offered SDP and the message is passed through the established session path to the originating end point.
Step 14-16.
When the originating endpoint has completed the resource reservation procedures, it sends the successful resource reservation message to S-CSCF#1 by the origination procedures and it is passed to the terminating end point through the session path.
Step 17-19.
The terminating endpoint acknowledges the result and the message is passed onto the originating end point via the session path.
Step 20-22.
Terminating end point generates ringing message and forwards it to BGCF which in tern forwards the message to SCSCF#1. S-CSCF#1 forwards the ringing message to the originator, per the origination procedure
Step 23.
When the destination party answers, the termination procedure results in a SIP 200-OK final response to the BGCF
Step 24-25.
The BGCF forwards this information to the S-CSCF#1 and then it is forwarded to the originating end point.
Step 26.
The 200-OK is returned to the originating endpoint, by the origination procedure from terminating end point.
Step 27.
The originating endpoint sends the final acknowledgement to S-CSCF#1 by the origination procedures.
Step 28.
S-CSCF#1 forwards this message to the terminating endpoint as per the PSTN terminating procedures.
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5.5.4  (S-S#4) Session origination with PSTN termination in a different network from the S-CSCF.Word‑p. 113
The Serving-CSCF handling session origination performs an analysis of the destination address, and determines, with support of applications or other databases, that the session is destined to the PSTN. The request is therefore forwarded to a local BGCF. The BGCF determines that the PSTN interworking should occur in another network, and forwards this to a BGCF in the interworking network. The BGCF then selects a MGCF in that network. The request is then forwarded to the MGCF.
Refer to Table 5.2 in clause 5.4a to see which origination sequences share this common S-S procedure. In addition the text below clarifies the role of the" Terminating Network".
MO#1
Mobile origination, roaming. The "Originating Network" of S-S#4 is therefore a visited network.
MO#2
Mobile origination, located in home service area. The "Originating Network" of S-S#4 is therefore the home network.
AS-O
Application Server origination. The" Originating Network" of S-S#1 is the home network. The element labelled S-CSCF#1 corresponds to the S-CSCF in Figure 5.16b.
Refer to Table 5.2 in clause 5.4a to see which termination sequences share this common S-S procedure. In addition the text below clarifies the role of the" Terminating Network".
PSTN-T
PSTN termination. This occurs when the MGCF is selected to be in a different network than the S-CSCF.
(not reproduced yet)
Figure 5.13: Serving to PSTN procedure - different operator
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Procedure S-S#4 is as follows:
Step 1.
The SIP INVITE request is sent from the UE to S-CSCF#1 by the procedures of the originating flow. This message should contain the initial media description offer in the SDP.
Step 2.
S-CSCF#1 invokes whatever service logic is appropriate for this session setup attempt
Step 3.
S-CSCF#1 performs an analysis of the destination address. From the analysis of the destination address, S-CSCF#1 determines that this is for the PSTN, and passes the request to the BGCF#1.
Step 4.
The BGCF#1 determines that the PSTN interworking should occur in interworking network, and forwards the request on to BGCF#2.
Step 5.
BGCF#2 determines that the MGCF shall be in the same network, and hence proceeds to select an appropriate MGCF. The SIP INVITE request is forwarded to the MGCF. The PSTN terminating information flows are then followed.
Step 6-8.
The media stream capabilities of the destination are returned along the signalling path, as per the PSTN termination procedure.
Step 9.
S-CSCF#1 forwards the SDP to the originator, as per the originating procedure.
Step 10.
The originator decides the offered set of media streams, confirms receipt of the Offer Response with a Response Confirmation, and forwards this information to S-CSCF#1 by the origination procedures. The Response Confirmation may also contain SDP. This may be the same SDP as in the Offer Response received in Step 12 or a subset.
Step 11-13.
S-CSCF#1 forwards the offered SDP to the terminating endpoint, as per the PSTN terminating procedure.
Step 14-17.
Terminating end point responds to the offer via the established session path towards the originating end point.
Step 18-21.
When the originating endpoint has completed the resource reservation procedures, it sends the successful resource reservation message to S-CSCF#1 by the origination procedures and it is forwarded to the terminating end point via established session path.
Step 22-25.
The terminating end point responds to the message towards the originating end point.
Step 26-29.
Terminating end point generates ringing message towards the originating end point.
Step 30-33.
Terminating end point sends 200 OK when the destination party answers the session.
Step 34-37.
Originating end point acknowledges the establishment of the session.
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