9.2.3 Basic CS network originated session

29.1633GPPInterworking between the IP Multimedia (IM) Core Network (CN) subsystem and Circuit Switched (CS) networksTS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

9.2.3.1 BICC forward bearer establishment

9.2.3.1.1 IM-MGW selection

The MGCF shall select an IM-MGW for the bearer connection before it performs the IM CN subsystem session establishment or the CS network side bearer establishment.

9.2.3.1.2 IM CN subsystem side termination reservation

The MGCF shall derive from configuration data one or several appropriate local codec(s) the IM-MGW may use to receive user plane data from the IM CN subsystem. The MGCF shall use the Reserve IMS Connection Point procedure (signals 2 and 3 in figure 37). Within this procedure, the MGCF shall indicate the local codec(s) and request a local IP address and UDP port from the IM-MGW. The local IP address and UDP port are used by the IM-MGW to receive user plane data from the IM CN subsystem. If DTMF support together with speech support is required, or if the resources for multiple speech codecs shall be reserved at this stage, the reserve value indicator shall be set to "true".

The MGCF may also indicate that the IP interface type is for MboIP.

The IM-MGW shall reply to the MGCF with the selected local codec(s) and the selected local IP address and UDP port.

The MGCF shall send this information in the INVITE (signal 4 in figure 37) to the IM CN subsystem.

9.2.3.1.3 IM CN subsystem side session establishment

The MGCF shall use the Configure IMS Resources procedure (signals 7 and 8 or 23a and 23b in figure 37) to provide configuration data (derived from SDP received in signal 6 in figure 37 and local configuration data) to the IM-MGW as detailed below:

– The MGCF shall indicate the remote IP address and UDP port, i.e. the destination IP address and UDP port for data sent in the user plane towards the IM CN subsystem,

– The MGCF shall indicate the remote codec(s), i.e. the speech codec(s) for data sent in the user plane towards the IM CN subsystem.

– The MGCF may indicate the local codec(s) and the local IP address and UDP port. The MGCF shall indicate the local codec(s) if a change is required.

– IF DTMF support together with speech support is required, the reserve value indicator shall be set to "true".

– The MGCF may indicate that IP interface type is for MboIP.

The IM-MGW shall reply with the selected remote codec(s) and reserve resources for these codec(s). If local codec(s) were received, the IM-MGW shall also reply with the selected local codec(s) and reserve the corresponding resources.

If the selected local codec(s) differ from the codec(s) received in the SDP of signal 6 in figure 37 (if any), the MGCF shall send the local reserved codec(s), and the local IP address and UDP port in the PRACK (signal 9 in figure 37) to the IMS.

If the selected local codec(s) differ from the codec(s) received in the SDP of signal 23 in figure 37 (if any), the MGCF shall send the local reserved codec(s), and the local IP address and UDP port in an re-INVITE or UPDATE (not depicted in figure 37) to the IMS.

9.2.3.1.4 CS network side bearer establishment

The MGCF shall request the IM-MGW to prepare for the CS network side bearer establishment using the Prepare Bearer procedure (signals 11 and 12 in figure 37). Within this procedure, the MGCF shall request the IM-MGW to provide a bearer address, a binding reference and optionally notify when the bearer is established. The MGCF shall also provide the IM-MGW with the bearer characteristics that was received from the preceding node in the IAM. After the IM-MGW has replied with the bearer address and the binding reference, the MGCF provides the APM message (signals 13 in figure 37) to the preceding node. The MGCF may also provide the IM-MGW-id in the APM message.

9.2.3.1.5 Called party alerting

The MGCF shall request the IM-MGW to provide an awaiting answer indication (ringing tone) to the calling party using the Send Tone procedure (signals 20 and 21 in figure 37) , when the following condition is satisfied:

– the MGCF receives the first 180 Ringing message, unless this message or some previous SIP provisional response contained a P-Early-Media header field that authorizes early media.

9.2.3.1.6 Called party answer

When the MGCF receives a 200 OK message (signal 23 in figure 34), it shall request the IM-MGW to stop providing the ringing tone to the calling party using the Stop Tone procedure (signals 26 and 27 in figure 37).

9.2.3.1.7 Through-Connection

During the Prepare Bearer procedure, the MGCF shall either use the Change Through-Connection procedure to request the IM-MGW to backward through-connect the BICC termination, or the MGCF shall use this procedure to both-way through-connect the BICC termination already on this stage (signals 11 and 12 in figure 37). During the Reserve IMS Connection Point procedure, the MGCF shall use the Change IMS Through-Connection procedure to request the IM-MGW to backward through-connect the IMS termination (signals 2 and 3 in figure 37).

When the MGCF receives the SIP 200 OK(INVITE) (signal 23 in figure 37), it requests the IM-MGW to both-way through-connect the terminations using the Change IMS Through-Connection or Change Through-Connection procedures (signals 28 and 29 in figure 37), unless those terminations are already both-way through-connected.

9.2.3.1.8 Codec handling

The IM-MGW may include a speech transcoder based upon the speech coding information provided to each termination.

9.2.3.1.9 Failure handling in MGCF

If any procedure between the MGCF and the IM-MGW is not completed successfully, the default action by the MGCF is to release the session as described in clause 9.2.6. If the MGCF receives a Bearer Released procedure from the IM-MGW the default action by the MGCF is to release the session, as described in clause 9.2.7.

NOTE: As an implementation option the MGCF may also decide for example to only release the resources in the IM-MGW that caused the failure, possibly select a new IM-MGW for the connection and continue the call establishment using new resources in the selected IM-MGW but such handling is outside of the scope of the present document.

9.2.3.1.10 Message sequence chart

Figure 37 shows the message sequence chart for the CS network originating session with BICC forward bearer establishment. In the chart the MGCF requests the seizure of the IM CN subsystem side termination and CS network side bearer termination. When the MGCF receives an answer indication, it requests the IM-MGW to both-way through-connect the terminations. Dashed lines represent optional or conditional messages.

Figure 37/1: Basic CS Network Originating Session, Forward Bearer Establishment (message sequence chart)

Figure 37/2: Basic CS Network Originating Session, Forward Bearer Establishment
(message sequence chart continue)

9.2.3.2 BICC Backward bearer establishment

9.2.3.2.1 IM-MGW selection

The MGCF shall select an IM-MGW for the bearer connection before it performs the IM CN subsystem session establishment or the CS network side bearer establishment.

9.2.3.2.2 CS network side bearer establishment

The MGCF shall request the IM-MGW to establish a bearer using the Establish Bearer procedure (signals 2 and 3 in figure 38). The MGCF provides the IM-MGW with the bearer address, the binding reference and the bearer characteristics that were received from the preceding node in the IAM (signal 1 in figure 38).

9.2.3.2.3 IM CN subsystem side termination reservation

The MGCF shall derive from configuration data one or several appropriate local codec(s) the IM-MGW may use to receive user plane data from the IM CN subsystem. The MGCF shall use the Reserve IMS Connection Point procedure (signals 2 and 3 in figure 38). Within this procedure, the MGCF shall indicate the local codec(s) and request a local IP address and UDP port from the IM-MGW. The local IP address and UDP port are used by the IM-MGW to receive user plane data from the IM CN subsystem. If DTMF support together with speech support is required, or if the resources for multiple speech codecs shall be reserved at this stage, the reserve value indicator shall be set to "true".

The MGCF may also indicate that the IP interface type is for MboIP.

The IM-MGW shall reply to the MGCF with the selected local codec(s) and the selected local IP address and UDP port.

The MGCF shall send this information in the INVITE (signal 6 in figure 38) to the IM CN subsystem.

9.2.3.2.4 IM CN subsystem side session establishment

The MGCF shall use the Configure IMS Resources procedure (signals 9 and 10 or 22a and 22b in figure 38) to provide configuration data (derived from SDP received in signal 8 in figure 38 and local configuration data) to the IM-MGW as detailed below:

– The MGCF shall indicate the remote IP address and UDP port, i.e. the destination IP address and UDP port for data sent in the user plane towards the IM CN subsystem

– The MGCF shall indicate the remote codec(s), i.e. the speech codec(s) for data sent in the user plane towards the IM CN subsystem.

– The MGCF may indicate the local codec(s) and the local IP address and UDP port. The MGCF shall indicate the local codec(s) if a change is required.

– If DTMF support together with speech support is required, the reserve value indicator shall be set to "true".

– The MGCF may indicate that IP interface type is for MboIP.

The IM-MGW shall reply with the selected remote codec(s) and reserve resources for this codec. If local codec(s) were received, the IM-MGW shall also reply with the selected local codec(s) and reserve the corresponding resources.

If the selected local codec(s) differ from the codec(s) received in the SDP of signal 8 in figure 38 (if any), the MGCF shall send the reserved speech codec(s), and the local IP address and UDP port in the PRACK (signal 11 in figure 38) to the IMS.

If the selected local codec(s) differ from the codec(s) received in the SDP of signal 22 in figure 38 (if any), the MGCF shall send the local reserved codec(s), and the local IP address and UDP port in an re-INVITE or UPDATE (not depicted in figure 38) to the IMS.

9.2.3.2.5 Called party alerting

The MGCF shall request the IM-MGW to provide an awaiting answer indication (ringing tone) to the calling party using the Send Tone procedure (signals 19 and 20 in figure 38) , when the following conditions is satisfied:

– the MGCF receives the first 180 Ringing message, unless this message or some previous SIP provisional response contained a P-Early-Media header field that authorizes early media.

9.2.3.2.6 Called party answer

When the MGCF receives a 200 OK message (signal 22 in figure 38), it shall request the IM-MGW to stop providing the ringing tone to the calling party using the Stop Tone procedure (signals 23 and 24 in figure 38).

9.2.3.2.7 Through-Connection

During the Establish Bearer procedure, the MGCF shall either use the Change Through-Connection procedure to request the IM-MGW to backward through-connect the BICC termination, or the MGCF shall use this procedure to both-way through-connect the BICC termination already on this stage (signals 2 and 3 in figure 38). During the Reserve IMS Connection Point procedure, the MGCF shall use the Change IMS Through-Connection procedure to request the IM-MGW to backward through-connect the IMS termination (signals 4 and 5 in figure 38).

When the MGCF receives the SIP 200 OK(INVITE) (signal 22 in figure 38), it shall request the IM-MGW to both-way through-connect the bearer using the Change IMS Through-Connection or Change Through-Connection procedure (signals 25 and 26 in figure 38), unless those terminations are already both-way through-connected.

9.2.3.2.8 Codec handling

The IM-MGW may include a speech transcoder based upon the speech coding information provided to each termination.

9.2.3.2.9 Failure handling in MGCF

9.2.3.2.10 Message sequence chart

Figure 38 shows the message sequence chart for the CS network originating session with BICC backward bearer establishment. In the chart the MGCF requests seizure of the IM CN subsystem side termination and CS network side bearer termination. When the MGCF receives an answer indication, it requests the IM-MGW to both-way through-connect the terminations. Dashed lines represent optional or conditional messages.

Figure 38/1: Basic CS Network Originating Session, BICC Backward Bearer Establishment (message sequence chart)

Figure 38/2: Basic CS Network Originating Session, BICC Backward Bearer Establishment
(message sequence chart continue)

9.2.3.3 ISUP

9.2.3.3.1 IM-MGW selection

The MGCF selects the IM-MGW based on the received circuit identity in the IAM.

9.2.3.3.2 CS network side circuit reservation

The MGCF shall request the IM-MGW to reserve a circuit using the Reserve TDM Circuit procedure.

9.2.3.3.3 IM CN subsystem side termination reservation

The MGCF shall derive from configuration data one or several appropriate local codec(s) the IM-MGW may use to receive user plane data from the IM CN subsystem. The MGCF shall use the Reserve IMS Connection Point procedure (signals 2 and 3 in figure 39). Within this procedure, the MGCF shall indicate the local codec(s) and request a local IP address and UDP port from the IM-MGW. The local IP address and UDP port are used by the IM-MGW to receive user plane data from the IM CN subsystem. If DTMF support together with speech support is required, or if the resources for multiple speech codecs shall be reserved at this stage, the reserve value indicator shall be set to "true".

The MGCF may also indicate that the IP interface type is for MboIP.

The IM-MGW shall reply to the MGCF with the selected local codec(s) and the selected local IP address and UDP port.

The MGCF shall send this information in the INVITE (signal 6 in figure 39) to the IM CN subsystem.

9.2.3.3.4 IM CN subsystem side session establishment

The MGCF shall use the Configure IMS Resources procedure (signals 9 and 10 or 22a and 22b in figure 39) to provide configuration data (derived from SDP received in signal 8 in figure 39 and local configuration data) as detailed below:

– The MGCF shall indicate the remote IP address and UDP port, i.e. the destination IP address and UDP port for data sent in the user plane towards the IM CN subsystem.

– The MGCF shall indicate the remote codec(s), i.e. the speech codec(s) for data sent in the user plane towards the IM CN subsystem.

– The MGCF may indicate the local codec(s) and the local IP address and UDP port. The MGCF shall indicate the local codec(s) if a change is required.

– If DTMF support together with speech support is required, the reserve value indicator shall be set to "true".

– The MGCF may indicate that IP interface type is for MboIP.

If the selected local codec(s) differ from the codec(s) received in the SDP of signal 8 in figure 39 (if any), the MGCF shall send the reserved speech codec(s), and the local IP address and UDP port in the PRACK (signal 11 in figure 39) to the IMS.

If the selected local codec(s) differ from the codec(s) received in the SDP of signal 22 in figure 39 (if any), the MGCF shall send the local reserved codec(s), and the local IP address and UDP port in an re-INVITE or UPDATE (not depicted in figure 39) to the IMS.

9.2.3.3.5 Called party alerting

The MGCF shall request the IM-MGW to provide an awaiting answer indication (ringing tone) to the calling party using the Send TDM Tone procedure (signals 19 and 20in figure 39), when the following condition is satisfied:

– the MGCF receives the first 180 Ringing message, unless this message or some previous SIP provisional response contained a P-Early-Media header field that authorizes early media.

9.2.3.3.6 Called party answer

When the MGCF receives a 200 OK message (signal 22 in figure 39), it shall request the IM-MGW to stop providing the ringing tone to the calling party using the Stop TDM Tone procedure (signals 23 and 24 in figure 39).

9.2.3.3.7 Through-Connection

Within the Reserve TDM Circuit procedure, the MGCF shall either use the Change TDM Through-Connection procedure to request the IM-MGW to backward through-connect the TDM termination, or the MGCF shall use this procedure to both-way through-connect the TDM termination already on this stage (signals 2 and 3 in figure 39). During the Reserve IMS Connection Point procedure, the MGCF shall use the Change IMS Through-Connection procedure to request the IM-MGW to backward through-connect the IMS termination (signals 4 and 5 in figure 39).

When the MGCF receives the SIP 200 OK(INVITE) message, it shall request the IM-MGW to both-way through-connect the terminations using the Change IMS Through-Connection or Change TDM Through-Connection procedure (signals 25 and 26 in figure 39), unless those terminations are already both-way through-connected.

9.2.3.3.8 Continuity Check

If a continuity check on the connection towards the CS network is requested in the IAM message, the MGCF shall use the Continuity Check Response procedure towards the IM-MGW to request loop-back of a received continuity check tone on the TDM circuit. Upon reception of the COT message, the MGCF shall use the Continuity Check Response procedure towards the IM-MGW to request the removal of the loop-back. (Not depicted in figure 39)

9.2.3.3.9 Codec handling

The IM-MGW may include a speech transcoder based upon the speech coding information provided to each termination.

9.2.3.3.10 Voice Processing function

A voice processing function located on the IM-MGW may be used to achieve desired acoustic quality on the terminations. If the voice processing function is used, the MGCF shall request the activation of it in the termination towards the CS network using the Activate TDM Voice Processing Function procedure (signal 23 in figure 39).

9.2.3.3.11 Failure handling in MGCF

If any procedure between the MGCF and the IM-MGW is not completed successfully, the session shall be released as described in clause 9.2.6. If the MGCF receives a Bearer Released procedure from the IM-MGW the default action by the MGCF is to release the session as described in clause 9.2.7.

9.2.3.3.12 Message sequence chart

Figure 39 shows the message sequence chart for the CS network originating Session with ISUP. In the chart the MGCF requests seizure of the IM CN subsystem side termination and CS network side bearer termination. When the MGCF receives an answer indication, it requests the IM-MGW to both-way through-connect the terminations. The MGCF may request the possible activation of the voice processing functions for the terminations. Dashed lines represent optional or conditional messages.

Figure 39/1: Basic CS Network Originating Session, ISUP (message sequence chart)

Figure 39/2: Basic CS Network Originating Session, ISUP (message sequence chart continue)

9.2.3.4 Handling of Forking

The procedures described in clauses 9.2.3.1 to 9.2.3.3 shall be applied with the following additions.

9.2.3.4.1 Detection of Forking

According to SIP procedures, the O-MGCF inspects the tags in the "to" SIP header fields of provisional and final responses to identify the SIP dialogue the response belongs to. If responses belonging to different dialogues are received (signals 8 and 13 in figure 39a), the INVITE request (signal 6 in figure 39a) has been forked.

9.2.3.4.2 IM CN subsystem side session establishment

If one or more SIP early dialog exists and:

a) if the O-MGCF has not already received a P-Early-Media header field that authorizes backward early media (the received em-param(s) in the P-Early-Media header field includes "sendrecv" or "sendonly") on any of existing SIP early dialog, then upon reception of a provisional 18x response with an SDP answer creating a new early dialog (signal 13 in figure 39a/1) and:

1) with the P-Early-Media header field that authorizes backward early media (the received em-param(s) in the P-Early-Media header field includes "sendrecv" or "sendonly"), the O-MGCF shall use the Configure IMS Resources procedure (signals 14 and 15 in figure 39a/1) and shall provide to the IM-MGW:

– the remote IP address and UDP port, and the remote codec selected from the received SDP answer.

The IM-MGW may be configured to use the remote IP address and port information as source filter for incoming packets to prevent that backward early media from other early SIP dialogs interfere. The O-MGCF may also provide an IP address and port source filter that disallows backward early media from other early dialogs without an early media authorization in order to prevent that such unauthorized backward early media interfere with the authorized backward early media. (NOTE 1); and

– local configuration data, corresponding to the SIP early dialog of the SIP provisional response containing a P-Early-Media header field that authorizes backward early media; or

2) without the P-Early-Media header field or with the P-Early-Media header field that does not authorize backward early media (the received em-param(s) in the P-Early-Media header field includes "inactive" or "recvonly"), the O-MGCF may either:

– refrain from reconfiguring the IM-MGW; or

– use the Configure IMS Resources procedure (signals 14 and 15 in figure 39a/1) as detailed below:

i) the O-MGCF may compare the selected local codecs of the different dialogs (which the O-MGCF selects due to the received SDP answer and local configuration data). If different local codecs are selected for the different dialogs, the O-MGCF may include all these codecs in the "local IMS resources", and set the "reserve value" to indicate that resources for all these codecs shall be reserved. Alternatively, the O-MGCF may only include the codecs received in the last received SDP in the "local IMS resources"; and

ii) the O-MGCF may update the "remote IMS resources" with the information received in the latest SDP answer. The MGCF should provide the remote IP address and UDP port, and the remote codec selected from the received SDP answer and local configuration data. (NOTE 2); or

b) if the O-MGCF has already received a P-Early-Media header field that authorizes backward early media (the received em-param(s) in the P-Early-Media header field includes "sendrecv" or "sendonly") on any of existing early dialog, then upon reception of a provisional 18x response with an SDP answer creating a new early dialog (signal 13 in figure 39a/1) and:

1) with the P-Early-Media header field that authorizes backward early media (the received em-param(s) in the P-Early-Media header field includes "sendrecv" or "sendonly"), the O-MGCF may either:

– refrain from reconfiguring the IM-MGW; or

– use the Configure IMS Resources procedure (signals 14 and 15 in figure 39a/1) to provide to the IM-MGW the remote IP address and UDP port, and the remote codec selected from the received SDP answer and local configuration data, corresponding to the SIP early dialog of the SIP provisional response containing the P-Early-Media header field that authorizes backward early media. The IM-MGW may be configured to use the remote IP address and port information as source filter for incoming packets to prevent that backward early media from another early SIP dialog interfere. The O-MGCF may also provide an IP address and port source filter that disallows backward early media from other early dialogs without an early media authorization in order to prevent that such unauthorized backward early media interfere with the authorized backward early media. (NOTE 1, NOTE 2); or

NOTE 1: The O-MGCF can use the P-Early-Media header field (IETF RFC 5009 [89]) to determine whether the media associated with a forked dialog is authorized and thus eligible for a through connection. In the presence of backward early media for multiple dialogs due to forking, if the IM-MGW is able to identify the media associated with a dialog (i.e., if symmetric RTP is used by the peer and the IM-MGW can use the remote SDP information to determine the source of the media), then the O-MGCF/IM-MGW can selectively establish a through-connection for an authorized backward early media flow.

NOTE 2: This behaviour is beneficial if forking is applied in a sequential manner.

If the O-MGCF has not already received a P-Early-Media header field that authorizes backward early media (the received em-param(s) in the P-Early-Media header field includes "sendrecv" or "sendonly") on any of existing SIP early dialog, then:

a) upon reception of a SIP message with the P-Early-Media header field that authorizes backward early media (the received em-param(s) in the P-Early-Media header field includes "sendrecv" or "sendonly") on existing early dialog; and

NOTE 3: For the existing early dialog the P-Early-Media header field can be received within the UPDATE requests, 2xx responses to the PRACK and the UPDATE requests, and in the subsequent 18x provisional responses (as specified in IETF RFC 5009 [89]). Subsequent 18x provisional response is every 18x provisional response except the first 18x provisional response, received within the one early dialog. Reception of the first 18x provisional response which creates a new early dialog is described in at the beginning of this subclause).

b) if the O-MGCF has received an SDP body on the early dialog on which the SIP message with the P-Early-Media header field that authorizes backward early media was received,

the O-MGCF shall provide to the IM-MGW the remote IP address and UDP port, and the remote codec selected from the latest received SDP body corresponding to the early dialog on which the SIP message with the P-Early-Media header field that authorizes backward early media was received.

Upon reception of:

a) a SIP message with the P-Early-Media header field that removes authorization of backward early media (the received em-param(s) in the P-Early-Media header field includes "inactive" or "recvonly") on the early dialog for which:

1) the P-Early-Media header field authorizing backward early media was previously received (the received em-param(s) in the P-Early-Media header field includes "sendrecv" or "sendonly"); and

2) the O-MGCF provided to the IM-MGW the remote IP address and UDP port, and the remote codec selected from the latest received SDP body corresponding to this early dialog and this remote IP address and UDP port, are the most recently sent the remote IP address and UDP port to the IM-MGW; or

b) a 199 Early Dialog Terminated provisional response corresponding to the SIP early dialog for which:

1) the P-Early-Media header field authorizing backward early media was previously received (the received em-param(s) in the P-Early-Media header field includes "sendrecv" or "sendonly"); and

the O-MGCF shall check if the P-Early-Media header field that authorizes backward early media (the received em-param(s) in the P-Early-Media header field includes "sendrecv" or "sendonly") was received on any other early dialog and that the authorization of backward early media on that early dialog has not changed. If such early dialog:

a) exists the O-MGCF shall provide to the IM-MGW using the Configure IMS Resources procedure, the remote IP address and UDP port, and the remote codec selected from the latest received SDP body on that early dialog. If there is more than one such early dialog, the O-MGCF shall select the early dialog where the P-Early-Media header field was most recently received; or

b) does not exist and if:

1) the O-MGCF has already received the 180 Ringing provisional response on any of existing SIP early dialog; or

2) the received SIP message is the 180 Ringing provisional response,

the O-MGCF shall request the IM-MGW to provide an awaiting answer indication (ringing tone) to the calling party using the Send Tone procedure (signals 27 and 28 in figure 39a/2).

9.2.3.4.3 IM CN subsystem side session establishment completion

Upon reception of the first final 2xx response (signal 32 in figure 39a), the MGCF shall use the Configure IMS Resources procedure (signals 35 and 36 in figure 39a) as detailed below unless the IM-MGW is already configured accordingly:

– If the remote IMS resources configured at the IM-MGW do not match the remote resources selected for the established dialogue of the final response, the MGCF shall provide the remote IP address and UDP port from the latest received SDP of this established dialogue, and the remote codec selected from the latest received SDP of this established dialogue and local configuration data within the "remote IMS resources".

– If the local IMS resources configured at the IM-MGW contain more codecs than selected for the established dialogue of the final response, the MGCF should update the "local IMS resources" with the selected local codec derived from the latest SDP of this established dialogue and local configuration data. The "reserve value" may be cleared unless it is required for DTMF.

– The IM-MGW may be configured to use the remote IP address and port information as source filter for incoming packages to prevent that early media from other early SIP dialogues interfere with the media of the established dialogue. The MGCF may also provide an IP address and port source filter that disallows early media from other early dialogues in order to prevent that such early media interfere with the media of the established dialogue. If the MGCF has provided a source filter selecting media of another SIP early dialogue, it shall remove or update this source filter.

9.2.3.4.4 Message sequence chart

Figure 39a shows an example message sequence chart for a CS network originating Session Setup with ISUP, where forking occurs.

Figure 39a/1: CS Network Originating Session with forking, ISUP (message sequence chart)

Figure 39a/2: CS Network Originating Session with forking, ISUP (message sequence chart continue)