A.5.8 CAT with different media types during one SIP session when both UE#1 and UE#2 do not have required resources available

24.1823GPPIP Multimedia Subsystem (IMS) Customized Alerting Tones (CAT)Protocol specificationRelease 17TS

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

Figure A.5.8-1 shows an example call flow for CAT when UE#1 and UE#2 does not have required resources available, and the media type required by CAT service which is video in this example, and the media type required by the call which is audio, are different during one session.

Figure A.5.8-1: CAT with different media types during one session, UE#1 and UE#2 does not have required resources available

1 INVITE request (UE#1 to CAT-AS) see example in table A.5.8-1

UE#1 sends a SIP INVITE request to the intermediate IM CN subsystem.

Table A.5.8-1: INVITE request (UE#1 to CAT-AS)

INVITE tel:+1-212-555-2222 SIP/2.0

Via: SIP/2.0/UDP [5555::aaa:bbb:ccc:ddd]:1357;comp=sigcomp;branch=z9hG4bKnashds7

Max-Forwards: 70

Route: <sip:pcscf1.visited1.net:7531;lr;comp=sigcomp>, <sip:scscf1.home1.net;lr>

P-Preferred-Identity: "John Doe" <sip:user1_public1@home1.net>

P-Access-Network-Info: 3GPP-UTRAN-TDD; utran-cell-id-3gpp=234151D0FCE11

P-Preferred-Service: urn:urn-7:3gpp-service.ims.icsi.mmtel

Accept-Contact: *;+g.3gpp.icsi-ref="urn%3Aurn-7%3gpp-service.ims.icsi.mmtel"

Privacy: none

P-Early-Media: supported

From: <sip:user1_public1@home1.net>;tag=171828

To: <tel:+1-212-555-2222>

Call-ID: cb03a0s09a2sdfglkj490333

Cseq: 127 INVITE

Require: sec-agree

Recv-Info: infoDtmf

Supported: precondition, 100rel, 199

Proxy-Require: sec-agree

Security-Verify: ipsec-3gpp; q=0.1; alg=hmac-sha-1-96; spi-c=98765432; spi-s=87654321; port-c=8642; port-s=7531

Contact: <sip:user1_public1@home1.net;gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6 >;video;+g.3gpp.icsi-ref="urn%3Aurn-7%3gpp-service.ims.icsi.mmtel"

Allow: INVITE, ACK, CANCEL, BYE, PRACK, UPDATE, REFER, MESSAGE

Accept:application/sdp, application/3gpp-ims+xml

Content-Type: application/sdp

Content-Length: (…)

v=0

o=- 2987933615 2987933615 IN IP6 5555::aaa:bbb:ccc:ddd

s=-

c=IN IP6 5555::aaa:bbb:ccc:ddd

t=0 0

m=audio 3456 RTP/AVP 97 96

a=tcap:1 RTP/AVPF

a=pcfg:1 t=1

b=AS:25.4

a=curr:qos local none

a=curr:qos remote none

a=des:qos mandatory local sendrecv

a=des:qos none remote sendrecv

a=rtpmap:97 AMR

a=fmtp:97 mode-set=0,2,5,7; maxframes=2

a=rtpmap:96 telephone-event

Supported: The UE indicates support for 199 responses, reliable provisional responses and preconditions.

P-Early-Media: The UE indicates support for the P-Early-Media header.

Recv-Info: The UE indicates willingness to receive DTMF Info Packages in INFO requests.

Contact: The UE indicates support for video handling.

SDP: The SDP offer (SDP_O) contains a set of codecs supported by UE#1 and desired by the calling user for this session. The local preconditions are indicated as not fulfilled. In this example, only audio media description has been included in SDP_O in case of audio call.

2 INVITE request (S-CSCF to CAT-AS)

The S-CSCF forwards the SIP INVITE request to the CAT-AS.

3-4 INVITE request (CAT-AS to UE#2)

The CAT-AS forwards the request to UE#2.

5-6 183 (Session Progress) provisional response (UE#2 to CAT-AS) see example in table A.5.8-2

UE#2 sends a reliable SIP 183 (Session Progress) provisional response for the INVITE request to the CAT-AS.

The CAT-AS terminates the provisional response.

Table A.5.8-2: 183 (Session Progress) provisional response (UE#2 to CAT-AS)

SIP/2.0 183 Session Progress

Via: SIP/2.0/UDP pcscf2.visited2.net:5088;comp=sigcomp;branch=z9hG4bK361k21.1, SIP/2.0/UDP scscf2.home2.net;branch=z9hG4bK764XC12.1, SIP/2.0/UDP catas.home2.net;branch=z9hG4bK764Q32.1, SIP/2.0/UDP scscf2.home2.net;branch=z9hG4bK764z87.1, SIP/2.0/UDP icscf2_s.home2.net;branch=z9hG4bK871y12.1, SIP/2.0/UDP scscf1.home1.net;branch=z9hG4bK332b23.1, SIP/2.0/UDP pcscf1.visited1.net;branch=z9hG4bK240f34.1, SIP/2.0/UDP [5555::aaa:bbb:ccc:ddd]:1357;comp=sigcomp;branch=z9hG4bKnashds7

Record-Route: <sip:pcscf2.visited2.net:5088;lr;comp=sigcomp>, <sip:scscf2.home2.net;lr>, <sip:catas.home2.net;lr>, <sip:scscf2.home2.net;lr>, <sip:scscf1.home1.net;lr>, <sip:pcscf1.visited1.net;lr>

From:

To: <tel:+1-212-555-2222>;tag=2236

Call-ID:

Cseq:

Require: 100rel

RSeq: 9022

Allow: INVITE, ACK, CANCEL, BYE, PRACK, UPDATE, REFER, MESSAGE

Contact: <sip:user2_public1@home2.net;gr=urn:uuid:2ad8950e-48a5-4a74-8d99-ad76cc7fc74>;+g.3gpp.icsi-ref="urn%3Aurn-7%3gpp-service.ims.icsi.mmtel"

Content-Type: application/sdp

Content-Length: (…)

v=0

o=- 29879336157 29879336157 IN IP6 6666::eee:fff:aaa:bbb

s=-

c=IN IP6 6666::eee:fff:aaa:bbb

t=0 0

m=audio 8386 RTP/AVPF 97 96

a=acfg:1 t=1

b=AS:25.4

a=curr:qos local none

a=curr:qos remote none

a=des:qos mandatory local sendrecv

a=des:qos mandatory remote sendrecv

a=rtpmap:97 AMR

a=fmtp:97 mode-set=0,2,5,7; maxframes

a=rtpmap:96 telephone-event

SDP: The SDP answer (SDP_A_regular) contains a set of codecs to be used for the session. The local preconditions are indicated as not fulfilled.

7-8 183 (Session Progress) provisional response (CAT-AS to UE#1) see example in table A.5.8-3

The CAT-AS sends a reliable SIP 183 (Session Progress) provisional response to UE#1. The CAT-AS shall take into account, when creating the SDP message, the media indicated by UE#2 in the previously received 183 (Session Progress) provisional response.

Table A.5.8-3: 183 (Session Progress) response (CAT-AS to UE#1)

SIP/2.0 183 Session Progress

Via: SIP/2.0/UDP scscf2.home2.net;branch=z9hG4bK764z87.1, SIP/2.0/UDP icscf2_s.home2.net;branch=z9hG4bK871y12.1, SIP/2.0/UDP scscf1.home1.net;branch=z9hG4bK332b23.1, SIP/2.0/UDP pcscf1.visited1.net;branch=z9hG4bK240f34.1, SIP/2.0/UDP [5555::aaa:bbb:ccc:ddd]:1357;comp=sigcomp;branch=z9hG4bKnashds7

Record-Route: <sip:scscf2.home2.net;lr>, <sip:scscf1.home1.net;lr>, <sip:pcscf1.visited1.net;lr>

From:

To: <tel:+1-212-555-2222>;tag=2236

Call-ID:

Cseq:

Require: 100rel

Allow: INVITE, ACK, CANCEL, BYE, PRACK, UPDATE, REFER, MESSAGE

RSeq: 9022

P-Early-Media: sendrecv

Contact: <sip:catas.home2.net>;+g.3gpp.icsi-ref="urn%3Aurn-7%3gpp-service.ims.icsi.mmtel"

Content-Type: application/sdp

Content-Length: (…)

v=0

o=- 29879336156 29879336156 IN IP6 5555::ccc:aaa:abc:abc

s=-

c=IN IP6 6666::eee:fff:aaa:bbb

t=0 0

m=audio 49170 RTP/AVPF 97 96

a=acfg:1 t=1

b=AS:25.4

a=curr:qos local none

a=curr:qos remote none

a=des:qos mandatory local sendrecv

a=des:qos mandatory remote sendrecv

a=rtpmap:97 AMR

a=fmtp:97 mode-set=0,2,5,7; maxframes

a=rtpmap:96 telephone-event

a=content:g.3gpp.cat

P-Early-Media: The CAT-AS requests authorization for early media.

SDP: The SDP answer (SDP_A_regular) contains a set of codecs to be used for the session and, in this example, includes a SDP "a=content" attribute with a "g.3gpp.cat" value for each media description. The local preconditions are indicated as not fulfilled.

9-12 PRACK request (UE#1 to CAT-AS, and to UE#2)

UE#1 sends a SIP PRACK request, which acknowledges the SIP 183 (Session Progress) provisional response, to the CAT-AS, which is further sent to UE#2.

13-16 200 (OK) response to PRACK request (UE#2 to CAT-AS, and to UE#1)

UE#2 sends a SIP 200 (OK) response for the SIP PRACK request to CAT-AS, which is further sent to UE#1.

17-18 UPDATE request (UE#1 to CAT-AS) – see example in table A.5.8-4

UE#1 indicates that it can send and receive media as the necessary resources are available.

Table A.5.8-4: UPDATE request (UE#1 to CAT-AS)

UPDATE sip:user2_public1@home2.net;gr=urn:uuid:2ad8950e-48a5-4a74-8d99-ad76cc7fc74 SIP/2.0

Via: SIP/2.0/UDP [5555::aaa:bbb:ccc:ddd]:1357;comp=sigcomp;branch=z9hG4bKnashds7

Max-Forwards: 70

Route: <sip:pcscf1.visited1.net;lr>, <sip:scscf1.home1.net;lr>, <sip:scscf2.home2.net;lr>

From: <sip:user1_public1@home1.net>;tag=171828

To: <tel:+1-212-555-2222>;tag=2236

Call-ID:

Cseq: 129 UPDATE

Require: sec-agree

Proxy-Require: sec-agree

Security-Verify: ipsec-3gpp; q=0.1; alg=hmac-sha-1-96; ealg=aes-cbc; spi-c=98765432; spi-s=87654321; port-c=8642; port-s=7531

Contact: <sip:user1_public1@home1.net;gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>;+g.3gpp.icsi-ref="urn%3Aurn-7%3gpp-service.ims.icsi.mmtel"

Content-Type: application/sdp

Content-Length: (…)

v=0

o=- 2987933615 2987933616 IN IP6 5555::aaa:bbb:ccc:ddd

s=-

c=IN IP6 5555::aaa:bbb:ccc:ddd

t=0 0

m=audio 3456 RTP/AVPF 97 96

b=AS:25.4

a=curr:qos local sendrecv

a=curr:qos remote none

a=des:qos mandatory local sendrecv

a=des:qos none remote sendrecv

a=rtpmap:97 AMR

a=fmtp:97 mode-set=0,2,5,7; maxframes=2

a=rtpmap:96 telephone-event

SDP: The SDP offer (SDP_O2) contains a set of codecs supported to be used for the session. The SDP content is based on information received from the UE#2. The local preconditions are indicated as fulfilled.

19-20 UPDATE request (CAT-AS to UE#2) – see example in table A.5.8-5

CAT-AS indicates to UE#2 that UE#1 can send and receive media as the necessary resources are available.

Table A.5.8-5: UPDATE request (CAT-AS to UE#2)

UPDATE sip:user2_public1@home2.net;gr=urn:uuid:2ad8950e-48a5-4a74-8d99-ad76cc7fc74;comp=sigcomp SIP/2.0

Via: [5555::ccc:aaa:abc:abc]:1357;comp=sigcomp;branch=z9hG4bKnashds8

Max-Forwards: 66

Route: <sip:scscf2.home2.net;lr>

From: <sip:user1_public1@home1.net>;tag=171828

To: <tel:+1-212-555-2222>;tag=2236

Call-ID:

Cseq: 56101 UPDATE

Require: sec-agree

Proxy-Require: sec-agree

Security-Verify: ipsec-3gpp; q=0.1; alg=hmac-sha-1-96; ealg=aes-cbc; spi-c=98765432; spi-s=87654321; port-c=8642; port-s=7531

Contact: <sip:user1_public1@home1.net;gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>;+g.3gpp.icsi-ref="urn%3Aurn-7%3gpp-service.ims.icsi.mmtel"

Content-Type: application/sdp

Content-Length: (…)

v=0

o=- 2987933615 2987933615 IN IP6 5555::ccc:aaa:abc:abc

s=-

c=IN IP6 5555::aaa:bbb:ccc:ddd

t=0 0

m=audio 3456 RTP/AVPF 97 96

b=AS:25.4

a=curr:qos local sendrecv

a=curr:qos remote none

a=des:qos mandatory local sendrecv

a=des:qos none remote sendrecv

a=rtpmap:97 AMR

a=fmtp:97 mode-set=0,2,5,7; maxframes=2

a=rtpmap:96 telephone-event

SDP: The SDP offer (SDP_O2) contains a set of codecs supported to be used for the session. The SDP content is based on information received from the UE#1. The local preconditions are indicated as fulfilled.

21-24 200 (OK) response (UE#2 to CAT-AS, and to UE#1) – see example in table A.5.8-6

UE#2 sends a SIP 200 (OK) response for the SIP UPDATE request to CAT-AS. The SDP shall contain information relevant to the media that will be used after the session is established.

CAT-AS shall store the relevant information in SDP for later use in step 35.

Table A.5.8-6: 200 (OK) response (UE#2 to CAT-AS)

SIP/2.0 200 OK

Record-Route: <sip:pcscf2.visited2.net:5088;lr;comp=sigcomp>, <sip:scscf2.home2.net;lr>, <sip:catas.home2.net;lr>, <sip:scscf2.home2.net;lr>, <sip:scscf1.home1.net;lr>, <sip:pcscf1.visited1.net;lr>

From:

To: <tel:+1-212-555-2222>;tag=2236

Call-ID:

Cseq:

Allow: INVITE, ACK, CANCEL, BYE, PRACK, UPDATE, REFER, MESSAGE

Contact: <sip:user2_public1@home2.net;gr=urn:uuid:2ad8950e-48a5-4a74-8d99-ad76cc7fc74>;+g.3gpp.icsi-ref="urn%3Aurn-7%3gpp-service.ims.icsi.mmtel"

Content-Type: application/sdp

Content-Length: (…)

v=0

o=- 29879336157 29879336157 IN IP6 6666::eee:fff:aaa:bbb

s=-

c=IN IP6 6666::eee:fff:aaa:bbb

t=0 0

m=audio 8386 RTP/AVPF 97 96

b=AS:25.4

a=curr:qos local sendrecv

a=curr:qos remote sendrecv

a=des:qos mandatory local sendrecv

a=des:qos mandatory remote sendrecv

a=rtpmap:97 AMR

a=fmtp:97 mode-set=0,2,5,7; maxframes

a=rtpmap:96 telephone-event

SDP: The SDP answer (SDP_A_regular2) contains a set of codecs to be used for the session. The local preconditions are indicated as fulfilled.

25-26 180 (Ringing) provisional response (UE#2 to CAT-AS)

UE#2 sends a SIP 180 (Ringing) provisional response for the INVITE request to the CAT-AS.

The CAT-AS instructs the MRF to reserve CAT resources upon receipt of 180.

27-28 UPDATE request (CAT-AS to UE#1) see example in table A.5.8-8

The CAT-AS sends an UPDATE request containing the SDP offer based on the CAT media information received from MRF.

Session between UE#1 and UE#2 is replaced with session between UE#1 and CAT-AS.

NOTE 1: The AS will remain in the signalling path, i.e. work as a B2BUA, in order to align and maintain information in SDP for further exchanged messages between UE#1 and UE#2.

Table A.5.8-7: UPDATE request (CAT-AS to UE#1)

UPDATE sip:user1_public1@home1.net;gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6 SIP/2.0

Via: SIP/2.0/UDP [5555::ccc:aaa:abc:abc]:8805;branch=z9hG4bK182D87.1

Max-Forwards: 70

Route: <sip:scscf1.home1.net;lr>, <sip:pcscf1.visited1.net;lr>

From: <tel:+1-212-555-2222>;tag=2236

To: <sip:user1_public1@home1.net>;tag=171828

Call-ID:

Cseq: 12814111 UPDATE

Require:

Contact: <sip:user2_public1@home2.net;gr=urn:uuid:2ad8950e-48a5-4a74-8d99-ad76cc7fc74>+g.3gpp.icsi-ref="urn%3Aurn-7%3gpp-service.ims.icsi.mmtel"

Content-Type: application/sdp

Content-Length: (…)

v=0

o=- 29879336156 29879336158 IN IP6 5555::ccc:aaa:abc:abc

s=-

c=IN IP6 5555::ccc:aaa:abc:abc

t=0 0

m=audio 8386 RTP/AVPF 97 96

b=AS:25.4

a=curr:qos local sendrecv

a=curr:qos remote sendrecv

a=des:qos mandatory local sendrecv

a=des:qos mandatory remote sendrecv

a=rtpmap:97 AMR

a=fmtp:97 mode-set=0,2,5,7; maxframes

a=rtpmap:96 telephone-event

m=video 7398 RTP/AVPF 98

b=AS:75

a=curr:qos local sendonly

a=curr:qos remote none

a=des:qos mandatory local sendonly

a=des:qos mandatory remote recvonly

a=rtpmap:98 H263

a=fmtp:98 profile-level-id=0

SDP The SDP offer (SDP_O_cat) contains a set of codecs to be used for CAT. The SDP content is based on information received from MRF, in this example, includes an additional video media description and related attributes. The local preconditions are indicated as fulfilled, and the remote preconditions are indicated as not fulfilled because UE#1 is expected to reserve additional resources for video media required by CAT-AS.

29-30 200 (OK) response (UE#1 to CAT-AS)

UE#1 sends a SIP 200 (OK) response for the SIP UPDATE request to CAT-AS. UE#1 includes a SDP answer (SDP_A_cat) in the 200 (OK) response to the corresponding SIP UPDATE request. UE#1 starts to reserve CAT resources. The CAT-AS should not instruct MRF to start playing the CAT media unless the remote preconditions are indicated as fulfilled in SDP answer included in this 200 (OK) response or in SDP offer included in consequent UPDATE request.

31-32 UPDATE request (UE#1 to CAT-AS) see example in table A.5.8-8

UE#1 indicates that it can receive CAT media as the necessary resources are available.

The CAT-AS instructs MRF to start playing the CAT media.

Table A.5.8-8: UPDATE request (UE#1 to CAT-AS)

UPDATE sip:user2_public1@home2.net;gr=urn:uuid:2ad8950e-48a5-4a74-8d99-ad76cc7fc74 SIP/2.0

Via: SIP/2.0/UDP [5555::aaa:bbb:ccc:ddd]:1357;comp=sigcomp;branch=z9hG4bKnashds7

Max-Forwards: 70

Route: <sip:pcscf1.visited1.net;lr>, <sip:scscf1.home1.net;lr>, <sip:scscf2.home2.net;lr>

From: <sip:user1_public1@home1.net>;tag=171828

To: <tel:+1-212-555-2222>;tag=2236

Call-ID:

Cseq: 129 UPDATE

Require: sec-agree

Proxy-Require: sec-agree

Security-Verify: ipsec-3gpp; q=0.1; alg=hmac-sha-1-96; ealg=aes-cbc; spi-c=98765432; spi-s=87654321; port-c=8642; port-s=7531

Contact: <sip:user1_public1@home1.net;gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>;+g.3gpp.icsi-ref="urn%3Aurn-7%3gpp-service.ims.icsi.mmtel"

Content-Type: application/sdp

Content-Length: (…)

v=0

o=- 2987933615 2987933616 IN IP6 5555::aaa:bbb:ccc:ddd

s=-

c=IN IP6 5555::aaa:bbb:ccc:ddd

t=0 0

m=audio 8386 RTP/AVPF 97 96

b=AS:25.4

a=curr:qos local sendrecv

a=curr:qos remote sendrecv

a=des:qos mandatory local sendrecv

a=des:qos mandatory remote sendrecv

a=rtpmap:97 AMR

a=fmtp:97 mode-set=0,2,5,7; maxframes

a=rtpmap:96 telephone-event

m=video 7398 RTP/AVPF 98

b=AS:75

a=curr:qos local sendonly

a=curr:qos remote recvonly

a=des:qos mandatory local sendonly

a=des:qos mandatory remote recvonly

a=rtpmap:98 H263

a=fmtp:98 profile-level-id=0

SDP: The SDP offer (SDP_O3) contains a set of codecs supported to be used for the session. The SDP content is based on information received from the CAT-AS in Step 27. The remote preconditions are indicated as fulfilled.

33-34 200 (OK) response to UPDATE request (CAT-AS to UE#1)

CAT-AS sends a SIP 200 (OK) UPDATE response for the SIP UPDATE request to UE#1.

35-36 180 (Ringing) provisional response (CAT-AS to UE#1)

CAT-AS sends a SIP 180 (Ringing) provisional response for the INVITE request to the UE#1.

37-38 200 (OK) response to INVITE request (UE#2 to CAT-AS)

The called party answers the call. UE#2 sends a SIP 200 (OK) final response for the SIP INVITE request to the CAT-AS.

The CAT-AS instructs the MRF to stop CAT media.

39-40 UPDATE request (CAT-AS to UE#1) see example in table A.5.8-9

The CAT-AS sends an UPDATE request containing values received in the 200 (OK) for UPDATE request from UE#2 in step 22.

Session between UE#1 and CAT-AS is replaced with session between UE#1 and UE#2.

NOTE 1: The AS will remain in the signalling path, i.e. work as a B2BUA, in order to align and maintain information in SDP for further exchanged messages between UE#1 and UE#2.

Table A.5.8-9: UPDATE request (CAT-AS to UE#1)

UPDATE sip:user1_public1@home1.net;gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6 SIP/2.0

Via: SIP/2.0/UDP [5555::ccc:aaa:abc:abc]:8805;branch=z9hG4bK182D87.1

Max-Forwards: 70

Route: <sip:scscf1.home1.net;lr>, <sip:pcscf1.visited1.net;lr>

From: <tel:+1-212-555-2222>;tag=2236

To: <sip:user1_public1@home1.net>;tag=171828

Call-ID:

Cseq: 12814111 UPDATE

Require:

Contact: <sip:user2_public1@home2.net;gr=urn:uuid:2ad8950e-48a5-4a74-8d99-ad76cc7fc74>+g.3gpp.icsi-ref="urn%3Aurn-7%3gpp-service.ims.icsi.mmtel"

Content-Type: application/sdp

Content-Length: (…)

v=0

o=- 29879336156 29879336158 IN IP6 5555::ccc:aaa:abc:abc

s=-

c=IN IP6 6666::eee:fff:aaa:bbb

t=0 0

m=audio 8386 RTP/AVPF 97 96

b=AS:25.4

a=curr:qos local sendrecv

a=curr:qos remote sendrecv

a=des:qos mandatory local sendrecv

a=des:qos mandatory remote sendrecv

a=rtpmap:97 AMR

a=fmtp:97 mode-set=0,2,5,7; maxframes

a=rtpmap:96 telephone-event

SDP: The SDP offer (SDP_O_regular2) contains a set of codecs to be used for the session. The SDP content is based on information received from previous 200 (OK) response.

41-42 200 (OK) response (UE#1 to CAT-AS)

UE#1 sends a SIP 200 (OK) response for the SIP UPDATE request to CAT-AS. If the resources indicated for the media cannot be reused by the one reserved for the CAT media, UE#1 shall reserve necessary resources prior to sending the response.UE#1 includes a SDP answer (SDP_A3) in the 200 (OK) response to the corresponding SIP UPDATE request.

43-44 200 (OK) response to INVITE request (CAT-AS to UE#1)

The CAT-AS sends the SIP 200 (OK) response for the (initial) SIP INVITE request to UE#1.

45-46 ACK request (UE#1 to CAT-AS)

UE#1 sends a SIP ACK request, which acknowledges the SIP 200 (OK) final response, to CAT-AS.

47-48 ACK request (CAT-AS to UE#2)

CAT-AS sends a SIP ACK request, which acknowledges the SIP 200 (OK) final response, to UE#2.

NOTE 3: The ACK request from CAT-AS towards UE#2 may be sent after receiving 200 (OK) from UE#2 in step 18.