J.2 EPS aspects when connected to the IM CN subsystem

24.1733GPPIMS Multimedia telephony communication service and supplementary servicesRelease 18Stage 3TS

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

J.2.1 Procedures at the UE

J.2.1.1 Service Specific Access Control

The following information is provided by lower layer:

– BarringFactorForMMTEL-Voice: barring rate for MMTEL voice;

– BarringTimeForMMTEL-Voice: barring timer for MMTEL voice;

– BarringFactorForMMTEL-Video: barring rate for MMTEL video; and

– BarringTimeForMMTEL-Video: barring timer for MMTEL video.

Upon request from a user to establish a multimedia telephony communication session as described in clause 5.2, the UE shall:

1) if the multimedia telephony communication session to be established is an emergency session, then skip the rest of steps below and continue with session establishment as described in clause 5.2;

2) retrieve SSAC related information mentioned above from lower layers;

NOTE 1: The values of SSAC related information retrieved from lower layers can depend on whether the UE has an Access Class with a value in the range 11..15 or not. Determination of the values of the SSAC related information is described in clause 5.3.3.10 of 3GPP TS 36.331 [26].

3) if video is offered in the multimedia telephony communication session:

A) if back-off timer Tx is running, reject the multimedia telephony communication session establishment and skip the rest of steps below; or

B) else, then:

I) draw a new random number "rand1" that is uniformly distributed in the range 0 ≤ rand1 < 1; and

II) if the random number "rand1" is lower than BarringFactorForMMTEL-Video, then skip the rest of steps below and continue with session establishment as described in clause 5.2;

NOTE 2: If the BarringFactorForMMTEL-Video is set to 1, the session is exempted from barring.

III) else, then;

i) draw a new random number "rand2" that is uniformly distributed in the range 0 ≤ rand2 < 1; and

ii) start back-off timer Tx with the timer value calculated using the formula:

Tx = (0,7 + 0,6*rand2) * BarringTimeForMMTEL-Video; and

iii) reject the multimedia telephony communication session establishment and skip the rest of steps below;

4) if audio is offered in the multimedia telephony communication session:

A) if back-off timer Ty is running, reject the multimedia telephony communication session establishment and skip the rest of steps below; or

B) else, then;

I) draw a new random number "rand3" that is uniformly distributed in the range 0 ≤ rand3 < 1; and

II) if the random number "rand3" is lower than BarringFactorForMMTEL-Voice, then skip the rest of steps below and continue with session establishment as described in clause 5.2;

NOTE 3: If the BarringFactorForMMTEL-Voice is set to 1, the session is exempted from barring.

III) else, then;

i) draw a new random number "rand4" that is uniformly distributed in the range 0 ≤ rand4 < 1; and

ii) start timer Ty with the timer value calculated using the fomula:

Ty = (0,7 + 0,6*rand4) * BarringTimeForMMTEL-Voice; and

iii) reject the multimedia telephony communication session establishment;

NOTE 4: If the multimedia telephony communication implemenation and the access stratum protocol implementation are located in separate physical entities, it is expected that the interconnecting protocol supports the transfer of information elements needed for the service specific access control enforcement.

Service Specific Access Control is not activated when the UE is in other radio accesses (e.g. UTRAN/GERAN). And when UE camping on E-UTRAN moves to other radio accesses (e.g. UTRAN/GERAN), back-off timer (Tx or Ty or both) shall be stopped if running.

J.2.1.2 Smart Congestion Mitigation

The following information is provided to the non-access stratum:

– MO-MMTEL-voice-started;

– MO-MMTEL-voice-ended;

– MO-MMTEL-video-started;

– MO-MMTEL-video-ended;

– MT-MMTEL-voice-started;

– MT-MMTEL-voice-ended;

– MT-MMTEL-video-started; and

– MT-MMTEL-video-ended.

Upon request from a user to establish an originating multimedia telephony communication session as described in clause 5.2, and if the session establishment is continued after performing the Service Specific Access Control as specified in clause J.2.1.1:

1) if only audio or only real-time text or only both audio and real-time text (see clause 4.2 for 3GPP systems) are offered in the multimedia telephony communication session, and no other originating multimedia telephony communication session initiated with offering only audio or only real-time text or only both audio and real-time text exists, the UE sends the MO-MMTEL-voice-started indication to the non-access stratum and continue with session establishment as described in clause 5.2;

2) if video is offered in the multimedia telephony communication session, and no other originating multimedia telephony communication session initiated with offering video exists, the UE sends the MO-MMTEL-video-started indication to the non-access stratum and continue with session establishment as described in clause 5.2.

When a terminating multimedia telephony communication session starts (i.e. an INVITE is received), the terminating multimedia telephony communication session is offering only audio or only real-time text or only both audio and real-time text (i.e. in the SDP offer in the INVITE request), and no other terminating multimedia telephony communication session started with offering only audio or only real-time text or only both audio and real-time text exists, the UE sends the MT-MMTEL-voice-started indication to the non-access stratum.

When a terminating multimedia telephony communication session starts (i.e. an INVITE is received), the terminating multimedia telephony communication session is offering video (i.e. in the SDP offer in the INVITE request), and no other terminating multimedia telephony communication session started with offering video exists, the UE sends the MT-MMTEL-video-started indication to the non-access stratum.

When an originating multimedia telephony communication session ends (i.e. a response to a BYE or a failure response to the initial INVITE request is transferred), the originating multimedia telephony communication session was initiated with offering only audio or only real-time text or only both audio and real-time text (i.e. in the SDP offer in the initial INVITE request), and no other originating multimedia telephony communication session initiated with offering only audio or only real-time text or only both audio and real-time text exists, the UE sends the MO-MMTEL-voice-ended to the non-access stratum.

When an originating multimedia telephony communication session ends (i.e. a response to a BYE or a failure response to the initial INVITE request is transferred), the originating multimedia telephony communication session was initiated with offering video (i.e. in the SDP offer in the initial INVITE request), and no other originating multimedia telephony communication session initiated with offering video exists, the UE sends the MO-MMTEL-video-ended indication to the non-access stratum.

NOTE 1: If the UE supports other 3GPP specific mechanisms for communicating with the non-access stratum protocol implementation, e.g. DHCP discovery via PCO, then the UE is expected to support the transfer of information elements needed for the smart congestion mitigation enforcement.

NOTE 2: Adding or removing media during the multimedia telephony communication session has no impact on the information relating to smart congestion mitigation.

J.2.1.3 MMTEL request timeouts

If the UE supports timer RequestTimeout, then upon a request from a user to establish an originating multimedia telephony communication session as described in clause 5.2, the UE shall start timer RequestTimeout when sending an initial INVITE request.

The UE may support being configured for the RequestTimeout timer with the operator’s IMS multimedia telephony communication service policy as specified in 3GPP TS 24.275 [36].

J.2.1.4 Abnormal cases

Upon request from a user to establish a MMTEL voice or an MMTEL video session as described in clause 5.2, if:

a) the UE receives, from the lower layers, a notification that the service request:

1) was not accepted due to congestion; or

2) resulted in starting timer T3325 (see 3GPP TS 24.008 [80]); and

b) an originating initial INVITE transaction, for which a 2xx response has not yet been received, exists;

then:

a) if a provisional response has been received, the UE shall cancel INVITE transaction; and

b) if an alternative radio access network is available:

1) if the service request was not accepted due to congestion, the UE shall attempt the MMTEL voice or an MMTEL video session on the alternative radio access network; or

2) if the service request resulted in starting timer T3325, the UE should attempt the MMTEL voice or an MMTEL video session on the alternative radio access network.