10.14 Ambient listening call

23.3793GPPFunctional architecture and information flows to support Mission Critical Push To Talk (MCPTT)Release 18Stage 2TS

10.14.1 General

The ambient listening call is a type of a private MCPTT call that only allows a "listened to" user to transmit media to a "listening" user such that there is no indication on the MCPTT UE of the "listened to" user about the call and the media transmission.

NOTE 1: "listened to" user refers to the user who is transmitting media in an ambient listening call.

NOTE 2: "listening" user refers to the user who is receiving media in an ambient listening call.

There are two types of ambient listening call as below:

– Remotely initiated ambient listening is initiated by the authorized user (e.g., dispatcher) who wants to listen to another user. In this case, the "listened to" user is the called party, and shall automatically accept the call without causing any indication about the call and transmit the media to the "listening" user.

– Locally initiated ambient listening is initiated by an authorized user who wants another user to listen to the MCPTT UE communication. In this case, the "listened to" user is the calling party and shall automatically transmit the media to the "listening" user without causing any indication about the call processing and media transmission.

10.14.2 Information flows for ambient listening call

10.14.2.1 Ambient listening call request

Table 10.14.2.1-1 describes the information flow ambient listening call request from the MCPTT client to the MCPTT server and MCPTT server to the MCPTT client.

Table 10.14.2.1-1: Ambient listening call request

Information Element	Status	Description
MCPTT ID	M	The MCPTT ID of the "listening" user
MCPTT ID	M	The MCPTT ID of the "listened to" user
SDP offer	M	Media parameters of MCPTT client.
Ambient listening type	M	The ambient listening type indicates remotely initiated ambient listening call or locally initiated ambient listening call.

10.14.2.2 Ambient listening call response

Table 10.14.2.2-1 describes the information flow ambient listening call response from the MCPTT client to the MCPTT server and MCPTT server to the MCPTT client.

Table 10.14.2.2-1: Ambient listening call response

Information Element	Status	Description
MCPTT ID	M	The MCPTT ID of the "listening" user
MCPTT ID	M	The MCPTT ID of the "listened to" user
SDP answer	M	Media parameters selected

10.14.2.3 Ambient listening call release request

Table 10.14.2.3-1 describes the information flow ambient listening call release request from the MCPTT client to the MCPTT server and MCPTT server to the MCPTT client.

Table 10.14.2.3-1: Ambient listening call release request

Information Element	Status	Description
MCPTT ID	M	The MCPTT ID of the "listening" user
MCPTT ID	M	The MCPTT ID of the "listened to" user
Ambient listening type	M	The ambient listening type indicates a remotely initiated ambient listening call or a locally initiated ambient listening call.

10.14.2.4 Ambient listening call release response

Table 10.14.2.4-1 describes the information flow ambient listening call release response from the MCPTT client to the MCPTT server and MCPTT server to the MCPTT client.

Table 10.14.2.4-1: Ambient listening call release response

Information Element	Status	Description
MCPTT ID	M	The MCPTT ID of the "listening" user
MCPTT ID	M	The MCPTT ID of the "listened to" user
Ambient listening type	M	The ambient listening type indicates a remotely initiated ambient listening call or a locally initiated ambient listening call.

10.14.2.5 Ambient listening call release notification

Table 10.14.2.5-1 describes the information flow ambient listening call release notification from the MCPTT server to the MCPTT client.

Table 10.14.2.5-1: Ambient listening call release notification

Information Element	Status	Description
MCPTT ID	M	The MCPTT ID of the "listening" user
MCPTT ID	M	The MCPTT ID of the "listened to" user
Call release reason	M	The reason for call release by the MCPTT server
Ambient listening type	M	The ambient listening type indicates a remotely initiated ambient listening call or a locally initiated ambient listening call.

10.14.3 Ambient listening call procedures

10.14.3.1 Remotely initiated ambient listening call setup

The MCPTT service provides the capability for an authorised user to initiate a remotely initiated ambient listening call at an MCPTT client.

Figure 10.14.3.1-1 illustrates the information flow for remotely initiated ambient listening call setup.

Pre-conditions:

– MCPTT client 1 is the client of the authorized user who is authorized to invoke a remotely initiated ambient listening call to be set up at the requested MCPTT client 2.

– MCPTT user 1 is the "listening" user at MCPTT client 1, and MCPTT user 2 is the "listened to" user at MCPTT client 2.

Figure 10.14.3.1-1: Remotely initiated ambient listening call

1. MCPTT client 1 initiates a remotely initiated ambient listening call by sending the ambient listening call request to the MCPTT server. The remotely initiated ambient listening call type is included.

2. The MCPTT server performs an authorization check for the authorized user 1 for the remotely initiated ambient listening call. If authorization fails, the MCPTT server provides a failure response to MCPTT client 1.

3. The MCPTT server sends the ambient listening call request to MCPTT client 2.

NOTE: MCPTT client 2 does not provide any indication of the ambient listening call request to its user.

4. MCPTT client 2 returns the ambient listening call response to the MCPTT server.

5. MCPTT server provides an ambient listening call response to MCPTT client 1, indicating whether the call is set up successfully or not.

6. The floor control server of the MCPTT server then sends a floor granted to MCPTT client 2 according to the ambient listening type received in step 1.

7. Accordingly, the floor control server of the MCPTT server sends a floor taken to MCPTT client 1.

8. After receiving the floor granted message at the MCPTT client 2, the media is transmitted from MCPTT client 2 to MCPTT client 1.

10.14.3.2 Locally initiated ambient listening call setup

The MCPTT service provides the capability for an authorised user to initiate a locally initiated ambient listening call at an MCPTT client.

Figure 10.14.3.2-1 illustrates the information flow for locally initiated ambient listening call setup.

Pre-conditions:

– MCPTT client 2 is the client of the authorized user who is authorized to invoke a locally initiated ambient listening call to be set up at the requested MCPTT client 1.

– MCPTT user 1 is the "listening" user at MCPTT client 1, and MCPTT user 2 is the "listened to" user at MCPTT client 2.

Figure 10.14.3.2-1: Locally initiated ambient listening call

1. MCPTT client 2 initiates a locally initiated ambient listening call by sending the ambient listening call request to the MCPTT server. The locally initiated ambient listening call type is included.

2. The MCPTT server performs an authorization check for the authorized user 2 for the locally initiated ambient listening call. If authorization fails, the MCPTT server provides a failure response to MCPTT client 2.

3. The MCPTT server sends the ambient listening call request to MCPTT client 1.

4. MCPTT client 1 returns the ambient listening call response to the MCPTT server.

5. MCPTT server provides an ambient listening call response to MCPTT client 2, indicating whether the call is set up successfully or not.

6. The floor control server of the MCPTT server then sends a floor granted to MCPTT client 2 according to the ambient listening type received in step 1.

7. Accordingly, the floor control server of the MCPTT server sends a floor taken to MCPTT client 1.

8. After receiving the floor granted message at the MCPTT client 2, the media is transmitted from MCPTT client 2 to MCPTT client 1.

NOTE: MCPTT client 2 does not provide any indication of the ambient listening call request to its user.

10.14.3.3 Ambient listening call release – server initiated

Figure 10.14.3.3-1 illustrates the information flow for ambient listening call release – server initiated when trigger by the MCPTT administrator. This procedure is applied for both remotely initiated ambient listening call and the locally initiated ambient listening call.

Pre-conditions:

– MCPTT client 1 is the MCPTT client of the authorized user, who initiated the ambient listening call at MCPTT client 2.

– There is an ongoing ambient listening call between MCPTT client 2 and MCPTT client 1.

– MCPTT user 1 is the current user at MCPTT client 1 who is listening, and MCPTT user 2 is the current user at MCPTT client 2 who is being listened to.

Figure 10.14.3.3-1: Ambient listening call release – server initiated

1. The ambient listening call release is triggered by the MCPTT administrator or by one of the following events:

– the MCPTT server receives MCPTT call requests towards MCPTT client 2; or

– the MCPTT client 2 initiates MCPTT call requests;

2. The MCPTT server sends an ambient listening call release request to MCPTT client 2.

3. MCPTT client 2 stops transmitting media to MCPTT client 1.

NOTE: MCPTT client 2 does not provide any indication of the ambient listening call release to its user.

4. MCPTT client 2 provides an ambient listening call release response to the MCPTT server.

5. The MCPTT server sends an ambient listening call release notification to MCPTT client 1 together with a reason code identifying that the call was released.

6. MCPTT client 1 notifies the authorized user 1.

10.14.3.4 Remotely initiated ambient listening call release – "listening" user initiated

Figure 10.14.3.4-1 illustrates the information flow for ambient listening call release – "listening" user initiated. This procedure is applied for both remotely initiated ambient listening call and the locally initiated ambient listening call.

Pre-conditions:

– MCPTT client 1 is the MCPTT client of the authorized user, who is authorized to release the ambient listening call at MCPTT client 2.

– There is an ongoing ambient listening call between MCPTT client 2 and MCPTT client 1.

– MCPTT user 1 is the "listening" user at MCPTT client 1, and MCPTT user 2 is the "listened to"user at MCPTT client 2.

Figure 10.14.3.4-1: Ambient listening call release – "listening" user initiated

1. The authorized user 1 at MCPTT client 1 initiates the ambient listening call release by sending an ambient listening call release request to the MCPTT server.

2. The MCPTT server provides an ambient listening call release request to MCPTT client 2.

3. MCPTT client 2 stops transmitting media to MCPTT client 1.

NOTE: MCPTT client 2 does not provide any indication of the ambient listening call release to its user.

4. MCPTT client 2 provides an ambient listening call release response to the MCPTT server.

5. The MCPTT server provides the ambient listening call release response to MCPTT client 1.

10.14.3.5 Ambient listening call release – "listened to" user initiated

Figure 10.14.3.5-1 illustrates the information flow for ambient listening call release – "listened to" user initiated. This procedure is only applied for the locally initiated ambient listening call.

Pre-conditions:

– There is an ongoing ambient listening call between MCPTT client 1 and MCPTT client 2.

– MCPTT user 1 is the "listening" user at MCPTT client 1, and MCPTT user 2 is the "listened to" user at MCPTT client 2.

– MCPTT client 2 is the MCPTT client of the authorized user, who is authorized to release the locally initiated ambient listening call at MCPTT client 2.

Figure 10.14.3.5-1: Ambient listening call release – "listened to" user initiated

1. The authorized user 2 at MCPTT client 2 initiates the ambient listening call release by sending an ambient listening call release request to the MCPTT server.

2. The MCPTT server provides an ambient listening call release request to MCPTT client 1.

3. The user 1 at MCPTT client 1 is notified about the ambient listening call release.

4. MCPTT client 1 provides an ambient listening call release response to the MCPTT server.

5. The MCPTT server provides the ambient listening call release response to MCPTT client 2.

6. MCPTT client 2 stops transmitting media to MCPTT client 1.

NOTE: MCPTT client 2 does not provide any indication of the ambient listening call release to its user.