22.4683GPPGroup Communication System Enablers (GCSE)Release 17TS
This use case describes communications in a speech group call.
A "traffic" group is enabled on the system, where "system" refers to the combination of group call application and underlying 3GPP network.
The list of UEs permitted to join the group is defined on the system.
The operating area of the group is defined on the system (e.g. by lists of radio sites which are allowed to support the group).
UEs are configured with lists of groups that they may join.
Mick is a dispatcher in control of "traffic" group.
Traffic group officers are members of "traffic" group and include Ned, Oscar, Peter and Roger.
Simon is another officer who is not authorised to use "traffic" group.
A.1.3 Service flows
A.1.3.1 Service Flow 1 – Joining a group
Traffic group officers including Ned, Roger and Peter (but not Oscar), who are within the defined operating area of the traffic group, select the group on their UE devices in order to join the group and communicate in the group. The group is also selected on Mick’s dispatcher terminal.
The UE indicates to the user if joining the group was successful.
Oscar is located outside the defined area of the traffic group. His UE indicates that he is out of area for this group.
Simon is within the defined area of the "traffic" group, but he is not authorised to use the group.
A.1.3.2 Service Flow 2 – Joining and leaving a group based on group operating area
Oscar has not changed the group setting on his UE. He moves into the group’s operating area. His UE automatically joins the group, and he receives an indication on the UE that he is now within the operating area of the group.
Ned moves to an area outside the operating area of the group. The system automatically removes his UE from the group. The UE indicates to Ned that he is now outside the operating area of the group.
A.1.3.3 Service Flow 3 – Call start
Ned moves back to an area within the defined operating area of the group. His UE automatically joins the group, and he receives an indication that he is now within the operating area of the group.
The group is not carrying traffic.
Oscar initiates a call to the group on his UE. The system sets the call up. Oscar receives an indication that the call is set up, and he may talk, within 300 ms of initiating the call setup. Other group members, irrespective of their location relative to Oscar and irrespective of the number of users who have joined the group, receive an indication that a call is starting within 300 ms of Oscar initiating the call setup.
The system efficiently allocates resources on all cells, and cells that provide service to many group members use the same resources for all members to receive the call.
Oscar speaks, and all group members including the dispatcher Mick hear the audio that he has spoken within minimal delay (within a few hundred milliseconds). The audio is a one way transmission, i.e. receiving parties hear Oscar’s spoken audio, but no-one hears audio from any of the receiving parties.
All receiving parties including the dispatcher, Mick, can see Oscar’s identity as the talking party on their UEs up to the time Oscar disconnects his call to the group.
Differences in audio delay are not noticeable to officers standing near each other who are receiving service from the same cell as each other; i.e. they do not hear an "echo" effect from different UEs.
Any UEs which are permitted to be members of the group, but which have not joined the group do not take part in the call.
A.1.3.4 Service Flow 4 – Call interruption
While Oscar is still speaking, Ned attempts to speak (i.e. he makes a PTT request).
– If the system is configured such that the transmitting party cannot be interrupted, then Ned’s PTT request is rejected. He receives an indication on his UE of the rejection. Oscar continues to speak and to be heard by all receiving parties in the group.
– If the system is configured (such that interruption is possible, or is possible based on relative priorities of Oscar and Ned, and Ned has a higher priority rating), then the system sends a message to Oscar’s UE withdrawing transmit permission. The UE indicates this to Oscar, and Oscar’s UE no longer transmits. The system grants transmit permission to Ned’s UE, which indicates that his request is successful. Ned now speaks, and his speech is heard by the group from this point onwards. All receiving parties now see Ned’s identity as the talking party on their UEs.
If priorities are used, at least four priorities are needed for normal user, supervisory user, dispatcher and supervisory dispatcher.
A.1.3.5 Service Flow 5 – Talking party changeover
When the current talking party (Oscar in 4.1, Ned in 4.2) has finished speaking, an indication is sent to the UEs of all group members.
Peter then initiates speech transmission to the group. The group call goes on with Peter as the talking party.
A.1.3.6 Service Flow 6 – Late Entry and mobility
Roger has not previously selected the "traffic" group, and therefore has not been listening to audio. Whilst Peter is transmitting, Roger selects the "traffic" group on his UE. The UE signals to the system that it wants to join the group, and system grants the request. The UE joins the call, and Roger hears the audio being spoken by Peter, and Roger’s UE displays Peter’s identity as the talking party.
The system allocates resources efficiently to include Roger. If Roger is receiving service from a radio site which is also serving several other users, Roger’s UE uses the same resources that are being used for the other users.
Ned is receiving the call on one cell, but moves to another cell whilst Peter is transmitting. The system hands over Ned’s call to the new cell. If the new cell is serving several other users, Ned’s UE uses the same resources that are being used for the other users. Ned experiences minimal interruption to his received audio due to the cell change.
Whilst transmitting, Peter moves to another cell. The system hands over Peter’s call to the new cell and allows him to continue transmission with minimal interruption.
A.1.3.7 Service Flow 7 – Call release
Peter finishes transmitting and ends the call on his UE. The UE signals to the system that the transmission is ceased, and the system in turn signals this to all receiving parties in the call. Peter’s audio is no longer heard, and his identity is no longer displayed as the talking party.
After a short period of time, the system releases resources which had been assigned to the call.
The Group Call is cleared by the system after a short time after the last speaker and no one has spoken in the meantime (and the system may be configured with a timer to determine this time).
A.1.3.8 Service Flow 8 – Blocking of resources
Oscar initiates a new call. Traffic group members are spread across several cells.
Calling Party cell busy:
– Oscar’s cell does not have enough resources to support the new call.
– Oscar receives an indication that the call cannot be established, and will be queued.
– The system places the call in a queue until enough resources are available at least on Oscar’s cell.
– Once enough resources are available, the call is started, and Oscar receives an indication that he can start to transmit.
Receiving Party cell busy:
- Oscar’s cell has enough resources to support the call, but on one or more cells where group members are receiving service, the system does not have enough resources to support the new call. Two alternatives may follow:
– Queued configuration: If the system is configured to queue calls, the call is queued until the system has enough resources to support all the users of the group. Oscar receives an indication that the call has been queued. Once enough resources become available, Oscar receives an indication that he can start to transmit.
– Immediate call start configuration: If the system is configured to start providing that the talking party’s cell has sufficient resources, the call will be started and audio received by receiving parties in the call on cells where the system has enough resources to receive the call. If cells that did not have enough resources at the start of the call subsequently have the enough resources available e.g. due to a reduction in load, users on those cells are connected into the call as soon as is possible.
Whilst idle, the group is not consuming system resources.