Page 399 - From GMS to LTE

VoLTE, VoWifi and Mission Critical Communication 385

TS 23.179 that a higher‐priority user can override a lower priority user. In this case the
server removes the floor from the current user and assigns the floor to the higher‐priority
user. As the server is in control of which uplink media stream it wants to replicate to all
users, it is able to enforce the floor priority scheme required for a group.
At this point it is perhaps interesting to note that while a media bearer is established
for an ongoing group call at all times, bandwidth for the bearer is only required while
one of the users has the floor. While nobody has taken the floor no bandwidth is
required for the ongoing group call in the radio network, in the access network or in
the core.

5.7.8 MCPTT Group Call Types
As there is no one‐size‐fits‐all type of group call, several different kinds of calls have
been specified. The pre‐arranged group call type discussed above is likely to be the most
common one.
In addition restricted group calls have been defined in which MCPTT users individu-
ally join a group call without being invited. The establishment of a restricted group call
therefore does not result in other group members being invited.
To make announcements to a predefined group of users, the broadcast call has been
defined. Here, only the initiator of the group call can speak and all other participants are
in a listen‐only mode. Especially for this group call it is important that the initiator gets
feedback about the status of other subscribers who have joined the call. Once the origi-
nator of the call has finished their announcement the broadcast call ends.
Group calls used during emergencies should have a higher priority especially in the
radio network to ensure that voice transmissions of emergency groups take precedence
over other data and voice traffic. A normal group call can be upgraded to an emergency
group call while the call is ongoing. In addition, network access privileges of all users
subscribed to this group call are also elevated. In addition, emergency alerts have been
specified to alert group members of an emergency. The mere sending of an emergency
alert does not automatically set up an emergency group call itself as sometimes it is
sufficient to send an alert message without establishing a speech bearer.
And finally, another important MCPTT call type is a private call between two users.
While communication is only between two persons, only one party can speak at any one
time as floor control is applied to such a call in the same way as for a group call. This is
especially useful in automatic commencement mode as the terminating device can acti-
vate hands‐free mode and the originator can talk to the terminator without any action
being necessary on the terminating side. If the terminating user wants to respond they
can press their push‐to‐talk button and only then is their speech path sent back to the
originator.

5.7.9 MCPTT Configuration and Provisioning
Configuration information about each group call is stored in the MCPTT group data-
base. 3GPP TS 24.381 [36] specifies the provisioning interface which can be used by
MCPTT clients to request group call information, to change group call information
(e.g. to add or to delete themselves as group members) or to create new group calls.
Which actions clients are allowed to perform depends on the permissions given to their
subscription by the MCPTT system. Interaction with the MCPTT database is based on