Page 398 - From GMS to LTE

384 From GSM to LTE-Advanced Pro and 5G

call, the devices can be located anywhere in the network and do not have to be in a
certain coverage area as would be the case in an analog PTT network.
The MCPTT group database can contain several criteria that have to be fulfilled to
consider a group call to be established successfully. The easiest scenario is when no
establishment criteria are configured. In this case a group call can be set up successfully
without any devices joining the call when it is first established. Once the group call is
established, members can join and leave the group call at any time. When leaving, they
have to inform the group call management server so the number and identities of group
call members in the call can be tracked. A leaving notification is also required as the
MCPTT server has to stop sending the media stream in the downlink direction when
somebody has taken the floor. A group call can also be configured so that the server
only considers call setup as successful if at least one other member has joined the call
during setup. Another option mentioned in TS 23.179 is that group call establishment
is only considered successful if during a configurable time certain group members join
the call. This is required in many PMR use cases, for example, to ensure that a group
leader is always part of certain group calls. If required participants do not join the group
call during the setup phase during a configurable time, call setup fails. In addition, the
initiator can be informed of leaving and joining participants, which is important for
some roles in PMR organizations such as dispatchers, and for small teams that perform
a task requiring all team members to be part of the call at any time.
In many situations it is required that personnel of different PMR organizations tem-
porarily work together. The MCPTT specification therefore also contains methods to
let MCPTT users of one server in one network communicate with MCPTT users served
by another system in another network. This way, the police and fire brigades can use
different MCPTT servers or even different LTE networks and are still able to commu-
nicate with each other if the option is implemented in practice.
In practice it can also be the case that a single user would like to monitor several
group calls simultaneously. An MCPTT client can thus be engaged in several group
calls simultaneously. A media mixer component in the device ensures that the user can
hear when several persons are speaking at the same time in different calls.
A group call can end in several ways. Depending on the configuration of the group in
the network group database, the MCPTT server ends a group call when the initiator
leaves the call, after the call has been idle for a certain amount of time (floor idle time),
when the number of participants reaches a configured lower limit or when certain key
members of the call leave the call.

5.7.7 MCPTT Floor Control
Like in analog push‐to‐talk communication, only one person can talk at any one time in
an MCPTT group call. Unlike in analog communication where several participants can
press the push‐to‐talk button simultaneously and thus garble the conversation for eve-
ryone on the channel, ‘taking the floor’ is controlled by the MCPTT server. If a partici-
pant wants to talk and presses the PTT button, the MCPTT application on the device
sends a floor request message to the server. This is not done over SIP and IMS. Instead
the message is sent directly from the MCPTT client to the server.
If the floor is currently free the server then assigns the floor to the requester and
informs other parties that the floor is now busy. Furthermore it was specified in 3GPP