Page 397 - From GMS to LTE

VoLTE, VoWifi and Mission Critical Communication 383

packets in an individual IP unicast data stream to the device again while the eMBMS
channel in the previous cell remains in place for the other participants of the group.

5.7.6 MCPTT Group Call Establishment
Figure 5.20 shows the SIP signaling flow for establishing a ‘pre‐arranged group call’ as
per 3GPP TS 23.179 chapter 10.6.2.3. The IP Multimedia Subsystem (IMS) components
which forward the SIP messages to the MCPTT server are not shown in the figure. The
group call is pre‐arranged because the group call database that is part of the MCPTT
service contains a list of all subscribers who are to be notified of the establishment of the
group call. The group call establishment request SIP message contains all necessary
information for the MCPTT server to find the group’s configuration data in its database
and also a Session Description Protocol (SDP) part that describes the voice codecs and
other parameters the client can accept for the speech path. Further details on the
Session Description Protocol can be found at the beginning of this chapter in the VoLTE
section.
Depending on device settings, the group call can be automatically accepted by devices
of other group members when receiving an invitation to join a group with a SIP group
call request message. The SIP message contains not only the group call identification
but also an SDP part with the selected media codec parameters. Depending on their
configuration, devices can automatically accept an incoming group call and switch
themselves into hands‐free mode so the users can hear others without the need to hold
the device to the ear. By notifying devices individually of the start of a particular group

MCPTT Client 1 MCPTT Server +
Group Management Server

User Initiates Group Call

Group Call Establishment Request
MCPTT Client 2 and 3
Retrieves Group Parameters
Based on Group ID received
From the Group Call database

Group Call Request
Group Call Request

Group Call Response
Group Call Response
Group Call Response
Group Call Notify (Optional)

Media plane and oor control establishment. Floor control over the media plane.

Figure 5.20 Signaling for establishment of a ‘Pre‐Arranged Group Call’.