Page 395 - From GMS to LTE

VoLTE, VoWifi and Mission Critical Communication 381

it can remain competitive when slowed down by PMR quality, stability and maturity
requirements. One requirement that might help is that PMR applications could and
should be run on their own IMS core and that there are relatively few dependencies on
the network stack. This could allow commercial networks to evolve as required due to
competition and advancement in technology while PMR organizations can rely on dedi-
cated and independent core network equipment that evolves separately from the rest of
the network.

5.7.5 Mission Critical Push To Talk (MCPTT) – Overview
While a standard voice call establishes a bi‐directional channel between two subscribers
that allows both participants to speak and listen simultaneously, push‐to‐talk services
used by safety organizations work in a different manner. Instead of establishing a bi‐
directional channel, the push‐to‐talk (PTT) concept uses a unidirectional channel and
only one person can speak at a time, by pressing the push‐to‐talk button. All other par-
ticipants on the channel are in a listen‐only mode. Initially this behavior was due to limi-
tations in traditional analog radio services in which the push‐to‐talk button activates the
transmitter in a device. The advantage of PTT over a normal voice call for safety services
is that the radio channel is only busy when a participant has ‘taken the floor’, i.e. they are
talking, and thus a channel can be used for conversations between many different per-
sons simultaneously. All users on the same channel (frequency) can hear all conversa-
tions of others and thus form a group. Different analog radio channels can be used to
separate different user groups, e.g. police, fire departments, individual groups inside
individual organizations, etc. Person‐to‐person PTT is also possible in analog networks
if only two users share a single channel. Typically, analog PTT channels are not encrypted
which makes it easy to monitor such calls with inexpensive equipment. Furthermore, the
service was limited to the transmit and reception range of devices.
The TETRA digital PTT service and the Mission Critical Push To Talk (MCPTT)
service based on LTE work in a way similar to the analog PTT service from a user point
of view but offer solutions for many of the shortcomings of the original service. In addi-
tion, many useful features were added for special situations.
In LTE, MCPTT is based on the IP Multimedia Subsystem (IMS) SIP network that is
also used for the VoLTE service. The service specification can be found in 3GPP 23.179
[35]. Figure 5.19 shows the MCPTT Application Server (AS) and how it is connected to
the IMS infrastructure and the LTE network. As will be discussed in more detail below,
the IMS system is used for registering to the MCPTT service and for group call session
establishment signaling. Like in VoLTE, the voice packets are not traversing the IMS but
are exchanged directly between devices. In MCPTT, speech packets are directly sent
from an MCPTT client device who has been ‘granted the floor’, i.e. the user has received
the right to speak from the network, to the MCPTT speech server. The MCPTT speech
server then sends individual copies of the voice data stream to all participants who have
subscribed to the group. If there are only few participants in the group, each participant
receives an individual copy of the voice data stream. Only one participant can be granted
the floor at a time, i.e. only one person can speak in a group call and all others are in a
listen‐only mode during that time.
For larger groups which are mostly located in a small number of LTE cells, it is option-
ally possible to use the Multimedia Broadcast/Multicast Service (eMBMS) specified by