Page 401 - From GMS to LTE

VoLTE, VoWifi and Mission Critical Communication 387

listener. This creates a heavy load for the server and also for the network infrastructure,
especially near the server side and also close to the recipients if they are physically close
to each other.
IP multicasting addresses these two issues as follows. A device interested in receiving
a multicast stream informs its upstream IP router of its desire to receive a unidirectional
data stream sent to a multicast IP address. In other words, the device requests the router
to forward not only IP packets with its own IP address in the destination field but also
packets with a certain multicast IP address. The Internet Group Management Protocol
(IGMP) is used to request an IPv4 multicast stream. For IPv6 multicast requests, the
Multicast Listener Discovery (MLD) protocol is used. When the router closest to the
listener receives the request it will itself send a request to the next upstream router,
which will do the same, until the final router before the server is reached. If there is
more than one listener in a subnet, only one data stream is required from one router to
the next to serve all of them. If there are listeners in other subnets served by other rout-
ers, only one data stream from the server is required until some point in the middle of
the transmission chain. On that router, the data stream is replicated and a copy is sent
to each downstream router. This way, a tree‐like distribution chain for the media stream
is created and a significant reduction in traffic load is achieved.
Despite these advantages, IP multicast is not widely used across the Internet. Netflix
and other major streaming services use individual unicast streams per subscriber as
they offer content on demand. As a consequence such services would not benefit from
multicast transmissions. Some network operators offering live TV over cable or DSL
lines make use of multicasting, however, as the streams of TV stations are the same for
all subscribers.
As the downlink audio stream of an MCPTT group call also has to reach all partici-
pants of the group call simultaneously it is an ideal application for IP multicast
transmission, especially if many subscribers are located in the same cell, i.e. many sub-
scribers are listening to the same channel and have to receive a single data stream
simultaneously.
In practice multicasting in wireless networks is a challenge because the LTE air inter-
face was built around the concept of unicast bearers to individual devices which are
individually encrypted over the air interface. To reduce power consumption mobile
devices do not decode the full downlink channel but listen to downlink assignments on
the control channel to find out if and where in the channel their data is transmitted by
the eNodeB in the next subframe. For details see Chapter 4.
To enable multicast reception two new LTE air interface channels are required, the
Multicast Control Channel (MCCH) and the Multicast Traffic Channel (MTCH).
eMBMS‐capable devices can use IGMP or MLD to request multicast streams over an
established unicast bearer. The network’s response then contains a Temporary Mobile
Group Identifier (TMGI) and a flow ID that identify a multicast stream on the radio
level. For MCPTT, neither IGMP nor MLD is used as the MCPTT server informs mul-
ticast‐capable MCPTT clients about the TMGI and flow ID of the downlink audio
stream of a group call.
Whether and which multicast streams are active in a cell is announced by the eNodeB
in SIB 2, SIB 3 and the new SIB 13 system information blocks, which are regularly
broadcast. If multicasting is active in a cell an eMBMS‐capable device will, in addition
to monitoring the Downlink Control Channel for unicast assignments, also monitor the