Page 403 - From GMS to LTE

VoLTE, VoWifi and Mission Critical Communication 389

Entity (MCE) inform individual eNodeBs which multicast data streams they have to
request from the MBMS‐GW. In addition the MCE is required to arrange simultaneous
transmissions of the same multicast stream in neighboring cells.

5.7.11 Priority and Quality of Service
To ensure that MCPTT‐related traffic is prioritized oven Internet traffic and even over
VoLTE in the network and especially over the radio interface, new QoS Class Identifier
(QCI) values have been introduced in 3GPP TS 23.203 [38].
Normal Internet traffic is transported over a bearer with QCI value 9, which has a
priority of 9 in the network. The higher the priority value the lower is the priority of
such packets in the network. VoLTE SIP signaling uses QCI 5, which has a priority of 1,
while VoLTE speech packets are configured as QCI 1 by the network, which has a prior-
ity of 2. For MCPTT, QCI value 69 with a priority of 0.5 for signaling traffic and QCI
value 65 with a priority of 0.7 for MCPTT speech packets were introduced.
Different priorities are assigned to IP packets in LTE in two ways. Like in VoLTE,
different default bearers, which could best be described as ‘virtual network interfaces’
each with its own IP address, are to be used for different kinds of traffic. The second way
to increase (or decrease) the priority of some of the traffic flowing through one bearer
(i.e. one virtual network interface) is to establish a dedicated bearer alongside an exist-
ing default bearer. A ‘Traffic Flow Template’ (TFT) then describes which source
addresses, destination addresses and UDP/TCP ports are to be handled differently by
the network. Further details can be found in the VoLTE section of this chapter.

Questions

1. Name the major IMS network components and give a short description of their
function.

2. How is it ensured that a SIP message can only be sent by an authenticated device?

3. What are ‘Preconditions’ and how does the mechanism work?

4. Why are Asserted Identities required?

5. Why is header compression beneficial for VoLTE?

6. How are call forwarding settings managed in VoLTE?

7. How are emergency calls handled in VoLTE networks?

8. Describe the main steps in handing over an ongoing VoLTE call to Wi‐Fi.

9. Describe the difference between VoWifi cellular‐preferred and Wi‐Fi‐preferred.

10. Why is ‘floor control’ required in Mission Critical Push To Talk Communication?