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5

VoLTE, VoWifi and Mission Critical Communication

5.1 Overview

In Chapter 1, the structure of the GSM network was described in combination with
voice telephony. It is difficult to separate the GSM voice service from the GSM network
as the (voice) service and the network are completely integrated. Even in UMTS this is
still the case to a significant degree. For LTE, however, 3GPP decided, with only a few
exceptions, to completely separate the network from any kind of higher‐layer service,
including voice telephony. This is the reason why the description of LTE in the previous
chapter could be done without mentioning an integrated voice telephony solution.
Voice telephony, however, is still an important service and the CS‐Fallback solution
described in the previous chapter was only meant to be a temporary solution on the
path to an all‐IP network in which all services including voice telephony are based on
the Internet protocol. After many years this has finally been accomplished with the
VoLTE (Voice over LTE) IP Multimedia Subsystem (IMS) profile standardized in GSMA
IR.92 [1], which is based on the Session Initiation Protocol (SIP).
In practice, it can be observed today that LTE networks have not yet reached the same
level of geographic coverage as GSM networks and it is likely to remain that way for some
time to come. Therefore, a fallback for an ongoing voice call to a classic circuit‐switched
channel is still required. This functionality is referred to as Single Radio Voice Call Continuity
(SRVCC) and is a major differentiator compared to non‐operator‐based IP voice services,
which have to drop a call when running out of LTE or UMTS network coverage.
In addition to voice on LTE, some network operators have extended their voice service
to the Internet and refer to it as Voice over Wi‐Fi (VoWifi). As this voice service uses the
same IMS network as VoLTE, ongoing voice calls can be transferred between LTE and
Wi‐Fi when required. Again, this is a significant differentiator to other non‐operator‐based
IP voice services, which drop an ongoing call when a device changes between LTE and
Wi‐Fi for Internet access.
The final part of this chapter takes a look at the use of LTE and the IMS infrastructure
for public safety organizations such as the police, fire departments, medical services,
etc. Today, the majority of these organizations still use first‐ or second‐generation
analog or digital push‐to‐talk communication systems which are nearing their end of
life and hence a replacement technology is required. The successor to those systems
standardized in 3GPP using LTE and IMS technology is referred to as Mission Critical
Push To Talk (MCPTT).

From GSM to LTE-Advanced Pro and 5G: An Introduction to Mobile Networks and Mobile Broadband,
Third Edition. Martin Sauter.
© 2017 John Wiley & Sons Ltd. Published 2017 by John Wiley & Sons Ltd.